Don’t throw away your old calculator. Its Solar panel can be used to make a Broken beam alarm or Automatic LED lamp. Let us see two circuits exploiting the photovoltaic property of the tiny calculator solar panel .
About Solar Cell
Solar cells are photoelectric transducers generating electric current by accepting light energy. The photocell has many p-n junctions connected in series. One of these junctions is very narrow to allow the passage of light energy. When sunlight or high intensity light such as LASER, incident on the p-n junction, electron-hole pairsare formed. These charge carriers generate current proportional to the incident light energy. The sunlight incident on the semiconductor junction of solar cell collides with thevalance electrons. This causes formation of charge carriers which crosses the p-n junction in an opposing manner and creates a voltage across the p-n junction. Approximately 0.6 volt will be generated from each solar cell.
Calculator solar panel generates 2.4 volts but current will be a few milli amperes
About LASER
LASER is the Light Amplification by Stimulated Emission of Radiation. LASER is amonochromatic light with very high energy and travel long distance. The laser beam is highly penetrating and when hit on the matter, the laser photons interact with the matter and generate heat. If the matter is body tissue, laser will burn the tissue. Low cost and low watt Laser pointers are available now which cost Rs.25 only.
Laser pointer
Circuit 1- Laser based Alarm
This circuit exploits the Photovoltaic property of Solar panel used in calculators. Usually the calculator solar panel has 4 cells and each generates 0.6 volts giving a total of2.4 volts if the incident light energy is strong. Here a LASER Pointer is used to generate photo current from the solar panel. When the LASER photons hit the solar panel, it outputs around 2 volts that forward bias T1 and LED lights. When T1 conducts, base ofT2 will be grounded and it remains non-conducting so that buzzer will be off. When the LASER beam breaks by a moving person, T1 turns off and T2 turns on and buzzer beeps.
LASER diode requires 3 volts and 30-50 mA current which can be obtained from the same power supply. Fix the solar panel and Laser pointer facing each other with exact alignment. It is advised to fix the laser pointer and alarm circuit 1 foot above the ground level so that accidental looking into the laser beam can be avoided. Range of the ordinary laser pointer is 5-10 meters.
Caution: LASER beam is dangerous and can harm eyes. Do not look into the laser beam. The pencil beam is highly penetrating and can burn. LASER pointer should not be given to children. Use only low power LASER pointers.
Circuit 2. Automatic White LED lamp
Working of the circuit is similar to circuit 1. But day light is used to inhibit the circuit instead of LASER.When the day light ceases, White LED turns on.
Filed Under: Electronic Projects