Paul shows off his custom made pin state test rig for testing on a Teensy 3.0.

I wanted a board that shows the state of all I/O pins on LEDs with all of these properties….

• Works for Digital Input, Output, and Input Pullup
• Pushbutton to drive the pin high, very weak pulldown (much weaker than on-chip pullup)
• Works for analog pins (thumbwheel pot), but also works when the same pin is used for digital
• LED shows actual analog level – LED current should be linear to actual voltage
• LED current does not interfere with analog voltage
• LED does not interfere with weak signals, or input pullup or pulldown
• Inexpensive – under $0.50 per pin – many parts are ok, as long as they’re cheap!

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I realize this is probably excessive design for most people, but when you’re making a development board and testing nearly every major Arduino library, it’s amazing how handy tools like this can be.  So I thought I’d document this little circuit, and especially the 150K feedback resistor trick that makes it work so well with these higher impedances and the chip LMV358 opamp.

Check out the full post for more circuit details and photos.