Solar Sell Engine with a UJT and SCR, This circuit is an example of solving a simple problem with complex and expensive technology. The two "special" devices, the UJT and SCR are difficult and expensive to obtain however it is interesting to see how different designers tackle a problem. Once you know how the circuit is required to work, from the previous Solar Engine, you can see how the two devices will operate, in a very broad sense.
The actual operation is far more complex.
Let me explain:
As the voltage on the 4700u electrolytic rises, the timing circuit made up of the 22u and 100k allows the 22u to charge. The UJT does not work below about 2v and at about this level the 2N 2646 begins to "oscillate."
We say it oscillates but it is actually a relaxation oscillator that produces a spike at the output. The output is the 2k2 resistor.
When the 22u charges to about 66% of rail voltage, the emitter detects the voltage and the transistor "turns on." The resistance between the Emitter and B1 becomes very low and is effectively equivalent to a diode in forward-bias and the energy in the electrolytic produces a spike across the 2k2.
When the voltage on the electrolytic drops to about 20% of rail voltage, the UJT turns off.
This voltage may not be sufficient to turn on the SCR and so the cycle repeats.
The main electrolytic keeps charging and the spike across the 2k2 keeps increasing. Eventually it is sufficient to turn on the SCR.
Once the SCR has "fired," the LOAD will remain active and the supply voltage must drop to near zero to de-energise the LOAD.
In other words the motor stops and the cycle repeats.
