Substitute NPN tranistor for pushbutton switch

Discussion in 'The Projects Forum' started by Circuit Breaker, Dec 15, 2010.

  1. Circuit Breaker

    Thread Starter New Member

    Dec 15, 2010
    Greetings to all,

    Admittedly I am a novice at electronic circuits but I am fascinated by the components and how they all work together. So far I have a working knowledge of the most rudimentary components and can build simple circuits successfully if I am given the components, their values and a schematic of the circuit.

    With that having been said here is my project. Attached is a schematic for a time delay circuit I would like to incorporate into my project. The schematic shows a pushbutton switch to activate the circuit. I would rather activate this circuit using another circuit via an NPN transistor. My dilemma is I do not know what type of NPN transistor to use or how to incorporate it's leads into my circuit in replacing the pushbutton. The circuit I would use to activate the attached circuit is a commercially built current sensing detector whose output is (-) negative 12 volts. The output configuration is a push-pull and can sink and source current with a maximum output load of 400 ma. I'm in hopes that that is sufficient information.

    In summary, the two things I am looking for guidance are one, the type and capacity of the NPN transistor I should use, and two, how do I configure the leads of the transistor into the circuits to replace the pushbutton switch.

    My thanks in advance to anyone kind enough to read and respond to this post.

    Circuit Breaker
  2. wayneh


    Sep 9, 2010
    For a switching application like this I'd recommend a MOSFET such as IRF540. You'd need to verify it can handle your load. It needs 12v applied to it's G (gate) pin to open a current path to ground, the S (source). The D (drain) attaches to the bottom of your circuit, the lowest voltage on the current path to ground. Not sure I understand the -12v signal, but the MOSFET needs the gate-source voltage to be 12v to be fully "on".
  3. SgtWookie


    Jul 17, 2007
    Wait a minute - Wayneh, did you even look at his schematic along with his problem statement?

    An N-ch MOSFET won't work, neither will an NPN transistor.
  4. Wendy


    Mar 24, 2008