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Help with Automatic light PCB

Discussion in 'LEDs and Optoelectronics' started by Soren, Apr 24, 2016.

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  1. Soren


    Apr 24, 2016
    Hi, just recently I have etched an automatic light PCB. The circuit should turn on the led as the brightness applied to the LDR decreases, but instead, the LED just turns on when I attach the 9volt battery. Anyone got any ideas on what might be wrong with my circut?

    FullSizeRender.jpg image1.JPG
    Automatic Light 2_schem.png Automatic Light 2_pcb.png
    Last edited: Apr 24, 2016
  2. duke37


    Jan 9, 2011
    The circuit looks reasonable.
    I assume that you have twiddled the potentiometer.
    Try shorting out the LDR and see if the light goes off, this checks the output stage.
    You could attenuate the signal going into the Darlington by making R4, say, 10k and add a 10k resistor from base1 to ground.
    A Darlington is very sensitive with a gain of over a thousand so very little current is needed to turn it on. Normally a voltage of 0.6V is necessary to turn on a silicon transistor but this assumes a reasonable base current. In the Darlington case the voltage is lower since the current is so low.
  3. dorke


    Jun 20, 2015
    Welcome to EP.
    What type of LDR are you using?
    Please measure the voltage on the LDR with a DVM in light and dark conditions.
  4. Soren


    Apr 24, 2016
    I tried the circut in brighter conditions and the led turned on when I covered the LDR, so I can confirm that the circut works :). Thanks for the help.
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