March 5, 2014

Light Switch ON/OFF LED Indicator

When I was first learning electronics I was faced with a little problem. I wanted to have some kind of indicator to tell me if a certain light on a remote location would be ON or OFF. Sure, there are light switches with LED or neon indicators but none of them actually had one for the ON position and another for the OFF position.


I didn't know much yet so I went to talk with a teacher about it. He gave me this circuit for me to try:

 
So I made a list of all the components and went to the electronics shop to buy them.

Quick enough, I had everything that I needed so I started making the circuit. The first the part of it, is responsible for turning on the red LED when the light is ON, for example indicating that room is being used. This is it:


This part is composed of 3 branches in parallel:
  • First the LED and a resistor;
  • Then 3 diodes in series with the same polarity as the LED, responsible for guaranteeing a voltage of 2.1V (3x0.7V) for the LED branch and also as a pathway for the current to the light bulb.
  • And then the third branch which is just a single diode with reverse polarity, to protect the LED against the negative cycle of the AC wave.
It's important to make sure that the diodes can resist the voltages and currents that will be applied to them, that's why I'm using the diode 1N4007, which has a maximum RMS voltage of 700V, it can resist to a reverse voltage of 1000V and has a forward current of 1A. Which means you can't use a bulb that requires a higher current than 1A, or the diodes will probably blow up.

Then the second part of the circuit, which is responsible for lighting up the green LED when the light switch is OFF:


Now this part is more simple, it just has an LED in parallel with a diode in reverse polarity, to protect the LED from the negative part of the AC wave, then both of them in series with a power resistor. The resistor must have a power rating of at least 1W because it will be responsible for dissipating the power of almost all of the 230VAC, since the bulb is OFF and the LED only needs around 2.1V, the rest of the voltage will be consumed by the resistor, which means, it will heat up a lot.

By the way, in the image, the wires for the LED are organized like a coil just to fit in the picture, I made them long to be able to put the circuit on the back of the switch and the LED on the front, but their length was being a problem for the photo. They are not meant to be or act as an inductance.

Having created both parts, it's now time to put it all together:


These are the basic connections to be made. After connecting everything I put all circuitry inside the switch and tested it.

Lights OFF:


As you can see, the green LED lights up, indicating the light bulb is OFF.

Lights ON:


And when I turn the switch the light comes ON and the red LED as well.

I made this circuit back in 2006 and I left it working on a room for over 2 years. It never gave any problem, although during cold nights, I noticed the light switch was a bit warm because the power resistor heated up pretty well inside.

4 comments:

  1. This is a interesting project. Very simple circuit, but less efficient than a switch with light indicator using auxiliary switches. A cost analysis will bring the final answer.

    Thanks for sharing your ideas.

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    1. Thanks!
      Yes, you're right, the advantage is just its simplicity. The resistor is almost always "wasting" power, so it's not the most efficient circuit.

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  2. It might be me but the actual layout doesn't seem to be matching the blueprint layout. They seem reversed in the red led. Could be the reason you have green when off and red when on?

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    Replies
    1. If you look closely, the schematic represents exactly the layout presented on the 3rd image counting from the end. The red LED is meant to be on when the lamp is also on, and the green LED on when the lamp was off. To help understand the logic of it, imagine it is an indicator to know if a bathroom is free to use (green LED on, light bulb off) or if it was occupied (red LED on, light bulb on).

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