Logic Probe

http://hackaday.com/2016/12/24/dead-bug-logic-probe-in-a-magic-marker/

I like the dead bug design, and most of the features.

Things I want:

• Red for high, green for low
• Convert signal to tone (at high impedance)

Mark I

I like the dual opamp design. It's simple. If I have a working buzzer, I can wire that up in parallel with the ON LED. Otherwise, the lights are enough.

I was trying to design this so it would work at any voltage. I think I'd be better off designing it to work from ~2v to 12v.

Failure

I used a 5v regulator which was just dumb. I need ~7V just to power it. So don't do that. The probe was also sensitive enough that it tended to ground itself. So the probe needs a high value voltage divider (2x10K).

Mark II

I can do the whole thing with a quad op amp, a few resistors and a Zener diode (plus the LEDs).

The trick is to have the comparators output a LOW for TRUE. I can use the Zener diode with a 3rd op amp to create a simple 5v regulator. If the voltage is lower than 5v, the reference will act as a pull up, and it will still output 3.3v. Since the LEDs are diodes, they will only conduct when the logic goes low, and they'll be hooked up to 5v (or whatever).