Solar LED Light and Charging Circuit
Automatic On/Off without CDS sensor
Only uses one common transistor
Provides battery charging
Low part count.
I developed this circuit because I kept seeing solar LED lights for sale that use a CDS light sensor and just seemed to have more components than necessary. It just seemed like it did not need to be that complicated. This circuit does not do any voltage boost that would be necessary to drive a white led from a voltage source of less than 3 volts. If you use a 3.8 volt cell you can drive a white led with this circuit. If you use lower voltage cell(s) you can still drive many other color LEDs (i.e. green, red, orange...). NOTE: you must choose a solar cell(s) that is greater than your battery voltage + 1.4 volts.
The solar cell charges the battery through D2 and D1, the voltage on the base of the transistor during charging mode turns off the PNP transistor. When there is no light on the solar cell, the voltage at the D1, D2, and base goes low, when this happens current flows through the emitter/base of the PNP transistor and R1, thereby biasing the PNP transistor on. Now current will flow from the battery through R2 and turn on the LED. R2 is used to current limit the current thorough the LED.
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