I'm trying to build a motor that is activated when light shines on a photoresistor/transistor (can anyone explain the difference?), but I'm totally lost right now. I can only utilize wires, batteries, photosensors (e.g., photovoltaic cells, photodiodes, photoresistors, phototransistors), light bulbs, LEDs, homemade solenoids, switches, and up to three motors. No computers, integrated circuits or other electric components are allowed. Can anyone provide a starting point? All the designs online either use an IC or they use a transistor.


closed as unclear what you're asking by Olin Lathrop, Daniel Grillo, Matt Young, PeterJ, Gustavo Litovsky Dec 2 '13 at 20:14

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    \$\begingroup\$ "Are allowed" implies that this is an educational assignment of some sort. How did you not learn about the pieces you have yet? \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 2 '13 at 7:38
  • \$\begingroup\$ Presumably the reason you got the above homework was to learn about those components. You could technically do it with only two of those parts (not counting wires). \$\endgroup\$ – PeterJ Dec 2 '13 at 7:45
  • \$\begingroup\$ The reason that most online circuits use an IC or transistor is that motors tend to be of at least moderate power consumption and so current draw, whereas light sensors are usually rated for low current use at levels not usually useful for motors. With a low enough current / power motor you can use what you have in a simple circuit. If you want more power you need to get creative, eg the "homemade solenoid" could operate a switch to drive the motor. BUT if you give us a fuller description of why you want to do this and why the restrictions it would help. As an assignment it's a bit 'suspect'. \$\endgroup\$ – Russell McMahon Dec 2 '13 at 8:11
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    \$\begingroup\$ You'll find a few ideas here and also here \$\endgroup\$ – Russell McMahon Dec 2 '13 at 8:13
  • \$\begingroup\$ Can you use a bimetal contact strip, a magnifying glass and a relay? What about external power? Without external power you are probably limited to using photovoltaic cells and prayers. \$\endgroup\$ – Andy aka Dec 2 '13 at 11:25

In answer to your question about photo resistor and photo transistor.

A photo resistor is a light sensitive resistor that changes its resistance depending on the light level falling on it. The material commonly used is Cadmium Sulphide (CdS). In the dark it has a high resistance (>100k) and in the light it has a low resistance (< 1k). It is normally used in a potential divider circuit (top or bottom position) with a fixed or variable resistor depending on what you want to detect (light or dark).

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The photo transistor is a transistor that uses light to supply the base current in order to turn it on. This small photo electric current is then amplified by the transistor allowing a much larger current to flow through the collector emitter junctions. For more amplification (sensitivity) there are Darlington versions of the photo transistor (two transistors in one package).

enter image description here

As to the starting point in your project.

First build a circuit to detect a change in the light level using an appropriate sensor/circuit.

N.B. This circuit will not control enough CURRENT to switch the motor on and off. You will need to interface with another electronic device that requires a small input current that controls a larger current.

Finally you will need to protect this device from the back emf when the motor is switched OFF. This is done by connecting a DIODE across the motor in the opposite direction to the normal (forward) flow of current.

enter image description here

Your final circuit should look something like this - I leave it to you to do a bit of research to get it into a working circuit.

enter image description here


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