How to check the state of the relay contacts?

Discussion in 'General Electronics Chat' started by sr_raval, Oct 8, 2012.

  1. sr_raval

    Thread Starter New Member

    Apr 19, 2012
    Hello Everyone,

    I would like to check the state of relay contacts, means everytime i want to check the status of the relay either its ON or OFF, insort by this feedback path circuit i want to know the status of the relay via microcontroller.

    Help to find out the suitable circuit for this.
  2. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    The common way is to use a set of NO contacts on the relay (one contact at logic ground the other contact pulled up to logic high with a small resistor) as a feedback signal to a digital input. The relay digital input reads a logic level high when off and reads a logic level low when energized and the contacts are closed. You could also just measure the voltage across the relay coil (not quite as good as checking actual contact closure). This is easy if the relay is driven by a open-collector circuit or contact to ground as you only need to measure the voltage from ground to the bottom of the relay coil. (high voltage when off, low or ground level voltage when on)