Night detector power LED (with LDR)

[Panagiotis_S]

Hi everyone! I'm trying to make this circuit I've found on internet.

I've modified the circuit by using the following parts:

1. Power Source 5V (and not 9V)
2. Resistor R1 = 6.8 Ohms (and not 390 Ohms)
3. 1W LED with forward voltage at 3.2-3.4V and forward current at 300mA.

I've used the other components as the image above.
My problem is this: My BC557 transistor gets too hot when I turn on the circuit (and after few seconds it breaks). The LED stays also on all the time and when I change the value of the potensiometer's resistor, nothing changes. Do you know what causes this problem and what to do so that the project will work fine? Thank you very much.

 

[dorke]

Welcome to EP.
The BC557 is a 100mA transistor,do you think it will not break at 300mA?
Get a bigger transistor!!!

What LDR are you using?

 

[Panagiotis_S]

Thank you for your quick reply!

Can you please suggest me a suitable transistor to work with?
I'm using a 5528 LDR (R248).

 

[AnalogKid]

TIP30 or TIP32. Add a 220 ohm resistor in series with the base before the pot wiper. This prevents blowing the base-emitter junction with too much base current when the pot is turned to zero.

Note that the LED will not snap on crisply; it will wander up as the ambient light decreases. For more crisp action and no fluttering at dusk, consider a comparator circuit to drive the output transistor.

Your circuit will work much better with a darlington, but at 5 V there is not much headroom to work with.

ak

 

[Panagiotis_S]

Thank you!!! I will test it and I will reply if something goes wrong again.

 

[Edward John]

This LDR circuit diagram shows how you can make a light detector. An LDR or “Light Dependent Resistor” is a resistor where the resistance decreases with the strength of the light.