Solar Powered Pond

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Project Description

We had a small pond powered by an AC pump. I replaced this pump with a DC pump and it is now powered with solar cells.


Simplest way to set this up would be to connect the panel directly to the pump. I did not do this for couple of reasons. First, when a cloud passes by or simply somebody’s shadow falls on the panel, this would stop the pump. But more importantly, I was afraid that unless full power is provided to the pump when turned on, the lifetime of the pump would be reduced.

Instead, what I came up with is that the panel charges up a small 12 Volt battery until it is charged up and then the pump is turned on. But the pump cannot be continuously connected to the battery because it would drain the battery to such a low level that the battery life would be destroyed.

Another feature that would be useful is that even if there is not high enough charge in the battery for long term operation, a button is provided to request the pump to turn on for a duration of time, adjustable up to 30 minutes.

So the way the system operates is that the power from the panel is used to charge the battery until the battery voltage hits the upper limit, and after that the excess power is dumped to the pump. Once the pump starts operating, it continues to operate until the battery voltage goes down to the lower limit. Both the upper limit and the lower limit is adjustable.

Circuit Diagram

Pond Controller Schematic

Parts: U1 = 555, U2 = 555, U3 = 7805, D1 = 1N4001, Q1/Q2/Q3 = NPN transistor, SPDT 12V relay, R1/R2/R3/R4 = 1K, R5 = 550K pot, C1 = 1000uF, R6/R8 = 1K pot, R7 = 2.2K, R9 = 2.2K, S1 = momentary contact switch.

The first 555 timer is used to monitor the battery voltage. Pin 2 (TR) input controls the lower limit. To adjust the lower limit, wait until the battery voltage hits the desired lower limit, turn R8 until the relay contacts open. At that point the TR voltage should be 1.67 Volts (5 / 3). To adjust the upper limit, wait until the battery voltage hits the upper limit, then turn R6 until the relay contacts close thus activating the pump. At that point the THR voltage should be 3.33 Volts (10 / 3 ).

One potential problem here is that when the pump is started, due to the load from the pump, the battery voltage drops. If the lower voltage limit is not low enough, that would immediately stop the pump. The system then could go into oscillation turning on and off continuously.

The second 555 timer is used to request pump to operate on demand. Turn R5 to adjust the ON time for the pump.

Q2 is used to invert the output of the first 555.

The solar panel is connected to one side of diode D1 and the negative side of the rechargeable battery B1. The pond pump is connected to the battery B1 through relay contacts. S1 is a momentary closing switch.

It was hard to find a submersible 12 Volt DC pump. I ordered it from ebay for $32.99 shipping included. CP50-1240 12V DC Submersible Fountain Pump from Golden Gadgets.

Project Completion

Throughout the day I notice that the pump comes on and goes off automatically based on available power. The interesting part of this is that even when the weather is cloudy the battery is being charged and when the voltage hits the upper limit, the pump will come on and stay on for a while.

Requesting on demand works in the evening when we have dinner in the back yard after the sun goes down.

The circuit board needs to be put into a plastic box to protect it from rain.


Panel 1

Panel 2

The short video segment below shows the 12 VDC pump working the pond, if your browser supports this format.

Search this site for tags 555 and solar for other relevant projects.

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