DIY Simple logic probe

10 thoughts on “DIY Simple logic probe”

1. Very little thought went into this. It will only light on a logic one, not on 0.

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2. It will also only work in a circuit where there is enough current to light the LED, and it may also affect the state of the circuit being tested. Real logic probes have high impedance inputs. This would more accurately be called a power-good / polarity tester.

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3. If it’s stupid and works, it wasn’t stupid. I’m sure most TTL and CMOS logic gates can drive an LED. Just look at the Arduino. LED+resistor combos are normally driven without an amplifier. I call these things ‘poke-a-lights’. I’ve made two similar to this. The first one replaces the resistor with a low watt, 120VAC incandescent light bulb. It can do a quick check of polarity and voltage from 3v all the way up to line voltage AC. The light bulb resistance changes with the voltage applied, and keeps the current flowing through it and the LED mostly constant. The second uses a voltage divider and is designed for 12v systems. One lead of the LED goes to a probe, and the other lead goes to a resistive divider made with 2*1k ohm resistors. The resistors connect to the 12v and ground of the system under test. I keep it in my glove box and use it to check fuses. It can check fuses even if they’re un-powered. As I poke the probe on each end of the suspect fuse, the LED should show the same color (red or green, doesn’t matter) on both sides. This works because all fuses should have either 12v or else be grounded through the circuits it powers.

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4. I think this is a better one – it has tri-state indication – low = LED off, high = LED bright, open/high impedance = LED dim. Here’s the URL:
   http://pminmo.com/troubleshoot/lprobe.htm

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   1. That does look cool. Thanks for the link.

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