Connect with us

Night detector power LED (with LDR)

Discussion in 'LEDs and Optoelectronics' started by Panagiotis_S, Aug 12, 2017.

Scroll to continue with content
  1. Panagiotis_S


    Aug 12, 2017
    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.
  2. dorke


    Jun 20, 2015
    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?
  3. Panagiotis_S


    Aug 12, 2017
    Thank you for your quick reply!

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


    Jun 10, 2015
    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.

  5. Panagiotis_S


    Aug 12, 2017
    Thank you!!! I will test it and I will reply if something goes wrong again. :);)
  6. Edward John

    Edward John

    Jun 24, 2017
    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.
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day