Dark Detector


New Member
I have a few questions regarding this circuit:

1. What type of capacitors should the 10n and 100n components be?
2. The 1Mo variable resistor - Is it a linear type?
3. I intend using the circuit (or rather several circuits!) to control model railway lighting (all LEDs).
I would imagine that it is a simple case of exchanging the 10u capacitor and speaker. Am I correct?
4. What is the maximum number of LEDs per circuit? For realism, I intend 4-6 LEDs per circuit. Would that work?

Sorry for the elementary questions!

Thanks in advance,

Hi Don you don't need to use a 555 chip if you just want to switch some LEDs on automatically when it gets dark. You just need a light sensor, a small transistor and a couple of resistors. I made a fish tank light last year that worked great. I'll see if I can post the diagram.

By the way you can use any type of capacitor it's the micro farad value that is important, not what it's made of. Also linear or logarithmic pots make little difference for this type of circuit.

The BC547c transistor can switch a maximum load of 100ma which doesn't sound much but thats at least 4 LEDs singley or you could put them into arrays or banks of 4 (4 because of the 12v supply ie. 3v each) with a 40 ohm resistor in series then you could light 12 up with one transistor, but they would be bunched up in fours of course.
Google 'LED resistor calculator' it does it all for you.
All the components are easily available on ebay. Build it on a little piece of veroboard (also from ebay) The LDR i used had a resistanc range of 1.5k ohms when light to about 2M ohms in darkness.
Any more questions feel free, and good luck


New Member
Hi Rich,
Many thanks for the replies and the schematic; I shall give both circuits a try and report back...!
Incidentally, the LEDs I shall be using are 12v types, rated at 20mA each (according to the datasheet on Rapid Electronics' website).


You don't even need the 680 ohm resistor then.

I don't think the 555 circuit will work very well as a night light because it outputs a square wave intended for a loudspeaker.


New Member
Hi Rich,
I can report that your very simple circuit worked a treat!
I have not tried the 555 circuit, as the one you posted is more than adequate for my needs.

Thanks again!