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Led On Indicator

Discussion in 'LEDs and Optoelectronics' started by shakey1961, Apr 15, 2017.

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  1. shakey1961


    Apr 15, 2017
    Hi, first time here.

    This is probably simple for someone out there, but I can't get my head around it. I have some electronics knowledge, but not enough.

    I need an LED to light when a relay is in a certain state. The relay is a SPDT and the Com output is carrying 12v to a dash-cam power supply. In it's off state the relay will take the power from the car ignition, but when activated the power will come from a permanent 12v source i.e. I can turn the dash-cam on without the ignition being on.

    AS I am using a wireless switch I would like some way of wiring an LED in so I can see if the relay is on without the ignition being on, i.e. if I'm away from my car and it's on accidentally.

    As the power supply is being permanently fitted on Weds 19 April I need to get on with this quickly.

    Any ideas?

    Many thanks in anticiaption


    Jon Relay Switching.png
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Place a LED with a series resistor across the coil of the relay. When the relay coil is powered, the LED will light.
  3. shakey1961


    Apr 15, 2017
    Duh! Simple when you know how. Do I need a 2v or 12v LED and what Ohm resistor? 0.25w resistor?
  4. Alec_t


    Jul 7, 2015
    Anything sold as a "12V LED" will already have a series resistor built in. For a normal LED (~2V), choose a high efficiency type to avoid draining the battery. If, say, the LED is bright enough with only 2mA current, then the series resistor would be (12V-2V)/2mA = 5kΩ.
  5. shakey1961


    Apr 15, 2017
    So if I can get a 12v LED I just connect to across the coil? Other than that go for the 12v with the 5k resistor in series and connect across the coil?

    Brilliant. Thank you all. I shall tell you how I get on.
  6. duke37


    Jan 9, 2011
    Get the polarity correct.
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