Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Photoresistor problem

  1. Aug 13, 2008 #1
    I need a circuit diagram for running a motor using a battery pack (3 volts) that uses a photo-resistor for a switch. Really sorry for bothering you with such simple question but i am new to this thing.
    Thank-you. The attached was my previous circuit it did not work.

    Attached Files:

  2. jcsd
  3. Aug 13, 2008 #2


    User Avatar
    Science Advisor
    Gold Member

    The photoresistor is just that a resistor. It will limit the current through the motor. You can drive the motor with a transistor that is switched on by the photoresistor however.
  4. Aug 14, 2008 #3
    but the photoresistor has too much resistance even wen exposed to sunlight. The max the resistance has dropped to is abou a 100 ohm
  5. Aug 14, 2008 #4
    but the photoresistor has too much resistance even wen exposed to sunlight. The minimum the resistance has dropped to is about a 100 ohm
    Last edited by a moderator: Aug 14, 2008
  6. Aug 14, 2008 #5


    User Avatar

    Staff: Mentor

    So what do you think would happen if you connected the photoresistor between the + supply and the base of an NPN transistor, where the NPN's emitter is grounded, and its collector is connected to the - side of the motor.....?
  7. Jan 7, 2009 #6
    Photoresistors are not a good source for Vin on a NPN, you need a trimpot to do a voltage divider, so you can control the brightness of the led on the Collector side
  8. Jan 7, 2009 #7
    will that work properly without a second resistor between base and ground?
  9. Jan 7, 2009 #8


    User Avatar

    Staff: Mentor

    Hi Proton,

    This whole thread is from last summer, and wasn't very well developed. A photoresistor driving the base would be fine, probably, but without more design details it's hard to say for sure.

    Your comment refers to the common design practice of providing an impedance from the base to ground, so that if there is no active drive (like from light hitting the photoresistor), the transistor is off.

    Again, in this application, with so little real design data, we can't really help much on the design. In general, though, a pulldown resistor on the base would be something to think about, WRT response time, and other design considerations.
  10. Jan 15, 2009 #9
    As I read the word switch I see a binary off/on operation. This circuit begs for a photo-resistor, resistor voltage divider feeding a comparator with a pot for supplying the reference voltage. The output of the comparator drives the transistor which turns on (or off) the motor.
  11. Feb 23, 2010 #10
    I have a similar need for a circuit (except mine is a 1.5v motor). Can someone post a diagram as described by skeptic2?

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook