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Thumb gets Finger

Thumb and finger When our guinea pig lemon geranium - now 110 cm high and more than 70 cm wide - got its Thumb back after a day of tinkering, it proceeded to autonomously drink half that bottle!
The Green Thumb's experimental finger is a little pump. Two transistors and a reed relay allow the Thumb to switch the pump on and off even when it's almost dark. The motor was borrowed from a broken electric toothbrush, and works well on two NiMH penlites. These are continuously recharged by a 4 x 8 cm solar panel.

That lemon geranium is growing much faster than his pre-Thumb predecessors. The leaves are larger too. Apparently living on the edge of water stress is good for this plant. I know it's on the edge, because if I wait for two hours after the first red LED flash before fetching the watering can, a number of leaves have gone limp. The plant wastes little time pumping them up again when it gets water. But obviously it's better to be prompt, and having to watch the red LED like a hawk is a bit of a burden. Since the Green Thumb is perfectly capable of switching a pump...

Closer look at finger

The pump (green cylinder with white top) is a type used to tank model aircraft. The original motor was intended for 12V, making solar power difficult to arrange. An old toothbrush motor is much better for our purpose, as it needs only 2.4V.
The pump draws 1.6A when active. The solar panel (above the circuit board) provides about 20mA in good light, so two hours of charge may compensate a minute of pumping. The plant demands something like one minute of pumping a day, more on hot days - but then the NiMHs (in battery holder, below the circuit board) also get more charge.
The maximum current supplied by the solar panel is less than 50mA. As the capacity of the NiMHs is 1000mAh, there is no need to limit the charge current. But the panel will keep up with discharge only in summer. In darker times, an occasional recharge by other means will be necessary.

A Green Thumb briefly flashes its LEDs every second or so, but the state of the soil is continuously remembered by the flip-flop (1/2 74HC74). So all the output needs is a simple electronic switch. The two transistors drive a hand-wound reed relay (needing only 30mA at less than 2V to activate) when the output is high; in turn, the relay switches the pump. Since the pump starts immediately when the flip-flop changes state to red, it runs at least 1 second, until the Thumb sends its next measuring pulse is sent through the soil. That avoids excessive switching.
The 74HC logic operates correctly at about 1V and 1uA of supply current. That means the combination of Thumb and pump works even in near darkness. There may not be enough solar power to flash a LED, but the transistors still react and switch power from the NiMHs to run the pump.

Circuit diagram
Connection to Thumb Thumb and pump switch share ground (- connection of their solar panels) and a signal wire from the Thumb's inverting flip-flop output to the BC549C base resistor on the pump switch.

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