# Led On Indicator

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

1. ### shakey1961

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0
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

Regards

Jon

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,489
2,830
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

10
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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

2,947
803
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

10
0
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

5,364
771
Jan 9, 2011
Get the polarity correct.