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Reliable 555 timer guarantees no false trigger

05 Feb 2015  | John Dawson

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Circuits utilising the popular 555 timer circuits are generally 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 the figure sends a pulse to a silicon-controlled-rectifier (SCR) 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.


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



About the author
John Dawson contributed this article.


This article is a Design Idea selected for re-publication by the editors. It was first published on January 6, 2011 in EDN.com.




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