Continuity tester circuit - Page 1
Forum section: electronics / beginners

author: msknight
author_rank: Regular Contributor
timestamp: July 15, 2021, 06:21:27 am
content: I'm doing so much probing and learning that switching my DMM between ohms and continuity is getting a bit wearing. So I decided to throw together a continuity tester. Right? Yeah, well that's not turning out to be as easy as I thought.I've been following this page-https://www.homemade-circuits.com/make-this-simplest-continuity-tester/- and I do have a 555 so I threw that circuit together and it works, but it's got 9v across the probes and I am a bit concerned about this. So I thought I'd ask for some advice. I'm interested by the LM3909 circuit and pondered buying some chips and trying that out, but the ebay scalpers are asking silly money per chip.There is a more advanced circuit at the bottom of the page but there's no sounder(I'd rather not have to look up from where I'm probing to check an LED)and again it looks to be 9v.Any advice gratefully received.

author: ataradov
author_rank: Super Contributor
timestamp: July 15, 2021, 06:42:03 am
content: In order to test continuity you have to pass some current through the circuit so there will always be some voltage applied to the probes. The exact voltage depends on the purpose of the probe. Most general purpose multimeters apply around 1 V - 3 V.In your case the current is limited to 9V / 4700 Ohm = 2 mA. Most things will be fine with that. If you are worried, you can increase the value of the resistor to further limit the current.

author: msknight
author_rank: Regular Contributor
timestamp: July 15, 2021, 06:46:34 am
content: So increase the 4.7k if I want to be safer, but you see no problem with the circuit as is. Superb.This is where a beginner like me keeps falling down :-) Thanks for the help.

author: ataradov
author_rank: Super Contributor
timestamp: July 15, 2021, 06:48:47 am
content: Quote from: msknight on July 15, 2021, 06:46:34 amI mean, 9 V is higher than most testers. But I don't see that as being a huge issue. If you are probing something very sensitive, you should be careful anyway with any equipment.But if you want to be really safe, get another meter. It is handy to have two full meters. And good Uni-T meters are very cheap.It is often also useful to know if something is a dead short or just a low resistance.

author: kripton2035
author_rank: Super Contributor
timestamp: July 15, 2021, 08:05:42 am
content: here is a collection of continuity tester circuits. choose the best for you.http://kripton2035.free.fr/continuity-repos.htmlhttp://kripton2035.free.fr/Projects/shorty-display.html

author: Gyro
author_rank: Super Contributor
timestamp: July 15, 2021, 08:25:43 am
content: The ideal continuity tester puts out a low enough voltage on the probes that it doesn't turn on semiconductor junctions, and also has a low enough resistance threshold that it only registers direct connections, not lowish value resistors, most DMMs fail here.The attached (well written) project from an old 1989 copy of Everyday Electronics does the job well. It puts out less than 300mV on the probes and can be set to resolve to below 1R. It also has an isolation mode which checks for insulation above 1M. It has audible indication and decent protection against accidentally applied voltages.It's based on a bridge circuit and can be tweaked. It only requires a single opamp and 4000 series inverter package. 9V battery powered....

author: msknight
author_rank: Regular Contributor
timestamp: July 15, 2021, 09:29:19 am
content: Quote from: Gyro on July 15, 2021, 08:25:43 amCan I use an LM358L in place of the LM308 on that circuit, do you think? I have some of the former already.

author: Ian.M
author_rank: Super Contributor
timestamp: July 15, 2021, 10:45:26 am
content: If you've got an ESR meter, you can use that for most continuity testing if you are aware of it's limitations, namely that it cannot test inductors and will 'jump' across large capacitors. Its output should be low enough not to turn on silicon junctions significantly. It would be relatively simple to modify most analog ESR meters to add an adjustable threshold beeper.Caution: Many ESR meters are electrically rather fragile and will be damaged if you accidentally use them on a powered board or charged caps.Considering all the above replies, maybe the true answer is just to have at least two multimeters on your bench so you can leave one fitted with needlepoint probes and set on continuity?

author: msknight
author_rank: Regular Contributor
timestamp: July 15, 2021, 11:29:19 am
content: Quote from: Ian.M on July 15, 2021, 10:45:26 amTrue. Very true. But I need to learn, and solving the issues of reading circuit diagrams, ordering parts, putting things together on a bread board and then getting a project case and making it up properly... this seemed to be a perfect project to solve a minor need and also do a chunk of learning.I think I'm going to attempt Not1Xor1's continuity circuit -http://kripton2035.free.fr/Continuity%20Meters/continuity-eevbl.html- I think I can manage that and I have the chips. The only thing I think I'm going to come unstuck on, is the TL431.

author: Gyro
author_rank: Super Contributor
timestamp: July 15, 2021, 03:43:14 pm
content: Quote from: msknight on July 15, 2021, 09:29:19 amYes, you could give it a try. The input bias current is higher than the LM308 but the input offset voltage is about the same at typ. 2mV. Supply voltage is fine and the inputs are biased to mid rail so common mode range isn't an issue. The LM358 supply current is a bit higher, but probably not enough to significantly reduce battery life.It's not a particularly critical set of requirements for this one.

author: msknight
author_rank: Regular Contributor
timestamp: July 15, 2021, 07:22:48 pm
content: OK - just a sanity check on Not1Xor1's diagram. I've bought what I hope is the right part, just in a three pin format rather than an IC....I believe that pin 3 in the schematic is the cathode ... pin 2 is the Anode and pin 1 is the ref and I've bought the "LP" package. Do I have that right please?

author: floobydust
author_rank: Super Contributor
timestamp: July 15, 2021, 07:34:16 pm
content: A long time ago, in the Service/Repair Shop we had a daily challenge - you can only use a continuity tester to troubleshoot and repair. No multimeter use, for all the techs.We all made a Continuity Tester clone of a Radio Shack one, 3V battery and one LED.The LED lit: if leads shorted, 1/2 bright if a diode-drop seen, very dim with say 1MEG but could test resistors and capacitors, and it lit really bright if 12V-120V present.It was a fun challenge and surprisingly you can test transistors and open-circuits and voltage tracing.I'll look for the old one, it was a couple transistors and a small (input) light bulb which lit up with 120V and today you would use a PTC.I think this schematic is a Russian tester (1.35V battery and buzzer) but the circuit is really weird, that's why I kept it.

author: msknight
author_rank: Regular Contributor
timestamp: July 15, 2021, 07:43:19 pm
content: Quote from: floobydust on July 15, 2021, 07:34:16 pmLove it!!!

author: floobydust
author_rank: Super Contributor
timestamp: July 15, 2021, 07:45:10 pm
content: If you need to know the difference between 0 ohms and 12V power present, the transistor got way more base-current. No smoke or anything. Tracing 120VAC well, it could be done.

author: Gyro
author_rank: Super Contributor
timestamp: July 15, 2021, 07:57:41 pm
content: Quote from: msknight on July 15, 2021, 07:22:48 pmSorry, not wanting to disillusion you, but you're probably buying fakes or floor sweepings - it's rife with China ebay and Ali sellers. What's worse, you have to wait so long to find out!An (LM334) example from just the other day...https://www.eevblog.com/forum/beginners/rheostat-act-as-a-potentiometer/msg3606295/#msg3606295Buy your parts from a reputable UK distributor, RS, Farnell, CPC, Rapid - take your pick but for heavens sake buy genuine parts, otherwise you're doomed to disappointment. Make up a shopping list to try to get to the free postage limit.P.S. Do you really need 100 of them?

author: msknight
author_rank: Regular Contributor
timestamp: July 15, 2021, 08:05:08 pm
content: Quote from: Gyro on July 15, 2021, 07:57:41 pm100 of them, at that price and postage... well it comes down to the UK sellers and the postage issue. I've made up so many shopping lists to get the free postage limit, that I'm seriously stretching the cash this month. I've got enough reputation with e-bay that if things go west, they back me up and I get the few squids back.Single units were the same price from inside the UK. And to be fair to the Chinese sellers, I(touches a lot of wood)haven't had serious trouble with them to date... although there's always a first time.EDIT - Yes, there were other e-bay sellers located in the UK(supposedly)with the same quantity of produce at double the price, but their reputation was hideously small. I bought from someone in China with 160,999 ratings and 98.9% positive. ... I'll take those odds.

author: Gyro
author_rank: Super Contributor
timestamp: July 15, 2021, 08:12:51 pm
content: Oh well, your choice. I can only seek to inform. A bog standard TO92 transistor is an easy target for re-marking to turn a profit though.

author: Sbampato12
author_rank: Regular Contributor
timestamp: July 16, 2021, 01:22:09 pm
content: Quote from: Gyro on July 15, 2021, 08:12:51 pmIndeed.As a person that already fall sometimes in the re-marking/counterfait ICs and Transistors, I wouldn't. Even if I complete understand the $ side.In a hobby project, could be. But imagine in a prototype, you already have the doubts with new project, the errors with the measurements, adding a possibility to problems that could be caused by a fake PN... it is very hard to figure out when is a design flaw and a fake component problem...(don't even try 2N3055 and MJ15000 on ebay...)

author: msknight
author_rank: Regular Contributor
timestamp: July 16, 2021, 08:56:53 pm
content: Well, the way I understand it is that everything of this particular level is made in China - the only difference is where and who you get it from. I mean, where are Farnell, RS, etc. getting these components from themselves, if not China?I've hit good suppliers inside China and bad suppliers outside China. Even suppliers in the UK who turned out to be in China anyway. Even German suppliers who ended up being in China. The only thing I can go on, is reputation. Plus, there are a number of videos for testing these particular components so I'll know if they're fake right out of the gate.The problem with Farnell, et. al. is that I've got the postage issue. I have ordered from this Chinese supplier before... my first order with them in 2017... so I don't think I'm ordering completely blind.So my question to you is this... if you won't order from China, and you're hitting the postage issue with the big guns ... then how do you solve the problem?

author: ataradov
author_rank: Super Contributor
timestamp: July 16, 2021, 09:09:15 pm
content: It is not about ordering from China or manufacturing from China. It is about the supplier. Anything you order on eBay should be considered at least highly suspicious. Some products (like power transistors) are 100% fake.Order from reputable Chinese suppliers, and you will get reasonable assurance of quality along with not inflated prices. I use LCSC quite a bit and I have never got a bad part from them.

author: bdunham7
author_rank: Super Contributor
timestamp: July 16, 2021, 09:37:00 pm
content: Quote from: Gyro on July 15, 2021, 08:25:43 amThey do, but in many cases this is easily remedied by shunting them with a resistor just above the threshold. So I have a DMM with a 1mA test current, an open voltage higher than I'd like and selectable 1/10/100/1000 continuity thresholds. I shunt it with 10R, which gives me an OC voltage of only 10mV and a threshold error of about 10%, which still works fine for continuity purposes--1.1 vs 1 ohm doesn't matter too much. I would think that a purpose-made continuity tester might simply use this shunt technique to minimize OC voltage, which is a real issue when testing populated boards for short.

author: floobydust
author_rank: Super Contributor
timestamp: July 17, 2021, 01:59:07 am
content: Real-world wiring has oxidation, a passivation layer and sometimes water- so you need a decent test current and voltage. "Low power ohms" as it was called, a compliance voltage less than that of a semiconductor, is no good for field work where you can encounter galvanic potentials that are easily above say 0.4VLong cable runs pick up AC hum. All of this can fool a continuity tester.One reason field techs love the antique Bach-Simpson 260 is Rx1 test current is almost 130mA at 1.5V which guarantees the reading involves copper - not water, rust etc.I've had lots of multimeters give bogus ohmmeter readings on wire and cables with their piddly 0.3-0.8mA test current of the 200R range.Also, most of the continuity tester circuits out there are garbage in that they have no input protection should you ever test something with power on it.Even low voltage automotive, like trailer wiring it's pretty easy to bump 12V.

author: Gyro
author_rank: Super Contributor
timestamp: July 17, 2021, 08:32:11 am
content: Quote from: bdunham7 on July 16, 2021, 09:37:00 pmI agree, shunting methods work for some specific and controlled circumstances, for instance dry circuit testing where you want a very low test voltage to evaluate contact oxidation etc on relay contacts.The problem you have with using this method with a DMM for continuity testing is:1). Any shunt resistor below, say, 150R is going to continuously sound the DMM continuity bleeper - annoying, and the OP wants audible continuity testing.2). Energised circuit protection. A low value shunt resistor is just crying out to be fried if you accidentally apply it to an energised circuit, probably taking out any components that happen to get in the way at the same time. Fuse or PTC protection are both tedious and potentially expensive or slow acting. They both add series resistance to your probes, which could of course be compensated, but, in the case of a PTC thermistor have long recovery time.As I said, dry circuit testers definitely have their place for contact evaluation, but can be a pain for general continuity testing, unless specifically designed with sensitive thresholds and protection.The EE circuit that I posted uses less than 300mV for convenience but this could be made much lower with a smaller energising current on the bridge and higher precision opamp. The bridge approach also offers non destructive energised circuit protection to at least 30V, easily made much higher if you increase a few resistor wattages.

author: Gyro
author_rank: Super Contributor
timestamp: July 17, 2021, 08:48:57 am
content: Quote from: floobydust on July 17, 2021, 01:59:07 amAgreed - forelectricalcircuit testing. For instance my Kewtech Insulation and continuity tester puts out a minimum of 4V with a test current of 200mA as required by BS7671, precisely for the reason of making sure you're measuring Copper. It also includes HRC fuse protection for energised circuits (typically mains in this case).The problem with such testers for modernelectronicuse - and even my AVO 9 or your Simpson 260 is that 1.5V will turn on all of the semiconductor junctions, and 130mA could well take out a B-E junction of a small signal transistor, and iswellin excess of the 5mA limit of most IC ESD protection diodes. I'd be fearful of applying them to a populated board when trying to trace continuity or shorts.

author: Gyro
author_rank: Super Contributor
timestamp: July 17, 2021, 11:00:55 am
content: Quote from: msknight on July 15, 2021, 11:29:19 amJust looking back through the thread, I might have spotted it earlier, but you could have used one half of the LM358 for the bridge input circuit from the Everyday Electronics project, and the second half for the bleeper oscillator, lifted as-is from the Not1Xor1 circuit. No need for the TL431 or the CD40106.

author: msknight
author_rank: Regular Contributor
timestamp: July 17, 2021, 12:46:35 pm
content: Quote from: Gyro on July 17, 2021, 11:00:55 amYou draw it.... I'll build it ;-)

author: Gyro
author_rank: Super Contributor
timestamp: July 17, 2021, 01:08:29 pm
content: Quote from: msknight on July 17, 2021, 12:46:35 pmNow it's not that hard, It would be much more beneficial ifyoudrew it and asked for corrections.Take everything to the left of the output of IC1 from the, again, nicely documented, EE article (you don't need C3 as the LM358 is unity gain stable) and patch on everything to the right of the output of IC2/a (including D4) from Not1Xor1's design.

author: iMo
author_rank: Super Contributor
timestamp: July 17, 2021, 01:33:34 pm
content: I built myself (as a child) a continuity tester made of a single 40106 chip.A standard single gate oscillator, where probes were placed instead of the timing resistor (in the RC part).The probes were protected by series resistors and anti-serial zener diodes.The other gates were used as a buffer for a piezo element. Powered from a single CR2032 (no power switch) forever..When the probes were loose it produced maybe 1 tick a second or two. With probes shorted it beeped with a few kHz, thus I could judge on the resistance of the circuit based on the tone

author: msknight
author_rank: Regular Contributor
timestamp: July 17, 2021, 02:28:20 pm
content: Quote from: Gyro on July 17, 2021, 01:08:29 pmNot that hard for you... maybe! I'm just about to crack open BBox 1 and start learning electronics. I can just about read the diagrams, get the bits and wire it together. I'm not yet at the stage where I can understand to the point where I can create my own circuits. I can throw kits together, but don't ask me how or why they work. If something doesn't work when I've built it, I've got no skills to troubleshoot it.For years, I've been bodging things together and hoping they don't go bang. How I've made it this far without causing a fire, I don't know. Probably by sticking to 5v DC at a guessSo yes, some way down the road I'd be happy to take on the challenge. Right now, however, I'm not up to the task.

author: bdunham7
author_rank: Super Contributor
timestamp: July 17, 2021, 02:46:59 pm
content: Quote from: Gyro on July 17, 2021, 08:32:11 amIt works in my case because I have selectable thresholds in addition to a readout of the resistance. With a 10R shunt and 1R threshold selected, it works like a normal, audible continuity tester. With a fixed 150R threshold and 1mA current, you'd need to use a 200R resistor. This would keep things below the semiconductor threshold, but not the dry-contact/bad connection threshold of 20mV or so.In my case this is an easily inserted and removed shunt that I only use when appropriate. If I were concerned about protection, I'd only have to protect the shunt itself, as the meter is fully protected. I'd probably look for a small light bulb with a cold resistance between 10 and 20 ohms.

author: bdunham7
author_rank: Super Contributor
timestamp: July 17, 2021, 03:12:42 pm
content: Quote from: floobydust on July 17, 2021, 01:59:07 amI have one and also a BK lo-hi resistance tester that uses up to a 200mA current on the low ranges and up to 1000VDC on the megohm ranges. Neither one is very good for scanning DRAM modules for shorts.QuoteI made a powered test probe for automotive almost 30 years ago and the solution is still the best I've seen. It's commercially available, just not as popular as I would have expected. You supply the probe with +12V and ground, use an LM7805 with a 100R or so shunt to ground and then one of those bidirectional Red/Green LEDs (as big as you can find, or multiples) to the probe end. Green = ground, Red = power, dark = open.You need different instruments for different purposes. That's why we all have stacks and boxes of various test gear, right?

author: floobydust
author_rank: Super Contributor
timestamp: July 17, 2021, 04:38:20 pm
content: I didn't realize this continuity tester is for in-circuit use, based on OP.So the requirements are test voltage under 0.3V and current under 5mA? Could just add a switch for hi/low test.I like it when the LED is analog, can show varying resistance (brightness) verses on/off with a comparator. You can "see" the bad connection or poor probe connection.The TL431 pinout is given as top view, which is awkward for TO-92.On-semi datasheet TL431less confusing.

author: Gyro
author_rank: Super Contributor
timestamp: July 17, 2021, 05:51:16 pm
content: Quote from: msknight on July 17, 2021, 02:28:20 pmSorry for overestimating your position on the learning curve (or maybe deliberately setting you a bit of a stretch goal). The 'cut and shut' is literally what I put in my post anyway. It just takes the audio oscillator part the Not1Xor1 design and uses it with the Everyday Electronics one as you have an opamp spare (half an LM358) and you probably don't have a CD40106. At the end of the day, battery consumption is a little bit higher but that doesn't matter.The reason I pushed the Everyday Electronics one is (as I keep banging on about), it has a good low level description of how it works, how it was designed, and why components were chosen, apart from just being a good one. Back in the golden days, Everyday Electronics was launched as a beginners version of Practical Electronics which launched in the mid '60s. They needed another one for the people who hadn't 'grown up' with PE (which is where many of us oldies got the bug).Luckily you can read the full set of issues for free. At your knowledge level a lot of fundamental things haven't changed (if you ignore the arrival of the micro, Arduinos etc), and most of the 'old faithfuls (like the LM358) are still available. EE and PE eventually got merged into EPE which still lives today, although a lot of the articles are reprints from the Australian Silicon Chip magazine, which seems in rather better health.I really would recommend that you dig into the old EE issues, they had lots of bottom of the learning curve projects and teaching series (analogue and digital) which are just as relevant today, and arefarbetter written than a lot of patchy, superficial web content you get these days....https://worldradiohistory.com/Everyday_Electronics.htmP.S. You can find PE, earlier EPEs and other mags there too.EDIT: Just a note that safety standards have changed over the years if you are looking a any of the mains related projects!

author: msknight
author_rank: Regular Contributor
timestamp: July 17, 2021, 07:14:21 pm
content: Quote from: Gyro on July 17, 2021, 05:51:16 pmI'll get there :-) ... At the moment I have the BBox1 and 2 ... box 2 came with about 80% of the bits needed including the LM358(Box 2 has about three of them, and more than ten 555)-https://bitsnblobs.com/- unfortunately they're not selling the kits at the moment. I also have, as I was advised, an early edition of Malvino, which I'm looking forward to. The BBox 1 has loads of bits and pieces including a prototyping board, battery boxes, LED's, a few diodes, resistors, capacitors... I'm heading in to a well laid out learning curve. Coupled with the mixture of parts I bought in the Velleman kits, I'm actually feeling confident about going forward and learning.I did have a run in with one of the EPE issues... advised to me by someone I bumped into in a supermarket and overheard him talking... but when it came to the radio article all I could buy was the PCB. When it came to the other bits I was totally on my own and to an initiate like me, trying to track parts on some of these web sites was just too much. Almost put me off for life. In fact, what you say about it being reprints of older Australian articles, now makes sense on why it all fell to rats. There were parts in the list that I still can't get hold of, and some of which I had to find on fleabay.I am fairly good at following rote, however, so I've re-capped a walkman and also a boom box...- dead easy to go through the board, identify the caps, finally learned how to order them, and got the job done. But if it had failed, I'd have been off the deep end.I'm still bashing my head against a brick wall :-)

author: PA4TIM
author_rank: Super Contributor
timestamp: July 18, 2021, 01:47:14 pm
content: I made this one a few years ago:http://kripton2035.free.fr/Continuity%20Meters/ed-ultimateconti.htmlWorks very well, very low voltage because there is no voltage with open test leads (the oscillator starts when you short the input) and it only uses 1,5V. No powerswitch needed and I used the 200 ohm potmeter. This way you can set the range of resistance. The higher the resistance the higher the tone. I can set it so it covers 0 to over 100 ohm but also 0 to 5 ohm, or even less.

author: msknight
author_rank: Regular Contributor
timestamp: July 18, 2021, 02:27:09 pm
content: Quote from: PA4TIM on July 18, 2021, 01:47:14 pmI did see that one, and the links to the circuit design were not working, and a discussion about whether or not it worked. Obviously, it is working for you. I might try it.

author: PA4TIM
author_rank: Super Contributor
timestamp: July 18, 2021, 02:58:11 pm
content: I first made it with 2 smd transistors and it looked like it did not work. But using the oscilloscoop it worked but at a very high frequency (>50 kHz) . I then replaced the transistors by through hole ones (BSX20 and BC558)This caused the oscillations to be lower but still to high. I then took my drawer of speakers/buzzers etc and that solved the problem. I then selected the loudest one that gave the wanted frequency range. The thing I used is a 3cm diameter metal speaker, I think it came out something like a postcard, the ones that makes music if you unfold it. But in that drawer there are piezospeakers, magnettic speakers, piezobuzzers etc, I am not into audio so I do not know all the types and just tried which one worked best (some did not work at all). The speaker I used gave sound if I connected it to a function generator zo it is some type off speaker and not a DC powered buzzer.This speaker "problem" can be the cause it does not work for everybody. Often the problem with these sort of simple analog circuits is that it is sensitive for voodoo. But to be serious, as soon as things need to oscillate and you do not use a buffered and more or less isolated oscillator then everything in your circuit can and will influence the oscillator. But that is needed for this circuit. You want the oscillator frequency to change by the impedance of the circuit you are probing.

author: msknight
author_rank: Regular Contributor
timestamp: July 18, 2021, 03:15:51 pm
content: This is interesting. The 1N5400 doesn't seem to be around but the 1N5408 is listed as a replacement and I have some of those. I also read that the BC547 and BC557 can replace the 548 and 558 respectively. I have those. All I would need is the 200 ohm pot (which is listed optional) and the 1w fuse resistor and I can build this. And a range of buzzers, of course. - that is if the replacements are valid.

author: Zero999
author_rank: Super Contributor
timestamp: July 19, 2021, 03:46:22 pm
content: What's wrong with a piezo buzzer and a 9V battery? I suppose it's too basic and no fun.Here's a buzzer circuit I designed awhile ago. It will work off a single AA cell, down to 1V. I have built it before and it definitely works. A cheap 8 Ohm speaker will do.Here's an improved version of the comparator-based circuit. If a micropower comparator is used for U1, it can be left connected to a couple of AAA cells, without draining the batteries. The tripping voltage is below the forward voltage of a silicon diode. It goes beeps, when Rtest <100R, but doesn't stop beeping, until Rtest > 200R, which provides some noise immunity. The trigger resistance is roughly equal to R4/100 and the resistance, when the buzzer stops sounding is roughly double that. I haven't actually built this circuit, but it should work, as long as a low power piezo buzzer is used, which can work down to 2V.

author: Gyro
author_rank: Super Contributor
timestamp: July 19, 2021, 04:11:54 pm
content: Quote from: Zero999 on July 19, 2021, 03:46:22 pmNot wanting to be picky, but isn't that roughly the sort of threshold you get with an ordinary dmm continuity bleeper. If you're making one, it would be nice to have one that was more functional than what you already have. Admittedly the hysteresis might be nice.P.S. The stumbling block, as far as I can understand it, is that the OP wants to go with what they already have (LM358?) or the TL431s on order, so I guess, no micropower comparators.

author: Gyro
author_rank: Super Contributor
timestamp: July 19, 2021, 05:19:42 pm
content: Quote from: msknight on July 17, 2021, 02:28:20 pmIf you really have difficulty following my written explanation, then I have chopped up the Everyday Electronics and Not1Xor1 designs, as I described, to use a single LM358. I'll leave the printing and glueing to you....Edit: LM358 pin numbers corrected.

author: Zero999
author_rank: Super Contributor
timestamp: July 19, 2021, 08:01:07 pm
content: Quote from: Gyro on July 19, 2021, 04:11:54 pmAs far as I'm concerned, being able to leave it switched on indefinitely, without discharging the batteries, is a big enough advantage. The threshold could be altered by changing R4, which could be achieved rotary switch for preset values, or a potentiometer.Quote from: msknight on July 15, 2021, 09:29:19 amDo you mean the LM358LV, or just the ordinary LM358? The LM358LV is a low power version, which would enable the circuit to be designed so it can be connected continuously, giving a standby battery life of a few years, from a couple of AA cells.

author: msknight
author_rank: Regular Contributor
timestamp: July 20, 2021, 08:36:43 am
content: Quote from: Zero999 on July 19, 2021, 08:01:07 pmIt just has an L after the code.I'm currently waiting for parts and it looks like the fusable resistor is going to beat the other bits by about a week... things appear to have got caught up in China, so that might determine which circuit I build.

author: msknight
author_rank: Regular Contributor
timestamp: July 20, 2021, 09:57:34 am
content: I think it might be LV - I've attached the best picture I can get of the codes...

author: Zero999
author_rank: Super Contributor
timestamp: July 20, 2021, 11:14:12 am
content: Quote from: msknight on July 20, 2021, 09:57:34 amAccording to the LM358LV data sheet, it isn't made in through hole, so it must be the standard LM358.Anyway, the good thing about the old LM358 is, its inputs can be taken to a higher voltage, than the supply, without being damaged, or drawing any current. This means it should be possible to design a circuit which only powers the op-amp, when the probes are connected to something. The following circuit only powers the LM358, when the resistance across the probes falls below a few tens of kOhm. This means it can be left connected to the battery, without draining it, as long as the probes aren't connected to anything. It uses a buzzer, with a built-in driver circuit, for simplicity.The spare op-amp in the LM358 can be used to make an oscillator, to drive a bare piezo transducer, which is a little cheaper and the tone can be varied, by changing C1 and R9, but it requires more parts.

author: msknight
author_rank: Regular Contributor
timestamp: July 26, 2021, 06:58:47 am
content: The design for the ultimate continuity tester calls for a 270nf capacitor. and I need through hole. I can find 220 and 330 in ceramic. I can't find 270 ceramic in anything other than surface mount. Do I use a 400v metallic film? How should I handle this please?

author: Zero999
author_rank: Super Contributor
timestamp: July 26, 2021, 07:42:47 am
content: Quote from: msknight on July 26, 2021, 06:58:47 amYes, 220nF or 330nF, 400V metallic film will be fine.

author: msknight
author_rank: Regular Contributor
timestamp: July 26, 2021, 08:10:45 am
content: Quote from: Zero999 on July 26, 2021, 07:42:47 amSo by that token, I could use a 220nf or 330nf ceramic and not have to get a specific 270nf in metallic then?If so... which would you recommend... 220 or 330... and why? -(never one to miss a learning opportunity)

author: Zero999
author_rank: Super Contributor
timestamp: July 26, 2021, 08:50:49 am
content: Quote from: msknight on July 26, 2021, 08:10:45 amBoth of those values are close enough. It will still work, but will oscillate at a slightly different frequency.Electronic components are made in standard values, known as the E series of preferred numbers. The E3 series has three values per decade i.e. 10, 22 and 47, the E6 series six: 10, 15, 22, 33, 47 and 68. Most components have a velue equal to the E series multiplied by a factor of 10, which is why we see 10nF, 22nF and 47nF, but rarely 50nF. The reason for this is it makes no sense to specify a value of say 90nF for a component with a tolerance of 20%. It's much easier to say 100nF.https://en.wikipedia.org/wiki/E_series_of_preferred_numbersMost of the time, it's normally acceptable to choose the nearest preferred value, unless it's a precision circuit, such as a volt meter, when changing a component value slightly, could significantly affect the result.

author: msknight
author_rank: Regular Contributor
timestamp: July 26, 2021, 08:58:00 am
content: Quote from: Zero999 on July 26, 2021, 08:50:49 amAh! Cool. ... so... if I read that right, I believe it will control the tone that comes out of the speaker... so a higher value capacitor should result in a lower tone generated. Am I reading that right please?

author: Zero999
author_rank: Super Contributor
timestamp: July 26, 2021, 10:32:35 am
content: Quote from: msknight on July 26, 2021, 08:58:00 amYou're right. A larger capacitor will take slightly longer to charge and discharge, thus lowering the frequency slightly.