This is a circuit that I designed for a Flood and Drain Aquaponics trial.

Hypothesis: If the sun is not shining the requirement to Flood and Drain diminishes and the requirement to have standby power during non-daylight hours is removed.

Prediction: Plant and fish will continue to do well if the Flood and Drain system pauses during hours of darkness.

Goal: To eliminate the requirement for a battery or mains power and minimise wear and tear on the water pump.

This circuit incorporates an adjustable timer (555) that cycles the pump on and off. Tests on this particular pump (12V bilge pump) determined that it can pump effectively at 8 volts. Increased source voltage results in more water flow but also suspect more wear and tear on the motor.

Therefore this circuit also clamps the Solar Panel to about 8.5V only when the pump is cycled on. This way the clamping transistor is only loaded when cycled on and only dissipates excess power not used by the pump.