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help I need a circuit layout designing

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New Member
What I require is a curcuit that will monitor whether a light bulb is on or off and switch a relay depending on the state of the bulb. this circuit can be powered by a 12v but but I require it to switch 240V.




New Member
Is this a school/college project?

Do a search on google.

Plenty of circuits out there.


Active Member
Hi leon,

Here is a circuit which will close a relay
when the light bulb is switched on.
You could use the contacts to drive
other items which are not connected to the mains.

Use a low voltage relay, the lamp wont notice
the drop.
The relay has to be AC working.

Regards, John



New Member
why not better use a 12v relay, in paralel with the bulb?
a 50mA relay can easyly switch 10A, so there is no problem like that.
or you can use an even smaller current relay, if you only need to swithch 1,2 A from 220V, you might use relays wich only draw 10mA, so no problem for the extra load.


Active Member
Two points,
if the relay is in paralell with the bulb,
it would operate whether the bulb works or not.

And those little 12v low current relays
are very rarely AC working.

Also i am starting to think that he wants the
circuit to be light sensitive.

So heres another offering below.

Pretty self explanatory.
The tube is to minimise unwanted light.
Aim the tube at the light source,
the thin paper should show when you're near enough.
Light sensors these days are good enough to
operate relays directly.

If the light sensor you choose is sensitive enough,
and lots are, then you can put another piece of
paper over the open end, and poke a hole in it with
a pencil, this would give greater accuracy of aim.

Best of luck with it.
light sensors are polarity consious,
so make sure its the right way round.




Active Member
You will need to put a transistor in between photo-sensor and relay because I don't think photo sensor will pass enough current to activate the relay.


New Member
and if it does, i dont think it can handle that kind of current.
and i dont understand what is the problem with the relay switching ac?


Active Member

The NTE3036 photo-transistor will handle 250 mA
that should be enough for most small relays.
Dont forget the 'quenching' diode across the
relay coil, cos that can have quite a kick
and may damage the sensor.

Putting the coil in series with the supply to the
lamp would mean using an AC relay (or rectifying)
there is no problem with the relay contacts
switching AC or DC,
its the relay coil.
Most of these smaller relays are DC only,
they buzz badly on AC,
you could of course rectify and smooth,
but getting an AC working relay in the first
place would be easier.

However, i think leon mean a light sensitive unit,
and since he hasn't been back,
i shant worry any more.

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