# Power indicator red/green LED



## Taylor622 (Jan 31, 2013)

Looking for a simple circuit to show 18 VAC power on or off using a 3 leg red/green LED. Wiring the green LED with a diode and resistor to the 18 VAC seems easy. But how to power the red LED when power is interrupted? I want to indicate green when a self-resetting circuit breaker is conducting current and red when the circuit breaker is open.


----------



## Shdwdrgn (Dec 23, 2014)

A simple circuit I've been using is an inverter. This can be made with a transistor (2n2222 or 2n3904 work well) and a pair of resistors (1k seems to work fine).










So for the green LED, attach the positive side to point A. For the red LED, attach the positive side to point Q. Vcc should be something like 5VDC, but it's not particular. You will need two more resistors to limit current to the LEDs -- their value depends on your Vcc voltage and the forward current rating of the LEDs.

As you can see from the logic table, Q is always the inverse of A, so one LED will always be on and the other will always be off. I have been using this particular combination for H-bridges to drive motors. These require the same logic - one input is high and the other is low.


----------



## gregc (Apr 25, 2015)

is this easier to understand?

drop the zener diode and series resistor on the lefts and connect the 47K to the 18VAC (maybe thru a diode)


----------



## J.C. (Dec 24, 2016)

this one I use for DC but could be used for AC you would have to use a larger value resistor on R2 for 18V , my guess would be 1.5K.


----------



## gregc (Apr 25, 2015)

J.C. said:


> this one I use for DC but could be used for AC you would have to use a larger value resistor on R2 for 18V , my guess would be 1.5K.


isn't this a fuse circuit, between the supply and load with LEDs to indicate when the fuse is blown?


----------



## J.C. (Dec 24, 2016)

gregc said:


> isn't this a fuse circuit, between the supply and load with LEDs to indicate when the fuse is blown?


 thought you wanted a indication of breaker open or closed , ok sub the term fuse for overload protection, will work the same.


----------



## Taylor622 (Jan 31, 2013)

Thank you JC. The "fuse" circuit is just what I'm looking for. The input voltage will be 6-18 VAC, the fuse will be a 5 amp self resetting thermal circuit breaker. Collecting parts now, I will post pictures when put together. Thanks for the guidance.


----------



## gregc (Apr 25, 2015)

Taylor622 said:


> Thank you JC. The "fuse" circuit is just what I'm looking for. The input voltage will be 6-18 VAC, the fuse will be a 5 amp self resetting thermal circuit breaker. Collecting parts now, I will post pictures when put together. Thanks for the guidance.


if that's all you want, why not just wire an LED and resistor on either side of the breaker? (or wherever you want an indication that a voltage is present).

the fuse circuit uses a common resistor. don't understand the purpose of R1?


----------



## J.C. (Dec 24, 2016)

Taylor622 said:


> Thank you JC. The "fuse" circuit is just what I'm looking for. The input voltage will be 6-18 VAC, the fuse will be a 5 amp self resetting thermal circuit breaker. Collecting parts now, I will post pictures when put together. Thanks for the guidance.


sorry the circuit I posted was the wrong one it is used in conjunction with a photo cell circuit in place of the fuse , this is the correct one , I used a 2mm by 5mm frosted depending on the red/green LED you get you might have to up value of R1 a bit , also you might want to play around with R2 value to get brightness you want 1.5 K is a starting point .


----------



## Shdwdrgn (Dec 23, 2014)

Now you have two resistors on the red LED and a single resistor on the green. And it looks to me like the red LED is just a power indicator while the green comes on when the fuse is good. I thought OP wanted either red or green to show, but not both?


----------



## J.C. (Dec 24, 2016)

Shdwdrgn said:


> Now you have two resistors on the red LED and a single resistor on the green. And it looks to me like the red LED is just a power indicator while the green comes on when the fuse is good. I thought OP wanted either red or green to show, but not both?



the red is (if you pull the green lead with fuse good there is just a minimum red with green overpowering red if fuse is good)mostly off when fuse is closed the LED's I used show green if fuse is good Led shows red if open, the value of R1 is dependent on turn on voltage of red LED. I've been working with this using a photo cell circuit in place of the fuse to show occupied tracks in hidden storage yard with the red green on both ends of ladder tracks , green indicates the switch is clear red meaning the switch is fouled might also add a green yellow on LED before switch to show the train is nearing the switch.


----------

