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Author Topic: Poor man's logic probe / 2 LEDs on one pin  (Read 481 times)
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Shelby Township, Disco, Michigan
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Here's a little circuit I put together that can be used as a Poor Man's logic probe. It can also be used as a 2 LED State display off one digital pin. When there is no signal, both LEDs will light up. When the signal is High, the Green LED lights up. When the signal is Low, the Red LED lights up. If you don't want both LEDs on, just add another LED on each side. I put this on a small perfboard with a 3 pin 90 degree header. So it can be plugged into a breadboard. If the red LED is brighter then the green, put a larger resistor on the red led side.


* 2 LEDs Logic Probe.jpg (40.68 KB, 872x762 - viewed 69 times.)
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how reliable is this ?  I have to maximum pin count and would like to both read and write to a pin, to sense if a voltage is present and then to light an LED.
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In theory there is no difference between theory and practice, however in practice there are many...
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does it also work for 3.3V signals?

something like
no led = 0V
1 led = ~3.0 V
2 leds = ~5.0 V
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Rob Tillaart

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I have not tried it on 3.3V. It should work, if not, try using smaller value resistors.
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