Plant watering timer

If you are like me, you hate doing the same chores every day, all your life. As much as you can automate such things, you should automate them. One of these chores is watering every plant around my apartment, specially those on the balcony! Long ago I made a small gizmo that takes care of this chore. It also keeps the plants alive when I travel away for one or two months.

I simply installed a water hose with a solenoid valve (I used a standard replacement valve for washing machines), and poked thin copper tubes into the hose, leading to each pot. Those plants that require less water get thinner copper tubes. And then I used the following timer circuit to control the valve. Problem solved. The plants are happy.

The full resolution schematic is here.
This circuit starts with a 555 timer connected as astable multivibrator, running at one cycle every several seconds. A CMOS ripple counter divides this time down to several hours. When the time is over, the counter triggers another 555 timer, which is a monostable multivibrator and can be set for times of a few seconds to about two minutes, controlling the solenoid valve via an optotriac (as shown) or a relay (like implemented later). A small regulated power supply feeds the circuit. Both the interval between activations, and the duration of the watering, are user-adjustable over a wide range. One LED blinks to show the heartbeat of the first timer, the other LED lights when the valve is activated.

After years of service, the valve I was using clogged with silt. Being sealed shut, it was not repairable, so I replaced it by a different one. This second valve was too inductive, and the optotriac proved incapable of properly controlling it! Instead of searching for a different valve, I opted for replacing the optotriac by a small relay. That's the version that is still running today at my home.

I also added a pushbutton from pin 2 of the second 555 to ground, for manually triggering the monostable for testing of the copper tube outlets.

Of course, watering plants is not the only application for a timer like this. Use it for whatever you fancy!

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