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Total Newbie Question: Using a relay as a switch?

Discussion in 'General Electronics Discussion' started by BaconCatBug, May 28, 2017.

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  1. BaconCatBug

    BaconCatBug

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    May 25, 2017
    Hi all. Once again I have a complete newbie question here,.

    I wear headphones and more often than not I cannot hear when someone is knocking at the door. I have a wireless doorbell that lights up a piddly little red LED for about 5 seconds to visually catch my attention but as it's so nondescript I often don't notice it. A lot of the equipment tailored towards the hard of hearing with lights and such are horrendously overpriced because they can get away with the pricing. Instead I am hoping it would be possible to cut out that LED and replace it with a relay which will light up a string of flashing battery powered multicoloured LEDs instead.

    This is a crude image of what I was hoping to do: http://imgur.com/a/r64un

    My actual question is what kind of relay should I use or should I be using something else? I guess I am looking for advice on what part exactly to get. Is it an SPNO or SPNC relay I would need?

    Apologies in advance for the total newbieness.
     
    Last edited: May 29, 2017
  2. Joosh

    Joosh

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    0
    Feb 25, 2017
    I'm gonna say something like this may be useful to you: https://www.google.co.uk/search?q=relay+optocoupler+board

    Low voltages. I'm assuming the doorbell circuit is battery powered too? Isolated doorbell from your flashing LED power supply. And cheap!

    Most of them have NO and NC contacts so you can play about. Generally would use NO for your application - the flashing LEDs would be off until your doorbell circuit turns on the relay via the optocoupler.

    Edit: You could just use one of those small blue relays on it's own if your doorbell circuit is capable of powering it.

    Josh
     
  3. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    A junction transistor or MOSFET will be able to drive a more powerful light. A suitable power supply will be required.
     
  4. BaconCatBug

    BaconCatBug

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    May 25, 2017
    Any chance you could link something suitable? I was thinking of picking this up from fasttech because it's a dollar so even if it doesn't work it's no big loss https://www.fasttech.com/products/5986401
     
  5. Audioguru

    Audioguru

    3,084
    691
    Sep 24, 2016
    A red LED uses only 1.8V. The 12V relay needs at least 9V that your doorbell does not have.
    The input to a transistor can behave exactly like the LED in the doorbell circuit but the bright lights it drives need an extra power source.
     
  6. BaconCatBug

    BaconCatBug

    4
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    May 25, 2017
  7. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    It may do but will only drive 70mA.
    It seems to be fussy with control voltage of 1 to 1.3V.
    Decide on the light you wish to use, find its voltage and current requirements, find a power supply with ample spare capacity.
    The switch can be a MOSFET, junction transistor (prefered) or Darlington. Plenty of choice, small and cheap.
     
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