This is a watchdog reset circuit that works off of the serial data line between a PC and a TNC. The watchdog resets itself whenever it sees activity on the transmit data line (indicating NOS or your BBS software is alive and kicking). If there is no activity over a very long period of time (on the order of tens of minutes), the circuit will switch a transistor on to trigger whatever circuitry you wish. Usually this will be connected to your CPU reset switch The circuit differs from the traditional 555 watchdog timer because such timers would require an extremely large value resistor and capacitor in the RC network to achieve very long timeouts Adjusting the timeout interval in such a circuit would also require a lot of patience. Instead, this circuit uses a binary counter as the timer. The circuit can be assembled using readily available parts and may be adjusted quickly using an audio frequency counter
