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A small fan, driven by a hobby motor. Powered by 2x AA rechargeable batteries. The batteries are hooked up to a solar panel, which recharges them during the day.
The circuit is switched by a CdS cell (light switch) which means the fan switches on when it is dark enough, at night, hopefully.
Only after I'd built the (incredibly ugly - sorry, no photos available) prototype did I realise I could've bought a pesonal fan for $5 and hacked it instead.
Here's the schematic. Feel free to use it or improve upon it to your own satisfaction.

Step 1: Parts, Clarification, That Is.

The transistor is an ordinary NPN 100mA 45V.
The CdS I hacked from a solar garden light.
The solar panel came from the same place.
It's output is (rough estimate) 4.5V, 60mA.
I used an old hobby motor with a range of 2-6 volts. No special reason, it was handy. And the propellor came from a model plane.
That leaves the diode, 1N4004, and the two resistors, 1k and 10k.


Step 2: Other Uses

This isn't really a step, more a note about other things I've done with this - putting a white LED driver circuit in place of the motor.
A variation of the idea of the solar garden light I got the parts from originally.
What's the purpose of the 1K resistor? It will work just fine without it. Change the 10K fixed resistor to a potentiometer to be able to adjust the point at which the transistor turns on. Swap the positions of the 10K and photo-resistor to create a "light on" switch. Also, the diode doesn't have to be a 1N4004. Any 1N400x series diode will suffice.
<p>You need to be careful with the potentiometer: if the user sets the resistance to 0 , then there will be a short through the base-emitter junction, which will probably destroy the transistor. So you can adjust the turn-on point with a pot, but you should include a fixed resistor also, e.g., the 1K as shown. </p>
Or if you give it a thermistor, you can have a temperature controlled fan.
Hi there,<br>I have a question here what is the meaning of LDR.Thanks u.<br>
Could you swap the light sensor with a switch?
I don't really understand are these just the Blueprints for the fan..?
whats the input into the fan; voltage?
To the motor? Two NiCd cells @ 1.2V each, 2.4V.
I used a similar circuit in my rechargeable battery powered laser for an alarm system.<br/><br/>you can check my Instructable out to see how you can even further enhance this circuit at:<br/><br/><a href="https://www.instructables.com/id/Laser_Beam_Alarm_System_with_Rechargeable_Battery_/">https://www.instructables.com/id/Laser_Beam_Alarm_System_with_Rechargeable_Battery_/</a><br/>
I guess no one will do this thing on a pcb, but just in case, here's a pic generated from croc tech 3d I edited it a little to make it more clear...

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