I'm a beginner and trying to figure out how this circuit works (its a simple circuit that glows an LED when dark) :


schematic http://makezineblog.files.wordpress.com/2014/10/wp20_schematic_w_circle-transistor.png?w=1410&h=827 My confusion is when the light is present, wont the current still flow through the collector and base of 2N3904 ? This is connected in parallel ? How does this circuit work ? How is the LED portion of the circuit not closed when light is present ?



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    \$\begingroup\$ If you want people to invest time to answer this question, then you will also have to put some effort into. Asking people to watch a video will probably mean that this will get closed. Summarize the video and do some screen captures or links. \$\endgroup\$ Oct 31, 2014 at 20:49
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    \$\begingroup\$ Better still, draw out the schematic yourself and try to work through what voltages and currents are where when there is light and when there isn't. Show all your thinking and ideas in your question, and we can help you understand it better. \$\endgroup\$
    – Majenko
    Oct 31, 2014 at 21:00

1 Answer 1


When light is present, the phototransistor will conduct better than in dark conditions. This results in a lower voltage on the base of the transistor (the phototransistor and resistor form a so called voltage divider).

When the voltage on the base of the transistor is lower than about 0.7V (transistor base-emitter voltage) + 1.8V (LED forward voltage) = 2.5V, the transistor will turn off and the led will be off.


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