I have this decade old wall clock in my house and it has this little component peeping out its dial. I was curious as to what this component is and what it does in a clock.

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I was into Arduino for a brief period, and a photoresistor looked similar to this. Is this a photoresistor?

If yes, Why it's present in the clock? In 10 years I haven't seen the clock glow or light up from anywhere. What might be the reason for using the photoresistor in a wall clock like this?

  • 3
    $\begingroup$ it's a photo resistor ... the clock is probably quiet at night $\endgroup$
    – jsotola
    Apr 20, 2022 at 5:42
  • $\begingroup$ Ohh, yes that could be the case, since it played some music every hour. We switched the music off so I never had a chance to notice whether it did play at night or not. I'll test it now. $\endgroup$ Apr 20, 2022 at 6:08

1 Answer 1


That is a light-dependent resistor, commonly made of cadmium sulfide. When light shines on it, its resistance goes down by a significant amount, and as such these devices found use in film cameras to control the aperture opening or shutter speed as the amount of ambient light changed- so the film would always be properly exposed.

They are also used widely in light fixtures, to turn them on automatically at dusk and off again at dawn. In this application they are easily interfaced to another device called a silicon-controlled rectifier which does the actual switching in response to the resistance change.

General Motors luxury cars from the 50's and 60's used these things in their so-called "Magic Eye" option, which would automatically kick your headlights off of high beam when the Magic Eye detected the headlights of an oncoming car.


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