555 Fast Reset Timer

Published:2013/8/22 20:05:00 Author:lynne | Keyword: 555 Fast Reset Timer | From:SeekIC

A common problem with on-delay timers is the timing capacitor reset. Typical timers (555 included) rely upon both complete power loss and finite elapsed time to assure that the timing capacitor is fully discharged. If this important detail is ignored, the timeout period is reduced and inconsistent. This 555 timer circuit resets almost instantly when Vcc drops below +10V. This circuit avoids the use of the 555 pin 4 reset feature that often causes more problems than it solves – this anomaly was previously documented in this article: Quirky 555 Timer Reset Function. The initial application for this circuit was a reset timer in which the output was hard-wired across a reset pushbutton. The terminology is a little confusing because this is a reset timer that resets digital electronics, and the timer itself must reset instantly upon power loss by dumping the charge in the timing capacitor. When digital components (including some Bluetooth stuff) are interconnected in automotive applications, initialization is sometimes an annoying problem that is complicated if both pieces of equipment do not have the same reset criteria (e.g. one component resets upon complete power loss, while another component resets when Vcc drops only briefly or slightly). In this application, a 50sec reset duration was found to be effective (I cannot provide more information because I was helping someone of a different language in a distant country). More advanced software features provide a “warm” reset feature that automatically restores lost communications – in such a case, a reset timer is not required.



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