Nuts and Volts - August 2010

Q&A

■ WITH RUSSELL KINCAID

In this column, I answer questions about all aspects of electronics, including computer hardware, software, circuits, electronic theory, troubleshooting, and anything else of interest to the hobbyist. Feel free to participate with your questions, comments, or suggestions.Send all questions and comments to:

Q&A@nutsvolts.com

WHAT’S UP: Join us as we delve into the basics of electronics as applied to every day problems, like: Irrigation Timer ✓

● Video Switch Mailbag

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IRRIGATION TIMER

QI have a suggestion for a device that you might be interested in creating and including in your magazine. My wife has an extensive drip irrigation system that includes a line with spray nozzles for some hanging plants that she’d like to activate a couple of times or so a day for only brief intervals of less than a minute when the weather is hot.

The problem is that the standard irrigation timers don’t allow for activation periods of less than a minute and that would apply too much water if repeated. It would be easy to design a 555 timer that would get its power when the 24V AC solenoid power is turned on by the program. The timer would activate a relay that would pass that power on to the solenoid.

AOne minute is a long time if you are holding your breath and it is a long time for a 555, although it can be done. A more reliable design can be made using a countdown timer like the CD4541 (see Figure 1). The oscillator is built in, the countdown is programmable, and you can choose whether the output is high or low. In this case, I chose the output to be high (pin 9 high), the countdown to be 256 (A and B programming), and therefore set the clock period to be 1/256 minute = .234 sec.