If you need to monitor the activity of digital channels, you can use this circuit. It is implemented with a PIC16F628A and monitors up to 8 digital channels. It will be triggered (and retriggered), if a rising or falling edge was detected at an input and sets the corresponding output (RA0->RB0, RA1->RB1, ... RA7->RB7) to high for 35 seconds. This means, as long as there are signal changes on an input, the output will be high all the time. If there is no signal change for more than 35 seconds, and at startup, the output will be low.
