Pulse Filtering

Discussion in 'General Electronics Chat' started by jamesmcuk, Oct 20, 2012.

  1. jamesmcuk

    Thread Starter New Member

    Oct 20, 2012
    Hi guys

    Looking for a simple solution to a little issue. I have a control unit that outputs 12 volts every two seconds or so for only a fraction of a second. This output has a resistor between itself and Ground. The resistor is present so the unit knows the output is good (otherwise a fault relay kicks in). Originally the output should have been supplying a flashing light that under certain circumstances would be lit up all the time (constant 12 volts applied to the output).

    I want to use the output to drive a relay but I only want the relay to respond to the output when it supplies a constant 12 volts and not the little blips of power that is being delivered for the flashing and fault monitoring. At the moment the relay clicks each time the light would have flashed. Is there a simple way I can maybe use a capacitor to dump or damp these pulses and stop them activating the relay.

    Many thanks in advance

  2. crutschow


    Mar 14, 2008
    You need a resistor, capacitor, and a MOSFET to drive the relay (below).

    The R*C value and the MOSFET threshold voltage determines the delay before the relay energizes. For example 1 megohm and 0.1μF will give a delay somewhat less than 0.1s (depending upon the threshold voltage of the MOSFET).

    The MOSFET can be just about any N channel MOSFET with a minimum voltage rating of 20V and a minimum current rating of 0.5A

    Relay Delay.gif