Reliable 555 timer doesn’t falsely trigger
John Dawson, Opelika, AL; Edited by Martin Rowe and Fran Granville -January 06, 2011
Circuits employing the popular 555 timer circuits are often reliable under many conditions. When you use them in electrically noisy environments, however, the timer can produce a false trigger, no matter how well you filter its power-supply lines. The circuit in Figure 1 sends a pulse to an SCR (silicon-controlled-rectifier) crowbar circuit when the 555’s input pulls low due to a fault-detection circuit. The 555 timer chip is unpowered until a crowbar fault signal occurs. The logic-low signal forces the 74LS02 NOR gate’s output high, which provides enough power to operate to the 555 timer circuit. The timer triggers on power-up. Capacitor C2 holds the trigger signal low until it charges to 5V. The 555 timer’s output should drive a low-current device—in this case, a transistor switch. This circuit solves the problem of false triggers. The pulse transformers connect to two SCRs in series that pulse 1600 to 2000V dc to fire a crowbar for a 22-kV dc power supply. The SCR-controlled high-voltage power supplies are electrically noisy, causing many false triggers from the 555 timer circuit.