# Help with choosing transistor

Discussion in 'General Electronics Chat' started by abradatanu, May 28, 2014.

May 10, 2014
10
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I have almost zero electronics knowledge and I need some sort of circuit which does the following: when it detects light from a lamp it powers three 120mm fans at 12V which draw 0.3, 0.3 and 0.22 A each and when it no longer detects light it stops all three fans and starts a simple 12V motor.

My solution was to take two of those cheap garden solar lights and put one of their solar cells facing the lamp which in turn would have it's LED off and another solar light panel would go right on top of the first light's panel. This second garden light would now have it's LED on as it thinks it's night time. When the lamp is switched off, the first garden light no longer detects light so it turns it's LED on and so the second garden light will now turn its LED off.

I measured the voltage across the LEDs and it's about 1.5V. My plan is to hook the outputs of those 2 LEDs to two NPN or PNP transistors which could then switch on and off 12V required for the fans and motor.

I know this seems really stupid but as I said I have no experience with electronics and for my purposes this would suit just fine.

Now to my question, I know you can use transistors to switch on and off a higher voltage by using a low voltage but I just don't know how to do it and exactly what how many parts I need.

If there is a cheaper and easier way of doing this I would really appreciate someone explaining to me, in dumb terms, how to do it

2. ### inwo Well-Known Member

Nov 7, 2013
2,441
315
Here's a simple circuit from the web................
Use the N.O. and N.C. contacts for motor and lights.
LDR is any common photo resistor.
A similar circuit could use a pair of photo cells from your solar lights.

Last edited: May 28, 2014

May 10, 2014
10
0
Thanks for the diagram inwo

I found the website with this circuit where it sort of gives step by step instructions on how to do it.

Indeed this is a LOT easier and simpler to build than my initial "design".

I will buy the parts needed and hopefully this should go without any problems as it's such a simple circuit.

Thanks again