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.
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.
does it also work for 3.3V signals?
no led = 0V
1 led = ~3.0 V
2 leds = ~5.0 V
I have not tried it on 3.3V. It should work, if not, try using smaller value resistors.