Project 10: Solar Powered 'Alarm armed'-led
Charges during the day, scares burglars at night
Required parts: Solarcell, microcontroller (pC), 4K7 resistor (yellow, purple, red, gold), 100 ohm resistor, (brown, black, brown, gold), BAT85 diode, BC557 transistor, battery holder for two AAA batteries, two AAA 1,2V rechargeable batteries, wire jumpers, red led.
How it works: When the sun shines. the voltage generated by the solar cell will be higher than the voltage of the batteries, so a current will flow from the solar cell to the batteries. This current will charge the batteries. The BAT85 diode prevents discharging of the batteries trough the solar cell in low light conditions. The base of the transistor is tied to ground (-) by means of the 4K7 resistor. This causes the transistor to turn on and supply power to the ucontroller. The controller will behave identical to project 2, so the led will flash. However, note that the base of the transistor is also tied to the (+) of the solar cell, so as long as the sun shines, the base of the transistor is kept high enough to prevent turn-on of the transistor, so the led remains off at daytime.