I want to build a timer circuit (possibly using a 555 timer) based on the presence (or absence) of a Hall Effect signal. I want the timer to turn on an SPST relay when it receives a pulse from the Hall Effect and the relay needs to stay on for about 3 minutes after the Hall Effect signal is no longer present. The Hall Effect signal can vary from 1 pulse per second (pps) to 30pps, but regardless of the Hall Effect frequency I need the relay to stay on until the pulses disappear. In other words, the time delay doesn't start until the Hall Effect signal disappears and... if the Hall Effect signal re-appears, the timer stops (relay stays energized). I will be using this timer circuit in conjunction with another circuit that is already assembled and operational, so there cannot be any interference between the two circuits (possibly couple the timer circuit with a capacitor?) The entire system will be running on 9vdc. The Hall Effect Timer circuit will turning on the display on an anemometer, but only when the wind is blowing. Can someone help??