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 only 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.
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