# How do I setup a go/not LED switch?

I am designing a simple circuit that will light a LED when a given resistance is between 0 and about 1K. At about 1k the LED would not be lit at all. The test circuit should not have more than .5ma go though the tested resistor. The supply voltage is coming from a car battery so it will vary between 12.5 to about 11 volts.

What I have worked out so far with limited success is to setup a voltage divider between the supply and ground using a 39k resistor (on the positive side) and the unknown test resistor to ground. At their junction I have put a 1k resistor in series with a red LED. The anode of the LED goes to the supply.

The above circuit will test down to about 22k before the LED will begin to glow somewhat. when testing 0 ohms, the LED will glow about half full brightness. The current though the LED is about .48 ma. I can get by with the that amount of brightness but I would like to be able to test down to a lower level.

• Current source and comparator. Commented May 31, 2016 at 2:12
• You mean 500mA? So when the resistance is between 0Ω to 1KΩ, do you want the LED be fully bright? Or get dimmer as you go toward 1KΩ? Commented May 31, 2016 at 5:55
• No I had the right current .5 ma . I like the circuit you put in; however, I had hoped to find something even simpler. Commented Jun 1, 2016 at 23:39
• I am so new to this forum I do not know how it works. I want to add more information. The circuit is to test Eletric 3 Commented Jun 1, 2016 at 23:42
• ... The circuit is to test Electric Matches. When a match is new (UN-burnt) the resistance is about 1-5 ohms. So when testing I can only use a current .5ma or less. More and the match may blow. Once burnt the match can test anywhere above 10k. The reason to keep the circuit so simple is that he want to expand the circuit to test 100 matches at one time. Thanks for you input! Commented Jun 2, 2016 at 0:14