Transistor help.... very basic, LDR/LED

I'm trying to make a very simple circuit to light an LED based on light from an LDR. I have all the components, it's a white LED, I'm using two AA batteries, and the LED glows brightly when I attach it to the batteries, but when I attach it to the LDR, it is either dim or dimmer due to the resistance... I'm sure I need to use a resistor for this... I have some assorted PNPs and NPN2222As... I've looked at a couple tutorials, but none of them seem to work... does anyone know how to do this without any (or as few as possible) other components? Thanks

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royalestel9 years ago
Not sure I understanfd the problem. Is the problem that even when you shine light on the LDR, the LED does not light up enough to satisfy you?
perkinsb1024 (author)  royalestel9 years ago
Without a transistor, yes. With a transistor, I can't even get that far. I can't get any sort of variation out of the LED's brightness with a transistor. I don't know what to plug into what pins.
for this you probably want a bipolar transistor. Take my advice with a grain of salt because I'm still struggling to get transistors working, but you hook up the photoresistor to the base of the npn transistor, the anode of the LED to the emitter, the cathode of the LED to the negative of the powersupply. The Collector to the positive of the powersupply. The photo resistor to the plus of the power supply.

checkout this website, explain a lot of semi conductor stuff really good
http://www.allaboutcircuits.com/vol_3/chpt_4/1.html
Goodhart9 years ago
A photoresistor is a decently high resistance semiconductor that activates when light falls on the device is of a high enough frequency. This lowers the resistance of the device. They can be either extrinsic or intrinsic. Those that are intrinsic is not an efficient semiconductor. They need higher amounts of energy to activate the device. Extrinsic devices require much lower energy photons (lower frequencies) to trigger them.
Those that are intrinsic is not an efficient

Should read: Those that are intrinsic are not efficient