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  • 3 months later...

Looks like this has been here a while, so I will take a stab at it:

Depends upon the circuit it is measuring. If it is not too fragile of a circuit, I guess you could just make a simple probe from an LED, a 1.5 battery, and two probes. If the probes are connected across an open circuit, the LED will not light. If it is not an open circuit, it will light with some exceptions. Depends upon the resistance of your circuit. So you would have to figure this factor in when making the probe as the LED needs enough to light it. On the other side of this fence, you do not want to add too much to the circuit which will kill the components in it.
You really have not given enough information about the circuit to return with much more than this.

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  • 3 months later...
  • 1 month later...

MP,
Most LEDs will not light with only 1.5V, and worse when the battery is exhausting.
Voltages of LEDs:

Common Ultra-bright
Red:1.7V 2.2-3.5V
Yellow: 1.9V 2.5-3.8V
Green: 2.1V 3.1-5.1V
Blue: - 3.1-5.1V
White: - 3.1-5.1V

That's why the little flashlights have an inverter inside, and why National Semi uses a votage-boosting circuit in their LM3909 flasher chip here:
www.national.com/ds/LM/LM3909.pdf
You probably meant to use a common LED with 2 batteries, a current-limiting resistor and probes.

Jo_the_dog,
Do you know how high is a resistance that is infinite?
Would 1,000,000 megohms fail?
Please tell us your maximum resistance that will pass, and how much voltage is allowed for the test. There is a commercial meter called a Megohmer, for measuring the resistance of insulation, that uses a battery of about 90V (ouch).

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It seems that you want to check if the entire circuit has continiuity or not (passing through all components).
I have seen such an instrument,known has MEGGER, but which is used to check the house wiring.
In your case such an instrument will be hard to make because if it is passing through all components what will happen it meets a capacitor.There will be no continuity.

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Nitpicking.
I have lighted LEDs on my bench with a AA battery is why I mentioned it. So if you cannot get this to work with your LEDs, use two batteries. It is still a simple circuit.
...and, yes, as I mentioned, it will depend a lot upon the circuit that it is used on.

Note that the original post did not give any feedback to my comment, "You really have not given enough information about the circuit to return with much more than this." So, I will stay with my original comments.

MP

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siddharth, a continuity tester is not usually used to check through all components such as resistors and capacitors. This is a different device. A continuity tester is usually used to check that a non powered trace on a pc board goes from point A to point B with no interruption. Even when components are placed on the board, this works because electricity will take the least resistive path. As with anything, there will be the one occasion where it does not work.

MP

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MP you are right. But then one will have to make a list of those times when the circuit will fail. Moreover with time new circuits come,so one will have update it now and then.
Back to the circuit, you made a simple circuit using a battery and LED.Won't this circuit fail easily when it meets resistance of quite high values.Ofcourse the least resistance path will be taken but then chances are that it will face high resistance.
MP i feel this is bound to be expensive circuit since you have mentioned that one does not know where infinity is.

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Siddarth, I am sure there is a tool in existence like what you have described. Yes, this battery/LED circuit would just be a basic continuity tester for one leg of a non powered circuit. There are a lot of variables one would have to consider to go beyond that. Interesting, though.

MP

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Siddharth, thought you might be interested in this circuit:
http://www.web-ee.com/Schematics/AudibleCircuitTracer/AudibleCircuitTracer.htm
It is a circuit "tracer", which would be more of what you were talking about.

MP

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