Continuity tester circuit

[msknight]

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.

[ataradov]

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.

[msknight]

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.

[ataradov]

So increase the 4.7k if I want to be safer, but you see no problem with the circuit as is. Superb.

I 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.

[kripton2035]

here is a collection of continuity tester circuits. choose the best for you.
http://kripton2035.free.fr/continuity-repos.html
http://kripton2035.free.fr/Projects/shorty-display.html

[Gyro]

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....

[msknight]

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.

Can I use an LM358L in place of the LM308 on that circuit, do you think? I have some of the former already.

[Ian.M]

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?

[msknight]

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?

True. 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.

[Gyro]

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.

Can I use an LM358L in place of the LM308 on that circuit, do you think? I have some of the former already.

Yes, 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.

[msknight]

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?

[floobydust]

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.

[msknight]

and it lit really bright if 12V-120V present.

Love it!!! 

[floobydust]

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.

[Gyro]

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?

Sorry, 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/#msg3606295

Buy 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? 

[msknight]

Sorry, not wanting to disillusion you, but you're probably buying fakes - 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/#msg3606295

Buy 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 actually need 100 of them? 

100 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.

[Gyro]

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. 

[Sbampato12]

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. 

Indeed.
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...)

[msknight]

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?

[ataradov]

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.

[bdunham7]

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.

They 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.

[floobydust]

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 0.4V
Long 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.

[Gyro]

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.

They 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.

I 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.

[Gyro]

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 0.4V
Long 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.

Agreed - for electrical circuit 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 modern electronic use - 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 is well in 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.

[Gyro]

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?

True. 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.

Just 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.