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Assembly and adjustment

The Green Thumb kit contains a drilled, tinned and partially goldplated PCB, which makes assembly very easy. But it is quite possible to build the circuit on for instance Protoboard. In that case, the sensor poles can be made of reasonably thick stainless steel wire.
Do clean the PCB after you've done your soldering, to avoid current leakage.
PCB layout

Given a suitably prepared PCB, building a Green Thumb is mostly a matter of soldering the components in place, while paying careful attention to the PCB layout and the photographs. Use a temperature-controlled soldering iron, or if that is not available, a small one of 16 watts or less. The solder should be resin-cored and have a diameter of about 1 mm.
Make good joints by first pushing the (fine) tip of the soldering iron against both the PCB and the connecting wire or pin. Then allow sufficient but not too much solder to flow. Keep the iron in contact for a total time of less than two seconds.
Make sure that the electrolytic capacitor (100uF) is connected as shown; its minus side (the `cup' of its circuit symbol) is marked with a black stripe. The connecting wires of resistors and caps are cut after soldering. Save three cuttings to serve as wire bridges. Straighten the pins of the adjustable resistor before pushing them in their holes. If necessary, ream out the holes a bit. Center the setscrew.

When all components (except the 12K resistor) and the three wire bridges are soldered in place, it's time to make the connecting wires for the solar panel. Both should be about 3 centimeters long. Strip about half a centimeter of insulation off the ends and tin the core. Also allow a little solder to flow on the contact patches of the solar panel. Then make the connections from the plus patch of the solar panel to the plus side of the electrolytic capacitor (100uF), and from minus to minus.
When the solar panel is connected and catches sufficient light, the red LED will blink. Temporarily solder the 12K resistor between the sensor poles (on the PCB, not on the gold plated poles themselves). Turn the setscrew of the adjustable resistor until de yellow LED blinks, then back until the red one just takes over.

Rubber support Remove the 12K resistor. Cut the self-adhesive rubber support to make it fit between the feet of the 74HC132. Stick on a bit of double-sided tape, and push the solar panel in position.

Push the sensor spike into the soil, close to the stem of a plant, without damaging important roots. Make sure that the solar panel faces the nearest window. If the red LED keeps blinking, your plant is probably thirsty!

Make sure that the electronics stay dry when you water the plant, and take the Green Thumb out of the pot before using a water spray on the leaves.

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