Micro-Power Solar Cell Oscillator
This is a Solar Cell powered oscillator which uses the least number of parts compared to any other oscillator, five parts counting the Earphone.
The oscillator configuration is push/pull. Output frequency is inversely proportional to light brightness. That is, high brightness yields low tones and low brightness yields high tones. Power output from the Solar Cell is low enough that no protection or current limiting components are required.
This is why there is a minimum of components. You can use a code Key if you like by inserting it in series with either Solar Cell lead. The keyed tone isn't the best, but what do you expect for a circuit designed for minimum component parts and low supply power! This circuit will oscillate with only about 0.05 volts applied.
The current at this voltage is only about 0.000005 amps (5 microamps). This not only shows how well the circuit works on low power, but also how well the Earphone works on low power.
If you want to measure the circuit current you can insert the Meter in series with the Solar Cell like you did the Key. If you bypass the Meter with the 3.3uF Capacitor (properly polarized) the circuit will retain the good low light characteristics which it had before you added the Meter. The full scale rating is about 250uA (microamps), so the top (blue) scale represents 25uA for each whole numbered calibration.
CAUTION: To remove possibly damaging stored current, temporarily short circuit across the 3.3uF Capacitor before placing it across the Meter. One of the biggest advantages to a circuit like this is: NO BATTERIES ARE REQUIRED!!