Bad timing

Trober Kelib -December 11, 2014

We went to Hawaii six years ago and to preserve the integrity of our front and back yard’s  flora, my wife purchased a water timer for some sprinklers. Recently, we wanted to set up the sprinklers for the same reason and the water timer did not work.

It had a new battery, still only three blinking LEDs, and no instruction manual available, not even on the manufacturer’s website. In fact the timer model was not even listed on their site. That was the first problem. I copied the manual from a similar but different model timer from the website and proceeded to program the recalcitrant timer without success and with lots of self doubt.

That manual did not explain what the three blinking LEDs meant, but did show that during programming only one LED at a time was supposed to be on. There was only one button to enter a selection and of course I suspected it was the culprit, but I could turn a red LED into green with that button so I concluded the button was OK.

By now this issue had become personal. The top lid would have to come off for a look inside. Maybe the water valve was stuck after six years with no use. There were no screws in the top lid so I suspected a glued lid. With a cutting saw blade I cut around the edge of the lid and found a potted electronic timer with two wires going to a small DC motor with a gear. Below the motor and gear was the water valve. Turning the gear showed no hang up, it all rotated smoothly.

I then clamped the top lid back on and tried to program the timer again and this time it worked. When the battery was removed and re-inserted the gear rotated all the way left and then right. I could select ON and OFF which rotated the gear correspondingly, presumably opening and closing the water valve underneath. And I could program the timer.

I decided to attach the timer to the outdoor water faucet and found that the screw threads on the timer and the faucet were different. After forcibly attaching the timer to the water faucet I received a cold shower when the timer showed that it was in the OFF position and I opened the faucet. Selecting ON or OFF on the dial on the lid made no difference: the water kept pouring out of the timer.

I was now curious why this timer started to work but not the valve. I cut the wires to the DC motor, inserted the battery, and the three blinking LEDs indicated that it was non-funtional. After connecting the motor, it was functional. This is brilliant, I thought. Inserting a battery triggers a self check that drives current into the DC motor in both directions and monitors if that motor actually turns until it reaches the end and stops.  But what good is that for the user?  If the self check fails the user cannot do anything to get it going anyway. And why did the self check fail before I turned the gear?

I believe the answer is that many low-voltage DC motors develop contact resistance between the brushes and the commutator on the rotor after a lengthy period with no use. By turning the gear, the brushes made contact and further rotation cleaned the commutator even more. This leaves me with a quandary. When designing a product with an inaccessible low-voltage DC motor, how can the designer guarantee that the motor will start after a lengthy period without use?

As to the water valve failing, it turned out to be a pilot operated spring-loaded diaphragm valve and the rotation of the gear on the motor lifted a 1.2-mm pin in the pilot so as to allow upstream pressure to reach the downstream side of the diaphragm. Nice in theory, but it did not work. I had to buy a new timer and this time I chose a different brand that made a nice audible click when turning the water valve between ON and OFF. Hopefully it has no electric motor, but we’ll see after the next idle period of six years if this timer is any better.

Trober Kelib has a bachelor’s degree in electrical engineering with experience in the repair of industrial and medical electronic devices, systems design engineering, and commissioning.  And he is still intrigued as to why things fail. 

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