I got tired of haveing to get out my multimeter every time I had to measure a simple on or off, so I decided to build a probe. This probe uses a 555 Timer Chip as an inverting schemit trigger, sourcing and sinking two LEDs.

Step 1: How It Works

I've made a simulation of it, when the input is 1 or high the output gets set to 0 or low, which allows the red(1) led to get GND an light up, then when the in put is 0 or low the output is 1 or high which allows the green(0) led to get VCC and light up.

ProbeVid.mp4
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Step 2: What You Need

To build this you will need:

1x 555 Timer Chip [Digikey]

1x 1k Resistor [Digikey]

2x Resistors for LED's (I used one 1k for the red led and none for the green led.) [Recommended values Digikey]

1x Red LED (for logic 1) [Digikey]

1x Green LED (for logic 0) [Digikey]

Step 3: Schematic

You can make it "dead bug" style like I did, or you could build it on a breadboard, or make a PCB. Just follow the diagram...

Step 4: Done!

Now you have your own logic probe built using a 555 timer!

It works pretty well, but when the probe tip is left "floating" (Not connected to anything) it will toggle somewhat randomly between 1 and 0, specially when you touch it. I attached a video of it measuring 1's and 0's on my Arduino with the blink scketch.