Need help with continuity tester circuit design

Thread Starter

Evil commando

Joined Apr 11, 2024
2
I need to made a dual led continuity testing circuit. My idea is to have two led’s. One Red and one Green.
so when to circuit is open the red LED will be on. When the circuit is closed The Green LED will be on.
My circuit design seems like both LEDs are illuminated when the circuit is closed. Instead of switching from the red to green. Both are illuminated.
Anyone have a circuit design that will switch off the red and turn the green on? Then when the circuit is open again just the red will be illuminated?

See attached for circuit photo.
 

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wayneh

Joined Sep 9, 2010
17,498
I'd use a LM393 (dual) or LM339 (quad) comparator. Take a look at the data sheet and the typical application circuits. It assumes one of two states very quickly and with great sensitivity. It's what I use in power supplies to detect and indicate when a load is attached.

You'll need to define what constitutes "continuity". Is a 100Ω resistor continuity? How about 1MΩ?
 
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MisterBill2

Joined Jan 23, 2018
19,042
The simplest design is not very efficient, but it should work. The two LEDs in series also a resistor, fed by a 3 volt battery pack.The probes across the red LEDA resistor across the green LEDso that with the RED LED lighted there is not enough to make the green LED glow. BUT when the probes sense continuity the red LED is bypassed and the green LED glows. I have not figured the resistor values yet, but that is the simplest cheap I can imagine to do it.
 

crutschow

Joined Mar 14, 2008
34,700
Below is the LTspice sim of a one transistor circuit that should work:
When there's no continuity (yellow trace low, SW open), the transistor and thus the red LED (red trace) is on since its forward voltage is less than the green LED's (green trace).
With continuity the transistor turns off along with the red LED, and thus the green LED goes on.

1712892969462.png
 
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MrChips

Joined Oct 2, 2009
30,984
You can eliminate the transistor if you can accept the opposite condition. The red LED comes on when there is continuity. The green LED is on when the circuit is open. Note that this circuit relies on the different forward voltage of red and green LEDs. It does not work if you interchange the LEDs.

Continuity Tester2.jpg
 

Ramussons

Joined May 3, 2013
1,417
I need to made a dual led continuity testing circuit. My idea is to have two led’s. One Red and one Green.
so when to circuit is open the red LED will be on. When the circuit is closed The Green LED will be on.
My circuit design seems like both LEDs are illuminated when the circuit is closed. Instead of switching from the red to green. Both are illuminated.
Anyone have a circuit design that will switch off the red and turn the green on? Then when the circuit is open again just the red will be illuminated?

See attached for circuit photo.
JFI. What is the simulation SW being used?
 

MrChips

Joined Oct 2, 2009
30,984
Revised circuit. In this circuit, you can interchange the LEDs or use LEDs of different colours. Adjust the value of R1 and R2 for different battery voltage.

Continuity Tester.jpg
 

MisterBill2

Joined Jan 23, 2018
19,042
You can eliminate the transistor if you can accept the opposite condition. The red LED comes on when there is continuity. The green LED is on when the circuit is open. Note that this circuit relies on the different forward voltage of red and green LEDs. It does not work if you interchange the LEDs.

View attachment 319822
GOOD JOB!!! This is an improved version of my circuit.
Years ago I designed and built a continuity tester that was able to withstand accidentally getting across the 120 volt mains, or DC, without any damage. i still have it, I should dig it out and trace out the circuit, because that level of survival is rare among continuity checkers. I think that the highest resistance for "continuity" was about two ohms. It was more complex, it did use an LM311 comparator and quite a few other parts.
 

Thread Starter

Evil commando

Joined Apr 11, 2024
2
I'd use a LM393 (dual) or LM339 (quad) comparator. Take a look at the data sheet and the typical application circuits. It assumes one of two states very quickly and with great sensitivity. It's what I use in power supplies to detect and indicate when a load is attached.

You'll need to define what constitutes "continuity". Is a 100Ω resistor continuity? How about 1MΩ?
1 ohm. basically building a cable test box. So I can plug both ends into it. I want two LED’s. Red and Green. If the cable is connected properly it will turn off the Red LED and the Green will come on. So each connection point will have two LED’s.
 

MisterBill2

Joined Jan 23, 2018
19,042
My very simple, very cheap, cable continuity tester used a six-volt wall wart, two cable connectors, and four six volt pilot light bulbs. It would verify adequate connection for 150 milliamps current, but it would not detect short ciruits between conductors. It was for checking modular phone cords.
Now I have another tester, made by "KLEIN TOOLS" It will check for both continuity and correct connections and shorted conductors on phone and data cables. It is the VDV Scout Pro. Not cheap, though.
 

Tonyr1084

Joined Sep 24, 2015
7,964
Not sure if a coin cell can deliver 15mA. If so, R2 might not be necessary. I've powered White Super Bright LED's directly from a coin cell with no resistor. They can burn constantly for weeks on end, maybe a month or more. (never tried it with a new coin cell, only a used up cell)
Screenshot 2024-04-15 at 9.15.05 AM.png
You need to be able to turn it off. SW 1 can be either a Normally Open (NO) momentary push button or (option 2) it can be a switch. When closed the circuit can only run through the RED LED. When TP1 & TP2 are shorted (continuity) the higher current draw of the GREEN LED should extinguish the RED LED.

Since the coin cell has a high internal resistance, a value I don't know, adjusting the values of the resistors is likely going to be necessary. The point is that the RED LED should draw less current than the GREEN LED. That way when you measure continuity the GREEN LED should light up and the RED should go out.
 

Tonyr1084

Joined Sep 24, 2015
7,964
OK curiosity got the better of me.
Screenshot 2024-04-15 at 9.33.31 AM.png
So R2 is probably not necessary, and R1 can be 100Ω. That should give you the desired response.
 

Tonyr1084

Joined Sep 24, 2015
7,964
These values are based off of what I have in stock. Your Vf will likely be different.
Screenshot 2024-04-15 at 9.48.43 AM.png
For a brighter LED use lower resistance. 15mA RED would require a resistor of approximately 87Ω. (using average internal resistance of the battery) You will probably need two resistors in series. 47Ω & 33Ω = 80Ω. At 80Ω your current would be around 16mA. Sufficient difference between the red and green should make this circuit fool proof.

The More You Know.
 
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Tonyr1084

Joined Sep 24, 2015
7,964
FYI: High resistance in the circuit you're running continuity on can affect how the circuit responds. Should your "continuity" measure a resistance of 100Ω or more the circuit will register an open circuit. In such cases an ohm meter would be the tool of choice.
 
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