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Using LED to monitor status of a relay


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#1 PaulD

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Posted 22 October 2009 - 12:22 PM

I have some relays that route 24VAC to a irrigation valve when activated and I would like to have a visual indication that shows which relays are activated. Relays are visible so I could easily do it with a LED bridged across the relay contacts if that would work. In effect, I want a LED to light up when 24 VAC is applied to it.
Any trick to that other than just wiring a LED between the common and hot leg? Prefer to avoid other parts such as resistors if possible. Any suggested model of LED that can handle 24VAC and is readily available at local Radio Shack? I checked the Radio Shack web site and many of the LED's do not provide info on their voltage limits

#2 Jreigner

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Posted 22 October 2009 - 01:50 PM

I have some relays that route 24VAC to a irrigation valve when activated and I would like to have a visual indication that shows which relays are activated. Relays are visible so I could easily do it with a LED bridged across the relay contacts if that would work. In effect, I want a LED to light up when 24 VAC is applied to it.
Any trick to that other than just wiring a LED between the common and hot leg? Prefer to avoid other parts such as resistors if possible. Any suggested model of LED that can handle 24VAC and is readily available at local Radio Shack? I checked the Radio Shack web site and many of the LED's do not provide info on their voltage limits

Your best bet would be to use a double pole relay and wire your led off of there. Wiring an led in series with your source voltage will only cause you problems(like when the led blows out and your irrigation doesn't turn on). I prefer to keep control circuits seperate from power circuits. Most LEDs will not operate on 24volts let alone ac power. Are you trying to monitor for failure or operation? You could also wire to your alarm if you have one, to provide a chime announcement when it switches on and then when it turns off- kind of like a driveway alert.

#3 WayneW

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Posted 22 October 2009 - 02:16 PM

I have some relays that route 24VAC to a irrigation valve when activated and I would like to have a visual indication that shows which relays are activated. Relays are visible so I could easily do it with a LED bridged across the relay contacts if that would work. In effect, I want a LED to light up when 24 VAC is applied to it.
Any trick to that other than just wiring a LED between the common and hot leg? Prefer to avoid other parts such as resistors if possible. Any suggested model of LED that can handle 24VAC and is readily available at local Radio Shack? I checked the Radio Shack web site and many of the LED's do not provide info on their voltage limits

What voltage are the relay coils? You could place the indicator lamp in parallel with the relay coil or in parallel with the sprinkler valves (24VAC). If the relay coils are 12VDC, that will be easier to drive a LED with.

Getting a LED to work on 24VAC will involve a resistor and maybe a diode to protect the LED from the reverse voltage as most LEDs operate at around 2VDC. Some LEDs has a built in dropping resistor. Maybe it is easier to just find a small 24V indicator bulb?

#4 PaulD

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Posted 22 October 2009 - 03:33 PM

What voltage are the relay coils? You could place the indicator lamp in parallel with the relay coil or in parallel with the sprinkler valves (24VAC). If the relay coils are 12VDC, that will be easier to drive a LED with.

Getting a LED to work on 24VAC will involve a resistor and maybe a diode to protect the LED from the reverse voltage as most LEDs operate at around 2VDC. Some LEDs has a built in dropping resistor. Maybe it is easier to just find a small 24V indicator bulb?


Relays are driven by 12 VDC and would be easy to tap into. The trigger is a 12VDC output (8 ea) on a HAI Expansion Enclosure intended to trigger relays. The relays themselves are on a HAI 19A00-1 8 Relay Module.

#5 Zac

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Posted 22 October 2009 - 03:41 PM

The 19A00-1 relay module already has an LED that indicates the status of each relay. Are you trying to add an additional LED in a remote location?

#6 Jensen Beach

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Posted 22 October 2009 - 03:45 PM

If you want or need an LED, I think you'll need to piece it together.
You could try this, pretty simple, about $3 for parts at RS.
Attached File  24VAC_to_LED.JPG   15.33K   30 downloads

#7 PaulD

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Posted 22 October 2009 - 07:31 PM

The 19A00-1 relay module already has an LED that indicates the status of each relay. Are you trying to add an additional LED in a remote location?


I have not hooked it up yet. Did not realize it has an LED. If so, that fixes my basic requirement.




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