automatic light switch

Automatic Light Switch with PhotoTransistor

This is an automatic light switch circuit that turns ON a light bulb in absence of light. The IC 4060 works as an oscillator and generates a signal that is applied to the base of T4 transistor. Phototransistor T3 BPW40 is in conduction when light is present and it keeps the T5 transistor’s base at ground potential.

When light goes out and it doesn’t reaches the phototransistor, it gets in insulation. T3 reaches positive potential through 22kohm. T4 and T5 both gets in conductance. The oscillator signal reaches the optic coupler and then turns on the lamp.

automatic light switch


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  • Kabir

    Please show me the circuit for stairs which will turn on at human presence nearby, and turn off 30 seconds after the person leave.

  • shravani

    in the above circiut the capacitor used is of 390nF and can we get 400v……is it feasible ??

    • Jim Keith

      400V is the voltage rating of the capacitor–the line voltage is either 115VAC or 220VAC –the article does not indicate. The capacitor size is not all that critical –470nf would work OK too.

      The opto coupled TRIAC is obsolete –you will need to find substitute.

      My recent “LDR Pole Light Switch” may be better project.

  • Ismaheel007

    hi am a new user in the house,just need some help about phototransistor and its uses.what can it be use for.tanx

  • rahul kumar

    im supported a eletronic projects. like a main supporter in elecronic

  • Darktempt

    hi there… the problem on using the photo sensor is… when it come w/ dust… that is the one big problem of other using that item second it only use 1 output…

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