Relay output on/off indicator. Red LED= output Off, Green LED= output On

Thread Starter

SockSPDT

Joined Mar 28, 2021
3
I have searched and found this helpful thread and it was an elegant fix for one project where i needed a go/no go indicator,
but i cannot figure out how to apply it to another project i recently want to do.

I have this remote relay, and it's currently switching an automotive battery powered 12v fluid pump on and off.

I'd like to have a bicolor red/green LED indicate red when the relay opens and shuts the pump off
and indicate green when the relay closes and the pump is on.

I've played around a bit running different options on this simulator.

Using the circuit in the post above, I found that i could use another relay in place of the SPDT switch.

Pump On
circuit1.JPG

Pump Off
circuit2.JPG

But to me, there should be a way to use a transistor instead of a mechanical relay, i just don't know enough about their application.
Also would be cool to use the same 12v source for the bicolor LED instead of the 5v shown that i copied from the original post, i suppose with the appropriate drop resistor.

TIA
 

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boostbuck

Joined Oct 5, 2017
42
Here's a simple indicator, using a 2-lead 2-color LED and a single transistor to switch. When the pump is powered the transistor switches on and robs the current from the lower red LED and supplies a conduction path for the upper green LED.

A practical circuit would require some protection from motor EMF.

PumpIndicator.jpg
 

Thread Starter

SockSPDT

Joined Mar 28, 2021
3
Here's a simple indicator, using a 2-lead 2-color LED and a single transistor to switch. When the pump is powered the transistor switches on and robs the current from the lower red LED and supplies a conduction path for the upper green LED.

A practical circuit would require some protection from motor EMF.

View attachment 233986
I've got this circuit operating with two discreet 5mm LED's (they fit the package better). For the red LED, I experimented with using a blinking LED instead of a steady one. It blinks as intended when the load is off, but it prevents the green led from illuminating when the load is on. Is it possible to adapt the circuit to support a blinking red LED.

From searching, a blinking LED uses a internal thermal wire to heat/cool to switch on/off so that seems like it's opening what would be the return for the green led?
 

boostbuck

Joined Oct 5, 2017
42
A thermal heater and bimetallic strip is an flashing mechanism used within incandescent bulbs, not within LEDs. Flashing LEDs have a built-in integrated circuit to perform the flashing.

A flashing LED has a rated operating voltage, which in your case should be 12v.

I expect the attached circuit will work as you want. The flashing LED doesn't require the 680R current limiting resistor.

pump.png
 

AnalogKid

Joined Aug 1, 2013
9,257
I can name that tune in zero transistors.

When the relay is open, D1red and D2 conduct through R1. The combined forward voltage of D1red + D2 is approx. 2.4 V. When the relay is closed, D1green "shorts out" D1red + D2 because its forward voltage is only 2 V, and conducts through R1.

Yes, this really works.

ak
Motor-Relay-Indicator-1-c.gif
 
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