Switching Delay Used by 555 Timer IC

Posted on Feb 4, 2014

The circuit described here was designed as an addition to a remotely controlled garage door opener. The problem was that a brief burst of interference, arising from a thunderstorm or a mains spike, was enough to trigger the mechanism, and the author found this a nuisance. The effect of the circuit is to enable the output from the receiver module o

Switching Delay Used by 555 Timer IC
Click here to download the full size of the above Circuit.

nly when a relatively long pulse (more than about 0. 5 s) is received. The circuit can of course also be used in other similar situations, such as electrically-powered shutters, alarms and the like. At the heart of the circuit is NAND gate IC1. C. The output of the circuit (after inverter IC1. D) only goes high when both inputs to IC1. C are at a high level. When the circuit is triggered T1 conducts, and the output of inverter IC1. A, and hence also pin 8 of IC1. C, go high. If we now arrange things so that for a preset time the other input to IC1. C remains low, the trigger signal will not be propagated to the out-put until this period has elapsed. In the case of the author`s garage door opener, this will only happen if the button on the transmitter is held down. The 555 timer is used to generate the delayed gating signal for IC1. C. It is wired as a monostable multivibrator in a similar fashion to the arrangement in the Economy Timer` circuit elsewhere in this issue. When the circuit is triggered T2 will briefly conduct as a result of the positive edge at the output of IC1. A. This triggers the 555 timer: its out-put will go high, and thus pin 9 of IC1. C will go low. Because of the propagation delays through the components a very short low pulse will appear at the output of IC1. C when the circuit is triggered. The RC combination at the input to IC1. D ensures that this does not affect the output. When the period of...

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