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How can I 'press' a push button with a relay?

Discussion in 'General Electronics Discussion' started by vasanthv16, Nov 27, 2013.

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  1. vasanthv16


    Nov 27, 2013
    The short form of the question is - how do I 'press' a push button with a relay. The button I am referring to is the Up/Down button on this thermostat.

    Here is the long version:

    I am looking at controlling my thermostat remotely. I could just go and buy a NEST or some similar thermostat, but the point really is to DIY. I am a guy with computer programming background. I don't have much insight into electrical/electronics. This is the rough idea I have so far:

    I have a non-programmable thermostat with Up/Down buttons to set the temperature. I am thinking of having a relay connected to each of these buttons. I can then control the relay programmatically and eventually access it from my cellphone, so that I can control the temperature from my cellphone. I have the latter part figured out (Cellphone > Web Server > Raspberry Pi > Relay). The main unknown for me is about how to 'press a button' on the thermostat with a relay.

    There are a couple of reasons I want to connect to the button and not directly to the underlying HVAC circuit - I am not comfortable doing that (did I say I am not an electrical/electronics guy?). The second reason is that it makes it easy for someone to override the system by simply pushing the Up/Down buttons.

    So, my question, again, is - how do I 'press' a button with a relay? I am looking at breaking open the thermostat to have a closer look at the push buttons, but am I even thinking in the right lines? Any ideas/suggestions/comments would be appreciated.

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    You can simply connect the contacts of a normally open relay in parallel to the pushbutton. If you actuate the relay, the contact is closed - the "pushbutton is pressed".
    You will need 2 relays, one for each contact..
    You may want to consider using photocouplers instead of relays. Photocoupülers are fast, silent and need little energy. You need to know the polarity of the signal you want to switch because photocouplers are unidirectional.
    Or use solid state relays. These are bidirectional.

    Before you go for electronic relays, check the maximum allowed voltage drop across the "closed contact". Electronic relays typically have a higher voltaeg drop than a mechanical switch or relay. You can check by using a resistor to short-circuit the contacts of the up/down switch. By varying the resistance you will find a point where the resistor is no longer recognized as "closed contact". measure the voltage drop across the resistor and compare that to the specification of the electronic switch (datasheet).
  3. vasanthv16


    Nov 27, 2013
    That was a lot of useful information. Thanks!

    I already have a Sainsmart relay lying around. If it turns out to be too noisy, I will consider switching to a photocoupler.

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