Light switch indicators

Thread Starter

Kardo22

Joined Mar 12, 2014
47
Hi,
Are there lights that can work with just the AC hot wire?

I have only hot wires going to the switches and I'd like to add an indicator light next to the switch that lights when the switch is on.
It would be a visual reminder to turn off the light.
There are switches that light when switch is off and only need hot so my way should be possible too?
 

LesJones

Joined Jan 8, 2017
2,726
The indicators that light when the switch is of are just a neon bulb (With current limiting resistor.) across the switch contacts. They are not just connected to the live wire. they get the path to the neutral via the load they are switching. A neon only takes a few hundred uA so the voltage drop across the load is insignificant. If you could tolerate a voltage drop of a few volts when the load was on then you could in most cases use a LED indicator to sence the current through the load but the circuit around the LED would have to be designed to match the current taken by the load.

Les.
 

AlbertHall

Joined Jun 4, 2014
10,554
If your light switches have no neutral and no earth connection (None of mine have these) then you could only have a indicator lit when the light is off (a neon connected across the switch).
 

AlbertHall

Joined Jun 4, 2014
10,554
If you could tolerate a voltage drop of a few volts when the load was on then you could in most cases use a LED indicator to sence the current through the load but the circuit around the LED would have to be designed to match the current taken by the load.
A neon needs in the region of 70V to strike so that would be quite a loss from the supply voltage.
You could conceivably use a torch bulb in series with switch but that would need matching the bulb to the load, possibly with a resistor too, but having matched them then it all goes wrong if you fit a more powerful or weaker bulb later.
 

LesJones

Joined Jan 8, 2017
2,726
Hi Albert, I was thinking more in terms of a bridge rectifier with say 4 diodes in series across it's output. This would give about 2.4 to 2.8 volts across them. This could then drive an LED (Assume a red LED with about 1.8 volts forward voltage.) via a resistor of 47 ohms to give an LED current of 10 to 20 mA. Even if the load current was low enough to use 1N4007 diodes or similar it would be difficult to make it fit inside the switch housing.

Les.
 
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