Electronics > Electronics

Photo resistor switch

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vipulan12:
i want to turn on a motor using a photo resistor for a rube Goldberg project, can anyone give me some advice
i have the circuit i planned up but i am having a hard time to get it working(attached)
it could use any battery but for now i am using 4 AA batteries in series so 6 volts

the concept behind the circuit is to allow light to lower the resistance of the photo resistor using a LED, with the LED on the resistance becomes approx. 475 ohms, this is too high(Resistance) for my toy motor, so that is why I connected the battery parallel to the motor with a resistor to allow more current to flow but(here's the catch) i need to supply enough current so that when the LED is on, both the current from the photo resistor and the power from the normal resistor can turn the motor on

but keep in mind that when the LED is off the motor should not turn on since the current from the normal resistor shouldn't be enough to turn the motor on independently

if what i am saying isn't making sense reply and if you think this isn't going to work please tell me
i have been experimenting for a while and  i cant get this to work

thanks any comments  is appreciated

 

johnwarfin:
the led idea is sound but a transistor is needed to increase drive of the ldr enough to power a motor. putting another led in place of the motor would be a good test for now. but the ldr will not pass anywhere near enough current to run most motors at reasonable speed w/o amplification.

base to ldr, collector to +, emitter to motor.

that resistor wont do much except waste power and generate heat.

waltr:
I don't see either the battery or an LED in the circuit you posted.

How much current does the motor draw?
Having a largish series resistance with the motor may or may not work depending on the motor's current draw (remember Ohms Law, the Voltage drop = I *R).

You probably need a transistor to turn on the motor driven by the photo-cell.

vipulan12:
i do have a couple of NPN transistors but i cant seem to find a datasheet ( 2N2222A338)
the battery is connected to the line where both the lines meet before the resistors
and i am not sure of the current draw but it needs approzimately 0.3 amps to run

johnwarfin:
2n2222 is good for almost an amp. most small motors draw less unless stalled.should be ok for testing.  fet is better because of lower on resistance.

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