Help with transistor switching

Discussion in 'The Projects Forum' started by mhighmark, Jul 18, 2014.

  1. mhighmark

    Thread Starter New Member

    Jun 21, 2014
    Hi guys,

    I'm not sure what I'm doing wrong, as I thought I had grasped how transistors work! But I'm new to electronics.

    I'm trying to use a 555 timer output to switch on a large magnetic valve. My circuit is running at 12v, 1A, and the motor requires the full power. I believe it draws a surge of several amps when it flips on.

    Basically, in the image instead of lighting an LED, I need to turn on the valve for a period of time.

    Thanks guys,
  2. wayneh


    Sep 9, 2010
    Get an n-channel MOSFET such as IRF540N (very common and easy to find, nothing special). Connect the source pin to ground, the drain pin to the V- pole of your load, and the gate pin to the 555 output. The V+ side of your load will be connected to +12V. The MOSFET will act like a switch on the return-to-ground line of your load. When the 555 output is high, the switch will turn on the motor.

    It sounds like your load is inductive, so you should place a diode across its poles, reversed biased. This will dissipate the voltage spike that occurs when you cut power to an inductor. The voltage spike could otherwise damage your MOSFET.
    mhighmark likes this.
  3. shteii01

    AAC Fanatic!

    Feb 19, 2010
    I see lots of words and only 3 numbers. There is definite lack of numbers.
  4. ScottWang


    Aug 23, 2012
    You are missing the pin 4 to connecting to +V, both of two 555.

    And you are using two stage of one shot timer, the second timer will be triggered until the first timer completed its delay cycle and its output from high to low level, if that's what you want.