Here is a picture of the world's simplest logic probe.
And this is how you use it. Ground your body, and hold the negative lead...
...then touch the positive lead onto the node you want to test. If the LED lights up, that node is high.
Measuring the resistance of my body from one hand to the other gives a result of about half a megohm, depending on how tightly I hold the probes. LEDs these days are so sensitive that even the ten microamps or so that that resistance allows to flow at logic voltages is enough to light them quite brightly. It's not even any special kind of LED, it's just some random cheap-ass Chinese LED off Amazon. Neat, huh?
In case you're wondering the circuit board in the pictures is my parallel-port programmer for MX25L8005 1MB serial flash RAM chips. The empty 8-pin DIL socket is where you plug in the chip you're programming. The reason some of the components in the last pic look a bit weird is because the camera flash totally drowns out the light from the LED so I had to fuck about to make it show up without the rest of the photo being totally black.
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