Society of Robots - Robot Forum
Electronics => Electronics => Topic started by: Jdog on January 30, 2010, 12:32:36 PM
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For a part of a school event I have to activate a motor with a photoresitor. I have a photoresistor and even when light is being directly shined on it it has a resistance of 650 ohms. This is way too much resistance to power my 3 volt motor. I am not allowed to use transistors nor voltages above 10 volts. I originally tried just wiring the resistor in series with the motor and a ten volt battery but that didn't work because the motor was only getting around 14 miliamps when light was shining on the resistor, which is not enough to power it. I am allowed to use mechanical relays, but are there any out there that don't include transistors and can operate at a little less than 14 miliamps? Can anyone think of a way to do this?
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My only thought is using a motor that will run with a very low current. Solarbotics has this one:
http://www.solarbotics.com/products/tpm2/ (http://www.solarbotics.com/products/tpm2/)
The motors used in solar power bots might work.
http://www.solarbotics.com/products/k_pp/ (http://www.solarbotics.com/products/k_pp/)
Could you use a different photoresistor? I have ones that go below 200 Ohms when a flashlight is shined on them.
A relay the will pull in at low current may work with the photoresistor in series with the coil.
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I can use a different photoresistor, I'll try finding a better one.
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Hi,
I am allowed to use mechanical relays, but are there any out there that don't include transistors and can operate at a little less than 14 miliamps?
Yes, most small relays should work.
This would do (http://www.maplin.co.uk/module.aspx?TabID=1&criteria=usb&ModuleNo=2613&C=SO&U=Strat15).
You have to go with say 6V and remember to put protection diode over the relay (or the photo resistor might die over time) and you need eg. a zener diode to get the motor voltage down to the 3V it expects.
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You have to go with say 6V and remember to put protection diode over the relay (or the photo resistor might die over time) and you need eg. a zener diode to get the motor voltage down to the 3V it expects.
Unfortunately it says "Electric components shall be limited to batteries, wires, motors, switches, resistors, capacitors, commercial photocells (i.e., cadmium sulfide (CdS) cell, photoresistors or light dependent resistors), mechanical relays, lightbulbs, and LEDs. No computers or transistors will be permitted in the device."
Diodes are not allowed, but then again it really only needs to work a few times, and could I just use a resistor to get the motor voltage down? Or instead just use the 6 volts to power the coil in the relay and have the part that switches run at 3 volts?
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Hi,
Yes, you can use a resistor to drop the voltage to the motor, if you're just demonstrating the idea.