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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

water distributor

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This circuit was made up to water a vegetable patch and a couple of flower beds on a suburban block. It is powered from a 12VAC 30VA garden transformer plugged into a 240VAC digital timer set to turn on for 15 minutes every 24 hours.

A bridge rectifier (BR1) and 2200µF capacitor provides 17VDC power to the circuit. This is regulated to +5V by a 7805 regulator (REG1) to supply a PIC16F628A microcontroller.

The PIC outputs are wired into a Darlington transistor array to boost current drive to switch on solenoid valves. The 19mm solenoid valves (24VAC 8VA) are commonly available from hardware stores such as Bunnings (eg, Irritrol 1012316 or HR Products MV75B) and are powered from the 17V DC.

Note that AC solenoid valves also work well on DC.

When the digital timer turns on, power is applied to the circuit from the transformer and a power-up reset is applied to the PIC microcontroller via pin 4. The software routine (see below) causes the RB0 to RB2 outputs on the microcontroller to go high in sequence, then repeat in a continuous loop. Each output is on for five minutes but this could be easily changed to supply more or less water to individual beds.

The circuit can be expanded to turn on up to seven valves, limited by the number of outputs on the Darlington array. The digital timer can be set for 30 minutes if double the watering time is required.

David Bathgate,
Oatley, NSW. ($45)

Note: the software is listed below and can also be downloaded from the SILICON CHIP website.

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Auto-sensing master/slave power control

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