Ideas for a point switching circuit...

Discussion in 'The Projects Forum' started by vonsworld, Sep 19, 2014.

  1. vonsworld

    Thread Starter Member

    Apr 27, 2011

    I'd like to develop a circuit for switching the points on a model railway and wondered if someone might give me some ideas...

    The point motor needs a short burst of power only, say 15v dc and 1 amp, since a continuous current would burn out the point solonoid.

    Therefore the circuit really needs to mimic a push button switch.

    When a voltage is applied to the circuit it should only allow through a short burst of power to the point motor and then nothing (even though a voltage is still being applied to the circuit, ie. it is still switched on).

    It will then not allow another burst through until the power to the circuit is switched off and back on again.

    I can see this could perhaps be done using a capacitor, which will only allow a voltage through until it is fully charged. However when the voltage to the circuit is cut the capacitor will remain charged, and so will not work again when the voltage is re-applied.

    The capacitor would need to automatically discharge as soon as the circuit is switched off, so that it is then ready for the next switching action.

    Please could someone guide me in the right direction for a solution?

  2. ericgibbs

    Senior Member

    Jan 29, 2010
    What if a discharge resistor was connected across the capacitor.?
  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
    Used diectly , it will take a verry large cap., using an transistor or darlington config. will multiply C down to manageable size. A timer would be my pref. sharp on sharp off. How long is short?; maybe 2 sec?
    Motor or solenoid?
  4. crutschow


    Mar 14, 2008
    Below is the sim of a simple circuit that should do what you want. It outputs a single ~1.2s pulse upon power on and resets when the power goes to 0V (D2 rapidly discharges C3). The pulse ON time can be varied by changing the value of R5 and/or C3. The transistor can be just about any P-MOSFET with a 20V or greater and 2A or greater rating.

    Rload is just for simulation of the motor load and is not part of the circuit.

    Power on One-shot.gif
  5. SgtWookie


    Jul 17, 2007
    Bernard likes this.