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The circuit

Circuit diagram

About once a second, N1 provides a clock pulse. The square wave and the capacitor (100N) on the second sensor pole create an alternating current between the poles, which helps avoid corrosion and the forming of deposits. Together with the associated capacitors, The adjustable resistor (50K) and the soil resistance delay the pulses as they pass to the clock and data inputs of the flip-flop (1/2 74HC74). The flip-flop reacts only when clock input c1 goes from low to high. At that moment it latches the level present at input D (high or low) and changes its outputs accordingly. The same rising edge causes N3 and N4 to blink one of the LEDs (on time 0.05 seconds). If the soil has less resistance than the adjustable resistor, then the soil is moist enough and the yellow LED blinks. Otherwise, the red one takes over.

The ratio between the delays caused by the soil and the adjustable resistor does not depend on the voltage provided by the solar panel. So the Thumb provides good information regardless of the amount of light it catches, though a certain minimum is of course necessary to make a LED blink. The avarage power consumption is about 100 uA at 1.9 V.

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