The circuit consists of two stages. The first stage is a `switch` or cut-off device. It detects a voltage above 0.7v from the solar panel and the resistance between its collector-emitter terminals reduces to a very small value.
The 10k resistor allows the voltage on the solar panel to rise above 0.7v during bright sunlight, while the 100k discharges the 100p when the voltage is very low and the capacitor holds a charge to keep the transistor turned on when the voltage is `high`.
When the resistance between the collector-emitter terminals of the first transistor is low, it is turned ON, and the second transistor does not get enough voltage on its base for it to operate as an oscillator.
The SOLAR CELL actually consists of a number of cells as each cell only generates about 0.5v to 0.6v.
The Solar Cell in our model consists of 4 cells and produces approx 2v with bright sunlight.
The short-circuit current produced is about 30mA and although this is not the correct way to determine the current capability of the cell, it has been given to help you select a suitable cell (or set of cells). The current will drop considerably when the solar cell is connected to a 1.2v battery via a diode and our project delivered 8mA. The purpose of the first transistor is to keep the second stage OFF when the solar cell detects sunlight.
This allows the energy from the solar cell to be passed to the rechargeable battery.
The second transistor is an oscillator.
To see how it works we remove the first transistor.
The second transistor works by itself. The components in the circuit are: the transistor, the two chokes, the 6k8 resistor, the 1n2 capacitor and the LED.
An oscillator needs feedback called POSITIVE FEEDBACK. Positive feedback creates noise or "oscillation" in a correctly designed circuit.
An oscillator must be self-starting and the circuit in this project starts to oscillate by detecting a voltage on the base.
The transistor amplifies this and a larger waveform (larger voltage- change) appears on the collector. This is then passed to the base so that oscillation continues.
For oscillation to occur, the...