Reliable 555 timer doesn't falsely trigger

John Dawson

EDN

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.

Figure 1.	Powering the 555 timer from a NOR gate results in no false triggers from an electrically noisy environment.

The timer triggers on power-up. Capacitor C₂ holds the trigger signal low until it charges to 5 V. 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 2000 V 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.

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