Reverse Polarity with photoresistors

Other Hardware Sensors
1

So I had this brilliant idea...

Say I have a small DC motor. One positive and one negative terminal. Lets say this DC motor can move a toy car forwards and backwards, based on whether I flip-flop negative and positive or not.

Is there anyway to wire positive and negative to photoresistors in such a way that the toy will "chase" light? So if I have a couple photoresistors on the the front of the toy and shine a my light at it...it goes forward. Likewise, if I shine a light on the back, it goes backwards.

Purpose: A photovoric self-feeding little car. That moves towards the light.

I tried to wire up a circuit on my breadboard with some photoresistors and diodes. My thought was that electricity would follow the path of least resistance...aka, the brightest path in this case. But after about ten arrangements of diodes and resistors and wires....I couldn't encourage the DC motor to move AT ALL. My power source is sufficient 10v for a 4.5 volt motor. I CAN get the motor to move on a simple circuit with the resistors all working in parallel. So I know my components are working.

The way I have it wired has my positive battery terminal splitting in two ---< Both branches have TWO photoresistors. Those then lead into one of the two terminals (with a diode) on the motor. So I have each terminal of the motor being fed positive, essentially. I also connected each side to ground, through another diode and photo resistor.

I don't understand why it doesn't work!!! When I place two LED's in the circuit (reversed from each other) it works exactly right! One set of photoresistors will power the green LED, and the other powers the red. With no light, nothing moves. With light only on one side, only one LED turns on.

I'm baffled. The only thing I can think of is that I need less resistance and/or more power. Any ideas?

2

Your problem is, as you suspect, lack of power. LDRs have a fairly high resistance even in bright light, and while LEDs require very little current, motors require quite a lot of current and if the LDR's resistance did drop enough for the motor to drive, it would probably just burn out the LDR.

You need to use a transistor to amplify the current enough to drive the motor.

There is a design for a simple cross-over light sensor / motor light following robot using photo transistors, regular transistors to amplify the current and toothbrush motors in my book 15 Dangerously Mad Projects for the Evil Genius http://www.dangerouslymad.com

3

Many years ago I got an electronic kit for Christmas. In it was a photo resistor, I found that it changed resistance enough to trip a relay if it was just wired in series with the coil. I then arranged the normally closed relay contacts to light a flash light bulb when the relay was not energised. So normally the light was on but when you bring the photo resistor close to it the relay turns on which turns the light off which which turns the relay off which turns the light on ..... and so on. It was in fact what I now know to be a relaxation oscillator.
So you can trip a relay to control your motor even if you can't control the motor directly.