PV Gap

Ana Said:

thanks guys! The resistor turned on the light but I am trying to use a solar cell which doesn't ?? help?

We Answered:

Let me see if I understand your circuit. You've got an NPN transistor with the emitter grounded. The positive terminal of a solar cell is connected to the base of the transistor, and the negative terminal of the solar cell is connected to ground. The cathode terminal of an LED is connected to the collector of the transistor through a 200 ohm resistor. The anode terminal of the LED is connected to +9 volts DC. (The LED and the resistor may exchange positions so long as the LED polarity is not reversed.)

Around 40 mA should flow through the LED when the transistor is on, causing the LED to light. However, to turn on the transistor, the solar cell will have to source around 1 mA at 0.7 volts or more. There are different types of solar cells, but a silicon-based cell produces only around 0.5 volts, which is too low a voltage to turn on the transistor. However, two silicon solar cells in series will produce around 1 volt which is adequate and should solve your problem.

I have two comments about your circuit. First, you should have a resistor in series with the solar cell(s) to limit the current through the base of the transistor. Second, you should have a resistor from the base to ground to keep the transistor from turning on due to the collector leakage current, known as Icbo on the data sheet. The values for these two resistors depend on the transistor and solar cell parameters, which you have not specified.