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Circuits for the Hobbyist
For your electronics hobby entertainment; ENJOY! It is assumed that you have AT LEAST the equivalent of a Basic Electronics certificate for the electronics projects listed on this page. Other projects require more advanced electronics. A lot of these circuits assume the latter so I will no longer answer the tons of emails in regards to that. If you wish to learn more about electronics there is enough of that available on the internet. Circuits' Message Board Ask your questions here. Someone may answer them.

ScanMate Your (Radio) scanner Alternating On-Off Control buddy! 6-20-2002 Audio Pre-Amplifier #1 Simplest R/C Circuit Automatic 9-Volt Nicad Battery Charger Simplest RF Transmitter Basic IC MonoStable Multivibrator Simple Transistor Audio PreAmplifier Basic RF Oscillator #1 Single IC Audio Preamplifier Basic LM3909 Led Flasher Solar Cell NiCad Charger 7-24Battery Monitor for 12V Lead-Acid 2002 Battery Tester for 1.5 & 9V Solid State Relay Bench Top Powersupply, 0-30V/0-10A, Part 1 Third Brake Light Pulser Bench Top Powersupply, 0-30V/0-10A, Part 2 Toroids, RF/EMI Cores Bench Top Powersupply, 0-30V/0-10A, Part 3 Touch Activated Alarm System Birdie Doorbell Ringer Two-Tone Trainhorn 'Bug' Detector with Beep Universal Flasher Circuit Car Converter for 12V to 9V Variable Power Supply, 1 - 30V @ 1.5A Car NiCad Charger Wailing Alarm DC Motor Reversing Circuit Water-level Sensing and Control DC Motor Control Circuit Waterpump Safety Guard for Fish-pond Gel Cell Charger, I - Off-line Weller WLC100 Electronic Soldering Gel Cell Charger, II Station Clock Generator Xmas Lights Tester Christmas Lights Tester Zap Adapter Continuity Tester, Low-Voltage 1.5V Tracking Transmitter Continuity Tester, Smart 4-Transistor Tracking Transmitter Continuity Tester, Latching

Cut Phone Line Detector Dark/Light Activated Relay Electronic Dazer 6-13-2002 Fluid-Level Detector High Voltage circuits Interesting HV devices Lantern Flasher/Dimmer Led Flasher, 2 transistor Leds Flasher, alternately LED Pilot Light (AC or DC) Light Sensor With Hysteresis Logic Probe with pulse Logic Probe with pulse, CMOS Micro-Spy with FETs Micro-Spy with USW Micro-Spy with TTL Miniature FM Transmitter #1 Miniature FM Transmitter #2 Miniature FM Transmitter #3 Mini-Drill variable Powersupply Missing Pulse Detector (Basic) Morse Code Practice Keyer, I Morse Code Practice Keyer, II Motor Accu Lader (Dutch) Motorcycle Battery Charger No-Hassle Third Brake Light 9 to 9 pin (Female) Nullmodem Cable Practical Intercom Pulse Width modulator 07-2002 RF Transmitter, light sensing RJ45 Cable Tester Radio Shack Special 8-

9-V Stabilized Powersupply 30-Meter QRP Transmitter for Morse Code 555 Timer IC Tester 5-30-2002 555 Go No/Go Tester More advanced 741-Light Sensor

555 Timer/Oscillator 741 Op-Amp Capacitors 6-28-2002 Electronic Template MosFet Test Piezo Education/Tutorial PLL - Almost done! Resistor Color Code Tutorial SCR Tester Triac Test UJT Test Coils Integrated Circuits Make Your Own Shunts Relays, Relay Drivers, Solid-State "Green" means on-line, "Red" means offline

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Circuits Archive - Older circuits. Most are working, some are not. Could be still useful. Radio Shack Partnumbers - Most common order numbers for my circuits Tandy Corporation - European/Australian counterpart of Radio Shack TUP/TUN/DUS/DUG European transistor replacement system Tomi Engdahls' Page - Solid electronics projects! Jan Freak's Page in the Netherlands - Well thought out information. Dutch language only Bowden's Hobby Circuits - Collection of circuits, for everyone. Circuit Exchange International - Andy's website. Good selection of excellent circuits Electronic Tutorials - Collection of electronics tutorials. Dolbowent.Com - Electronic Surplus and Engineering Support. Jordan's Electronics Page - Lots of good circuits here also. LED Webpage. White Led's everywhere - Malcom's site in the UK. Guelph Amateur Radio Club - GARC--Official Homepage PA3BWK's Ultimate Morse Code Website - Wilko Hollemans site in the Netherlands Larry's Robotics & Electronics Page - Many good circuits ElectronicsZone - Naveen's Website Spark Museum - John D. Jenkins amazing collection of antique wireless & scientific instruments

DISCLAIMER: I take no responsibility whatsoever for the use and/or implementation thereof, or the misuse leading to damage to equipment, property, or life, caused by the above circuits. Check with local, provincial and federal laws before operating some of these devices. You may also check your life insurance and/or the fact if they cover death by electrocution if you intend to play with Micro-wave ovens and other lethal HV devices. Safety is a primary concern when working with high power circuits or con/inverters. Play it safe!

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Copyright © 1995 - Tony van Roon. ALL RIGHTS RESERVED. Last Updated: August 7, 2002

Alternating ON-OFF Control by Tony van Roon

Use this circuit instead of a standard on-off switch. Switching is very gentle. Connect unused input pins to an appropriate logic level. Unused output pins *MUST* be left open!. First 'push' switches ON, another 'push' switches OFF. You can use 1/4 watt resistors if they are metal-film type. Any proper substitute will work for Q1, including the european TUN's. For C2, if you find the relay acts not fast enough, leave it out or change to a ceramic cap between 10 and 100nF.

Parts List
All resistors are 1/2 Watt and 5% tolerance. R1 = 10K R2 = 100K R3 = 10K C1 = 0.1µF, Ceramic C2 = 1µF/16V, Electrolytic D1= 1N4001 Q1 = 2N4401 (ECG123AP, NTE123AP, etc.) IC1 = 4069, CMOS, Hex Inverter (14069), or equivalent S1 = Momentary on-switch Back to Circuits page
Copyright © Tony van Roon

Audio Pre-Amplifier

Additional Notes (N/A) Back to Circuits page

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Published & Translated with permission of Jan Hamer, The Netherlands. Good care given to your NiCad batteries will ensure a long life. However, they do need to be handled and charged with special care. It is therefore important to first discharge the NiCad to 1 Volt per cell, ensure that the battery is discharged, and then start the charge cycle. Manufacturers

recommend a charge current of 1/10th the capacity for a duration of about 15 hours uninterrupted. In reality, we learn some hard lessons when we forget to switch the charger off after the 15 hours and find that one or more cells inside the battery no longer accept a charge. That is the very reason that the circuit above is fully automated. The only thing to do is connect the battery and press the 'Start' button. When the discharge cycle is finished the circuit switches over to charge for 15 hours. After the 15 hours the circuits maintains a trickle charge to keep the battery 'topped-up'. Before I go into the schematic details I like to explain some of the component descriptions in the schematic. Jan Hamer lives in the Netherlands and so the circuit details are based on european standards. 120E, 150E, etc. The 'E' just stands for Ohms so 120 ohm, 150 ohm. The original circuit specified the HEF type of cmos IC's which are not readily available in most of Canada. So just get any other type of CMOS chip like the MC4011, MC4020, MC4047 from Motorola. Any other type will do fine too. The BC548B is replaceble by a NTE123AP (NOTE: make sure it is the 'AP' type, the regular NTE123A is a total different transistor), ECG123AP, and the 2N3904 will work also. Watch for the correct pin locations since the BCE may be reversed with this european type. The LM317T is a TO-220 type and replaceble with a ECG956 or NTE956. The LM339N can be replaced with a ECG834 or NTE834 Although this circuit looks quite impressive and maybe a bit difficult it is certainly not difficult to understand. The circuit needs to be hooked-up to a DC supply voltage of between 16.5 and max 17.5 volt, otherwise the CMOS IC's will go defective. Because I didn't feel like to design a seperate powersupply for this circuit I connected it to my fully adjustable bench top powersupply. First we connect a 'to-be-charged' 9-volt nicad battery to the appropriate connections. Then hook it up to the powersupply. Upon connection the 1nF capacitor starts up the two RS Flip-Flops formed by IC1a, IC1b, IC1c, IC1d, and pulls pins 3 and 10 'high' and pins 4 and 11 'low'. The clock pulses are created by the freerunning multivibrator IC4. IC4's frequency is determined by the 10uF capacitors, the 220K resistor and the 100K trimpot. The clock runs continuesly but the counter behind, IC5, is not counting yet because pin 11 (the master-reset) is kept high. When the 'START' button is pressed, output pin 4 from IC1a goes high and biases TR4, which is made visible by the Red LED (D9) which remains lit. The NiCad is now being discharged via this transistor and the 100 ohm resistor. The 10K trimpot (at the right of the diagram) is adjusted in such a way that when the battery voltage dips below 7 volt, the output of IC3 goes LOW and the output pin 11 of IC1a HIGH. At hte same time the output pin 10 of IC1d goes LOW, and the red LED turns off. Because output pin 11 went HIGH the green LED (D8) lights up and at the same time the voltage level rises causing the battery to be charged. The chargecurrent is determined by the 120 ohm, 150 ohm, and the trimpot of 1K, at the right side of IC2. Actually we could have used one resistor, but the output voltage of different brands for IC2 may differ, by about 1.25 volt. Because the charging current is devided by value of the resistors, with the trimpot the current can be adjusted to the correct value of your own 9-volt NiCad. (In my case, the battery is a 140 mA type, so the charge current should be adjusted for 14 mA (c/0.1). At the same time the LOW of output pin 10 from IC1d starts the counter of the clock. On pin 9 of IC5 appear pulses which light up the red LED. This is implemented for two reasons, the clock-frequency can, with the 100K trimpot, be adjusted to the correct value; the red LED has to come ON for 6.59 seconds and for the same duration going OFF and except for that fact the green LED, who indicates the charge current, can be checked if the total charge-time is correct. When the counter has reached 8192 pulses ( x 6.59 = 53985.28 sec = 14.99 hours) the output pin 3 of IC5 goes high again, transistor Tr1 activates and resets the two flip-flops to the start position. The charging process stops and goes over to trickle charge via the 10K resistor and the D2 diode and keeps the battery topped-up. The adjustments of the project are really very simple and nothing to worry about. Turn the walker of the 10K pot in the direction of the 12K resistor, ground connection point of 10K resistor/diode D2, like the adjustment pin of IC2, apply a voltage of 7-volt to the battery connection terminals, switch the power ON and slowly turn the pot backward until the greeen LED starts to light up. Switch OFF the power and take away the connections you made to make the adjustment. Insert an amp-meter between the battery and the output connection and again switch the power ON. The battery will, in case it is not completely empty, totally discharged (to a safe level) and as soon as the 7 volt margin is reached goes over to the charge cycle. The charge current is at this time adjusted via the 1K trimpot (which is connected in series with the 150 Ohm resistor and in parallel with the 120 ohm resistor) accurately to the desired value. Addendum: It is strongly recommended to include small 100nF ceramic capacitors over the powersupply lines feeding EACH CMOS IC to keep possible interference to a negliable value.

If you have improved upon or know ways to improve it, Jan Hamer will appreciate your feedback. Klick on his name at the top of this page or contact him via his website specified below. Thanks! Please visit Jan Hamer's website in the Netherlands! Return to Circuits Page
Copyright © 1995 - 2001 Tony van Roon

Basic IC MonoStable Multivibrator by Tony van Roon

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Posted with permission of Jan Hamer This simple circuit makes it posible to monitor the charging process to a higher level. Final adjustsments are simple and the only thing needed is a digital voltmeter for the necessary accuracy. Connect an input voltage of 12.65 volt between the positive and negative poles and adjust the 10K trimmer potentiometer until Led 10 lights up. Lower the voltage and in sequence all other Led's will light up. Check that Led 1 lights up at approximately 11.89 volts. At 12.65 volt and higher the battery is fully charged, and at 11.89 is considered 'empty'. The green Led's indicate that the battery capacity is more than 50%, the yellow Led's indicate a capacity of 30% - 50% and the red Led's less that 30%. This circuit, with the components shown, uses less than 10mA. Ofcourse you can adapt this circuit to your own needs by making small modifications. The circuits above is set for 'DOT' mode, meaning only one Led at a time will be lit. If you wish to use the 'BAR' mode, then connect pin 9 to ground, but obviously with increased current consumption. The LED brightness can be adjusted up- or down by choosing a different value for the 4K7 resistor

connected at pin 6/7 You can also change the to monitoring voltage level. For example, let's say you wanted to change to 10 13 volt, you connect 13volt to the input (+ and -) and adjust the 10K potentiometer until Led 10 lights up. Change temporarily the resistors at pin 4 with a 200 Kilo-ohm potentiometer and reconnect a voltage from 10 Volt to the input. Now, re-adjust the 200K potentiometer until Led 1 lights up. When you are satisfied with the adjustment, feel free to exchange the 200K potentiometer with resistors again.(after measuring the resistance from the pot, obviously). The diode 1N4007 was included to protect the circuit from a wrong polarity connection. It is however strongly recommended to connect the monitor directly to the battery, in principle a connection to the cigarrette lighter would suffice but for reasons unknown at this time the voltage at that point is 0.2 volt lower than the voltage measured directly on the battery. Could be some residual resistance caused by ignition switch and path through the fuse? Back to Circuits Menu page
Page copyright © 2001 - Tony van Roon

Battery Tester for 1.5 and 9V by Matthew B.

Parts List:
R1 = 18K R2 = 240 Ohm R3 = 8.2K R4 = 3K R5 = 10 Ohm M1 = Panel Meter (Anyone will work)

Design Considerations:
You may have experiment with the values of R3 and R4 to get an accurate reading from the meter. Every meter is different, so a little bit of playing with the resistor values is required. Try using a variable resistor in place of R3 & R4 to get a value of resistance that works. If you have questions or suggestions please contact Matthew B. Back to Circuits page
Circuit Copyright © 2002 - Matthew B. ALL RIGHTS RESERVED Page design Copyright © 2002 - Tony van Roon

Notes
P1 is of experimental value. Start with 220 Ohms or so and modify to suit your needs. The transistor is a general purpose kind and is not critical, almost any pnp type will work. L1 is a bell-transformer which is usually already present in the house. If you wish, you could use a battery instead of the bell transformer. Just hookup a 9-volt battery to points 'A' and 'B' (A=+) the diode (D1) is to protect the circuit from accidental polarity reversal and is optional, but required for use with the bell transformer. T1 is a General Purpose PNP transistor and probably anything will work. L2 comes out of an old am transistor radio. They look like miniature transformers and are usually colored red or green. You have to fiddle with different transformers as the sound can vary depending on the value. The loudspeaker is a 8 Ohm type and must be larger than 200milli-Watt. I used a 2Watt type, but anything over 0.2W will do. It really sounds like a bird and when you release the doorbell button the sound slowly fades away. I have used this circuit in my house for over 20 years and even build the "Birdie" for others. Although an old circuit, the experimentation and the final results still give a punch. Remember to Have fun! Back to Circuits Page
Copyright © 1993 - Tony van Roon

Parts List
R1 = 390 ohm, 5% R2 = 390 ohm, 5% C1 = 1000uF, 6V C2 = 1.5uF, 6V C3 = 3.9nF C4 = 20pF, trimmer C5 = 10nF (0.01uF) IC = SN7413 or SN74LS13 (2) Please note: This circuit is not open for discussion. Although working perfectly, it was experimental. I will answer no emails in regards to this circuit. Back to Circuits page
Copyright ©1995, Tony van Roon

Parts List:
R1 = 560 ohm C1 = 1000µF/16V, Electrolytic C2 = 100µF/16V, Electrolytic C3 = 330nF, Ceramic Z1 = 9.1V, 0.4watt zener Q1 = ECG184, NTE184

Notes:
To get a more precise output voltage, replace zener diode Z1 with 10V and R1 with a 1Kilo ohm potentiometer. A Coolrib for Q1 is optional. Simple circuit to power your 9 volt cassette recorder and other stuff. Back to Circuits Menu
Copyright ©2001, Tony van Roon

Parts List:
Resistors are carbon, 1/4 watt, 5% tolerance, unless otherwise indicated. R1 R2 R3 *R4 *R5 *R6 *R7 = = = = = = = 22 ohm, 1W 270 ohm 220 ohm 715 ohm, 1% 3.57K, 1% 1.40K, 1% 1.47K, 1% C1 C2 D1 T1 U1 S1 = = = = = = 0.1µF, ceramic 0.1µF, ceramic 1N4001 TIP31A, B, C (or equivalent) NE555V (or equivalent) Toggle switch, ON-OFF

Description:
This circuit needs a regulated 10V-DC front end capable of supplying 2 Amps. Starts the charge cycle at 240mA and at full charge switches automatically to a float condition (trickle charge) of 12mA. The capacitors are the ceramic 50V (or better) type. Switching transistor T1 is an NPN, Si-Power Output/SW, with a TO-220 case and can be replaced with a suitable substitute like the NTE291, ECG291, etc. Timer/Oscillator U1 is a 8-pin NE555V and can be replaced with a NTE955M or ECG955M. Resistors R4, R5, R6, and R7 are 1% metal film types. They may not be available at your local Radio Shack/Tandy store and have to be ordered in. Try Electro-Sonic or Newark Electronics supply stores.

NOTE: For 6-volt, 1.2Ah Gel Cell type batteries only!
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Copyright © 2001 by Tony van Roon

o - Excellent clock generator to drive 4017 type cmos circuits. o - R1 = 10K to 10M, C1 = 100pF to 47uF. o - Fo is ±1Kz when R1=100K and C1=10nF. o - Input voltage can be from 5 to 15V. Please note: I will answer no email in regards to this circuit. Back to Circuits page
Copyright © Tony van Roon

Chrismas Lights Tester
© by Jan Hamer Like every year arround the same time, I hurried to get my Christmas tree all set up and the first thing we do when the tree is 'standing' we like to hang the lights in the tree. Okay, better first test them before putting all 50 of them in the tree. Yep! Working beautifully. I started a carefully planned organization of the lights so they would be evenly devided over the branches. Now the second string of lights, tested, yep working. In the tree with them. Putting the plugs into the receptacle and... oh no-- one series of them are on in full glory, all the others are out. Annoyed I tried to 'fix them' by trying to push each bulb further into their sockets. Still no go. It was a crime trying to pull all the bulbs out of their sockets to measure them for continuity. Funny enough, and against the law of nature, it was not even the last bulb in the string of 50 which was defective, but number 41. I put a new bulb in it, and yes here we go, they all light up beautifully. Alright! Happy again I again hung them in the tree. Finally the big moment arrived, as soon as I plugged them in they would shine in all their glory. Right? Oh no! The second I plugged in my lights only the first series of bulbs lighted up, same as before. All my work for nothing. Sigh... In the mean time is was already way past midnight and so I decided for my next attempt to wait till next morning. Irritated and very annoyed I went to bed. However, I was so irritated that I could not sleep immediately and so was thinking of a smart way to get to the defective bulb the easy way. All over sudden I got it; if the bulb was not lit, there was no current draw either and up to the defective bulb I would measure the 115V AC (phase). Now I knew the solution I almost fell asleep satisfied right away. The next day I had to get some groceries in I noticed new xmas lights for a small price. $5.95 for a string of 100 lights, and with a CSA and UL sticker. Wow, I thought for that kind of money I might as well forget the repair and buy a new set. So I did. Coming home I plugged the new lights into the receptacle and yes, all 100 were doing fine. Happy again with the new lights I again hung them in to the Christmas tree, not suspecting that this could be another rotten day. After fiddling with the lights to get them all neatly organized in the tree the moments had arrived to plug them in and awe at the fascinating beauty of those little lights. Yes? NO! Not again. Isn't this to explode out of your skin! Angry I was looking for a solution, but there was none. I finally decided to put a circuit together on pieace of experimenters board from Radio Shack.

The heart of this little "CIRCUIT" is established by a hex inverter IC, the MC14069. By positive feedback to the input, the first inverter acts as an analogue amplifier, which amplification can be adjusted a bit via the 50K trimpotentiometer. To get the correct polarity on the basis of the transistor a second and third stage inverter have been added the same way. The others I put to the positive input voltage of the 9-volt battery. When you touch a voltage carrying wire, with the antenna connected to pin 1 of the MC14069, the led will light up. The antenna is just a sturdy small piece of wire. Armed to the teeth with this little tester I re-investigated the cords. At the first try I ofcourse picked the wrong wire; the neutral (0). The moment I tried it on the other wire (phase) the led came on right away. I followed the cord from bulb to bulb sliding the piece of antenna wire over the cord until I hit the broken xmas bulb and the led went out. Aha! Finally got the bloody little sucker! The broken bulb showed voltage on one site of the wire (led on) and none at the other end of the bulb (led off). This little tester can also be used for other AC applications, like checking for broken wires behind the wall and stuff. If you have questions about this circuit, please direct them to Jan Hamer or visit his website in the Netherlands (if you can read Dutch). Published & Translated from Dutch into English with permission of Jan Hamer, The Netherlands.

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Page Copyright © 2002 - Tony van Roon

Parts List:
R1 R2 R3,R4 R5 R6,R7 R8 C1,C2 = = = = = = = 1K 2K2 22K 2K7 56K *See text 22nF D1,D2 = 1N4148 Z1 = 8V2, 1/4 watt T1 = 2N3905 (PNP) T2,3,4,5 = 2N3904 (NPN) 9volt Alkaline battery suitable loudspeaker housing & probes

An on-off switch is not necessary. D1 is used when the battery is brand-new and giving over the nominal 9 volt, T1, T2 and T3 acting as the switch for supplying power to the multivibrator.

Design Considerations:
Several simple circuits were tried -- a lamp, battery and probes still demanded the attention of the eyes; replacing the lamp with a buzzer was more successful but needed some three to four volts and gave no indication of a series semiconductor junction if the polarity was correct while the current flow was large enough to damage the more delicate devices within the circuit under test. An extension of the principle to

operate an astable (multivibrator) type of oscillator gave good audibility but would operate from zero through to several thousands of ohms and so was too general an indication. A set of specifications was becoming apparent; (1) probe current to be small; (2) probe voltage to be as low as possible, preferable less than 0.3V to avoid seeing germanium or silicon junctions as a continuous circuit; (3) no on/off switch to be used. The above circuit was the result and several have been designed and are earning their keep for both "heavy" electricians and electronic technicians.

How it works:
Starting with a 9 volt supply, when the probes are shortcircuited there is a 8.2 volt drop accross the zener diode Z1 leaving a maximum of 0.8 volt across R1. Aplication of Ohms' Law shows that a maximum current of 0.8/1,000 = 0.8 mA lows via the probes and this satisfies the first design requirement of low probe current. T1 is a silicon type and the base-emitter voltage will need to be about 0.5 to 0.6 volt to forward-bias the junction and initiate collector current. With a maximum of 0.8 volt availabe across R1 it is seen that if a semiconductor junction or resistor is included in the outside circuit under test and drops only 0.3 volt then there will be 0.5 volt remaining across R1, barely enough to bias T1 into conduction. Assuming that the probes are joined by nearly zero resistance, the pd across R1 is 0.7 - 0.8 volt and T1 turns on, its collector voltage rising positively to give nearly 9 volt across R3. T2 is an emitter follower and its emitter thus rises to about 8.3 volt and this base voltage on T3 (a series regulator circuit or another emitter-follower if you prefer it) results in some 7.7 volt being placed across the T4 - T5 oscillator circuit. All the transistors are silicon types and unless the probes are joined, the only leakage current flows from the battery thus avoiding the need for an On-Off switch. When not in use, the battery in the tester should have a life in excess of a year. My own unit lasted for more than 2 years with one Alkaline battery.

Descriptive Notes:
The output from the speaker is not loud but is more than adequate for the purpose. I used a small transistor radio loudspeaker with an impedance of 25 - 80 Ohms. The resistance should be brought up to 300 ohms by adding series resistor R8. Example, if your speaker is 58 ohms, then R8 = 242 ohms. An experiment worth doing is to select the value of either C1 or C2 to produce a frequency oscillation that coinsides with the mechanical resonant frequency of the particular loudspeaker in use. Having choosen the right value, which probably lies in the range of 10n - 100n, the tone will be louder and more earpiercing. A "freewheel" diode D2 is connected across the transducer since fast switching action of the oscillator circuit can produce a surprisingly high back e.m.f. across the coil and these high voltages might otherwise lead to transistor damage or breakdown. Zener diodes do not provide an absolutely constant volt-drop regardless of current; at the 0.8 mA design current an 8.2 volt diode will quite possibly give only about 8.0 volt drop since test current for zener selection and marking is typically 5 mA or more. A further possible source of error is the battery; the one

suggested, nominally provides 9V but a brandnew one may be as much as 9.2 - 9.6V until slightly rundown and this "surplus" voltage, combined with an "under-voltage" zener volt-drop will leave considerably more than the forecast voltage available at the probes. A silicon diode D1 is therefore connected in series with the zener to decrease the probe voltage by a further 0.6 volt or so. During your final testing and before boxing your circuit, the most suitable connection, A or B, is selected for the positive probe wire. The aim is to have the circuit oscillating with short circuited probes but to stop oscillation with the least amount of resistance or the inclusion of a diode (try both ways) between the probes. No sensitivity control is fitted because I don't think it is worthwhile nor necessary and would spoil the simplicity of the circuit. There is no easy way to proof the unit against connection to the supply. Be careful if checking AC line wiring and switch off first. In a similar way, if checking electronic apparatus for unwanted bridging between tracks, for instance or a suspected crack in a PCB (Printed Circuit Board) track switch off power first also. DISCHARGE ALL LARGE CAPACITORS. Good luck!

The pcb pattern above is shown full-size at 73mm x 33mm (2-7/8" x 1-1/4") Back to Circuits page
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Parts List:
R1 R2 R3 R4 R5 = = = = = 100K 10K 1K 100K 500 ohm Led1 = High Brightnes LED, RED, 5mm IC1 = LM741, OpAmp 9 volt Alkaline battery, case, probes

How it works:
Occasionally you need a continuity test between two points in an electronic circuit. Unfortunately, most continuity testers are prone to "lie". They don't do that deliberately, but if they see a small resistance, they still tell you that you have continuity. They just don't know any better. This unit is different. If you have continuity it will tell you so. And if you're reading even a low resistance through a component, the unit will tell you that as well. The unit uses two 741 op-amps. It offers a short-circuit test current of less than 200uA. It detects resistance values of less than 10 ohms. Nicest of all, it will not break down a PN junction. The device has come in handy in my own shop for debugging electronic circuits. In building this circuit, use good electronic practice, mounting the 741's in suitable ic sockets on perfboard. While there's nothing critical here, keep the work neat, and leads nice and short. When you're done, mount the unit in a small plastic box. A small dab of silicon rubber adhesive keeps the 9-volt battery in place at the bottom of the case, and will last a long time.

Just in case you're just starting out in electronics, here is how to get the -9, +9, and Ground connections. A small hole with a grommet keeps the leads (probes) together. Another hole with a grommet holds the LED in place on top of the box where it is plainly visible. This makes a nice one-evening project. Enjoy!

Caution:
There is no easy way to proof the unit against connection to the supply. Please, please be careful if checking AC line wiring and switch off first. In a similar way, if checking electronic apparatus for unwanted bridging between tracks, for instance or a suspected crack in a PCB (Printed Circuit Board) track switch off power first also. Always practice good safety and think-before-you-do! Back to Circuits Menu
Copyright © Tony van Roon>

Continuity Tester, Latching
By Tony van Roon "This Latching Continuity Tester can help you locate those difficult-to-find intermittent short and opens that other testers always seem to miss. It has been part of my workbench for many years and performs superb. I have solved many intermittend problem with this highly flexible unit." A continuity tester is a must on every service bench for testing cables, pcboards, switches, motors, plugs, jacks, relays, and many other kinds of components. But there are times when a simple continuity test doesn't tell the whole story. For example, vibration-induced problems in automobile wiring can be extremely difficult to detect because a short or open is not maintained long enough for a non-latching tester to respond. This latching continuity tester detects intermittent (and steady state) opens and shorts. The tester will detect and latch on an intermittent condition with a duration of less than a millisecond. In addition, it provides both visual and (defeatable) audio indicators, uses only one inexpensive and easy-to-find IC, and can be built from all new parts for about $35, or less if you have a well-stocked junkbox.

Circuit Elements:
The heart of the circuit is a 4093 quad two-input NAND Schmitt trigger, one gate of which is shown in Fig. 1-a. The gate functions as shown in Fig. 1-b. Nothing happens until the enable input goes high. When that happens, the output responds to the input as follows. As long as the input voltage stays between VH and VL, the output stays high. But when the input goes above VH, the output goes low. The output will not go high again until the input goes below VL. That characteristic is what gives the Schmitt trigger its ability to "square-up" a slowly changing input signal. The Schmitt trigger is ideally suited for our application because it is not dependent on edge triggering, and because both slow and fast signals trigger it when either threshold is exceeded.

We use two gates of the 4093 as a combination detector and latch. The gates are cross connected to form an SR (SetReset) flip-flop. When pin 12 goes low, pin 11 will go high. That high may be used to enable an LED or other indicator. Switch S1 is used to select whether the tester will provide ouput when it detects an open or a short. In the OPEN position, pin 12 is held low, so the output of the gate is normally high. When the test leads are connected across a short, pin 12 is pulled high, so the output drops low. The circuit works in the converse manner when S1 is in the CLOSED position. As shown in Fig. 2-a, we use another Schmitt trigger to build a gated astable oscillator. A gated astable oscillator produces output as long as the GATE input is high. Fig. 2-b shows the waveforms that are present at various points in the circuit. When the pin-8 input goes high, pin 10 goes low, and C1 starts discharging through R1. When VC falls below VL, the output of the gate goes high, so C1 starts charging through R1. When VC exceeds VH, the output again drops low. Oscillation continues in that way as long as the gate input remains high. The frequency of oscillation is given by a fairly complex equation that can be simplified, for purposes of approximation, as F = 1 / R1C1.

Putting it all together:
The complete circuit is shown in Fig. 3. In that circuit, IC1-a and IC1-b funtion as the flip-flop/detector. The output of IC1-a is routed through S4, AUDIO. When that switch is closed, IC1d is enabled and an audio tone will be output by BZ1. The frequency of that tone can vary from 1000Hz to well above the audio range (100KHz), according to the setting of R4. In addition, R4 varies frequency and volume simultaneously, so you can set it for the combination that pleases you best. Originally we used a PM (Permanent Magnet) speaker. When the detector has not been tripped, the full power-supply voltage is across the buzzer, but no current is drawn. The reason is that the piezo element is like a capacitor and does not conduct DC current. When the circuit is oscillating, the buzzer consumes a current of only about 0.5 milli-amp. The output of the flipflop/detector circuit also drives IC1-c. If S2 is in the AUTO position, the output of IC1-c will automatically reset the flipflop after a period of two to six seconds, depending on the position of R7. If S2 is in the MANUAL position, the LED will remain lit (and the buzzer will continue buzzing, if S4 is on) until manual RESET switch S3 is pressed

Construction:
Picture at the left shows the tester from the back. The hole is for the piezo buzzer. The circuit may be built on a piece of perfboard or Vero-board, or on a PCB. The PCB is designed to take boardmounted switches, which makes a neat package and eliminates a rat's nest. (see prototype picture below). Referring to Fig. 4, mount and solder the components in this order: diodes, fixed resistors, IC-sockets, capacitors, variable resistors, and then the pcb mounted switches. The regular ones will work too it just means more wire. Mount the buzzer and the LED last as

described below. Trimmer potentiometer R7 is manufactured by Piher (903 Feehanville Drive, Mount Prospect, IL 60056); it has a shaft that extends through the panel. If the Piher pot is unavailable, an alternate is available from DigiKey (701 Brooks Ave, South, P.O. Box 677, Thief River Falls, MN 56701). The disadvantage of the alternate is that it has no shaft, so it must be adjusted using a miniature screwdriver. The circuit board is held approximately 1/2inch from the cover by the shafts of the switches. The LED and the buzzer should be inserted in the appropriate holes in the pcb now. Then install the top cover, and adjust the height of the LED so that it protrudes through the top cover. Then solder its leads. Attach the buzzer to the top cover, using silicone rubber adhesive (RTV or double side foam tape. We mounted a pair of banana jacks on the top of our prototype's case, but you could solder the wires directly to the appropriate points on the circuit board, tie strain reliefs in the wires, and then solder alligator clips to the ends of the wires. However, a set of good leads are really not all that expensive and it does give the tester more flexible usage as you have the opportunity to use a variety of different leads to suit your purpose. The nine-volt battery is secured to the side of the case with a clip or use a holder. Your completed pcb should appear as in Fig. 5.

Usage Hints:
Set S1 for short or open depending on the condition to be tested. Then connect the test leads across the circuit to be tested. If an intermittent condition is detected, the LED will illuminate, and the buzzer will sound (if S4 is on). If you don't remove the test leads (assuming if S2 is set for AUTO Reset, the LED will flash (very fast)and audio will warble at a rate determined by the reset circuit. It is very important that the test leads make a positive connection with the circuit to be tested. In fact, clips should be used instead of test leads. There are good test leads available for about $15 which are hardened stainless-steel and have sharpened points which were my personal choice. This detector is so sensitive that, when it is initially connected across a long length of parallel wires or traces, it may latch due to capacitance between the wires. As a matter of fact, it happens with my model all the time. Just press the reset switch S3 when that occurs.

Parts List
R1 = 10K IC1 = 4093B Quad Nand Schmitt Trigger (NTE4093B/ECG4093B) R2,R3 = 470K D1,D1 = 1N914 or 1N4148 (NTE519/ECG519) R4 = 100K Trim-pot LED1 = Red, 5mm, High Brightness R5 = Not used BZ1 = Piezzo Buzzer R6 = 1.8K (1800 ohm) S1 = DPDT, miniature toggle, pcb mount R7 = 1M Potmeter (Lin) S2,S4,S5 = SPDT, miniature toggle, pcb mount R8 = 10M S3 = SPST, momentary push, normally open C1 = 0.1µF, ceramic Additionally: IC socket, plastic case (4.75" x 2.5" 1.5"),banana jacks, C2,C4 = 0.01µF ceramic wire, solder, battery clip, couple cold beers. C3 = 4.7µF, 16V, Elec.

I fully support this project. Most parts can be obtained via your local Radio Shack or Tandy store. I will answer all questions but via the message forum only. Tony's Message Forum can be accessed via the main page, gadgets, or circuits page. I'm fine-tuning this project at this time. There are a couple of extra holes on the pcb; ignore them. When you're done soldering everything up check your wiring before connecting the battery. Especially if you use non-pcb switches (which is okay) it is very easy to make a wiring error. Good luck and have fun building this most versatile project. For Radio Shack part numbers click on this RS data sheet.

Copyright and Credits:
The original project is copyright © by Eldon L. Knight (1986). Document updates & modifications, all diagrams, PCB/Layout by Tony van Roon. Photography by Yves Savoret. Back to Circuits page Copyright © 2002 - Tony van Roon

Parts List:
R1 = R2 = R3 = R4 = R5 = Mount R6 = R7 = R8 = P1 = P2 = (Rx) 47K 470 ohm 10 ohm 22K 3M3 100 ohm 330 ohm 100 ohm 100K, Lin 20M 10-turn optional C1 C2,C3,C4 C5,C6 D1,D2,D3,D4 T1 U1 LS1 J1,J2,J3 J4 LED1 = = = = = = = = = = 1uF/16V 0.1uF 1000uF/25V 1N4001 2N2222 LM1458 Speaker, 8-ohm Jacks, 3mm* Jack, 2mm* Bicolor LED* S1 S2 S3 Ry1 TR1 = = = = = on-on, SPDT Switch on-off, SPST Switch on-off, SPST Switch (115VAC) Reed Relay, 5V-1A Transformer, 12.6V CT, PC-

Socket for U1 (8-pin) >>Radio Shack

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