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Tristan (Tristan)
Intermediate Member
Username: Tristan

Post Number: 28
Registered: 4-2005
Posted on Tuesday, November 15, 2005 - 12:51 pm:   Edit PostDelete PostView Post/Check IPPrint Post   Move Post (Moderator/Admin Only)Ban Poster IP (Moderator/Admin only)

One of my friends had a project where he needed to use a LDR to make a model drawbridge go down when a toy car needed to pass and went over the LDR.
But there is a weird problem that he had that I still can't figure out. We both thought it would be easy enough to just connect the battery to the LDR to the Motor and back to the battery again. And add in a small curcuit to make it run for a short amount of time. But for some reason. (We have tried a ton of different ways to do it but it hasn't worked.) When you do that it won't run the motor or anything. And we have tried with all sorts of other things in place of the motor and it won't work with anything but a LED.
We have tried using diodes before the Motor and after. We have tried using a LED bfore the motor. And the LED lights up but the motor still doesn't move!.
Anyone know anything about this phenomenon?. I would like to know what caused it. Even the teacher of the class can't figure it out. And my Multimeter is busted so I can't run many tests.

Thanks

P.S.
I have tried using different parts. And I have tested them otherwise so it isn't anything defective.
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Simon Quellen Field (Sfield)
Senior Member
Username: Sfield

Post Number: 782
Registered: 12-2004
Posted on Tuesday, November 15, 2005 - 1:14 pm:   Edit PostDelete PostView Post/Check IPPrint Post   Move Post (Moderator/Admin Only)Ban Poster IP (Moderator/Admin only)

Cadmium sulfide photoresistors have as much as 500,000 ohms
in the dark, and 15,000 ohms in the light, and some go as low as
500 ohms in very bright light.

Small electric motors need at least 50 milliamps to start turning,
some need 100 to 200 milliamps to turn. The current needed will go
up when there is a load such as a drawbridge.

So what does this mean to your design? For the answer, we turn to Ohm's Law.

The current through the circuit will be the voltage divided by the resistance.
In the best scenario (good motor, excellent LDR, no load, 12 volt battery)
we have:

current = 12 volts / 500 ohms, or 24 milliamps.
That is unlikely to be able to turn the motor.

In the more likely scenario, you have 3 volts / 15,000 ohms,
or 2 milliamps which will just barely light a high brightness LED.

What you will want to do is to have the LDR control the base current of a
transistor, and have the transistor turn the motor on and off.
Do a Google search for "transistor switch".
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Tristan (Tristan)
Intermediate Member
Username: Tristan

Post Number: 29
Registered: 4-2005
Posted on Tuesday, November 15, 2005 - 3:17 pm:   Edit PostDelete PostView Post/Check IPPrint Post   Move Post (Moderator/Admin Only)Ban Poster IP (Moderator/Admin only)

That makes sense. I will do some more tests to make sure.
Thanks!

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