25 May

Watering system repair

Some time ago I picked up a Draper automatic watering timer for the greenhouse. The idea was to connect it to the water butt and a micro drip watering system, basically a set of small plastic fittings connected with tubes. There are different types of fittings but the ones that do the business have very small holes, the idea is to place these close to each plant so the water goes to the roots and is not wasted elsewhere and left to evaporate. The timer opens or and closes a valve at preset times. I managed to get this working reasonably well, although I wouldn’t recommend this type of watering system unless you have a decent water pressure, it clogs far to easily with low pressure.

Watering Timer

Watering Timer

Setting it up again for year 2, the timer didn’t work, displays and settings were all okay but the valve didn’t move. I took it apart to do some fault finding. I could get the valve motor to run by hooking it directly to the supply, the micro switch used as a position sensor was fine, so it was down to the electronics. Most of the work was done by an embedded micro controller, with just a handful of supporting components. It fairly easy to trace the circuit and figure out what was going on. the motor was switched by a pair of bipolar transistors, the main ‘power’ transistor was faulty. This was a surface mount pcb and the transistors were SO23 packages, very small devices, especially when I need to use glasses. In my spares box I had a selection of through hole mounted transistors, better for my eyesight but not so easy to use to replace a surface mount component. However I struggled on with an oversize soldering iron and a cheap magnifying glass and managed to get the thing fitted. All good for a couple of years, until now. Once again the timer had failed, although this time the motor was on permanently. I stripped it down again and my replacement transistor was blown. I decided to check the datasheet this time and realized that my replacement, despite being physically much bigger than the original, was rated at 100mA collector current, a cheap DC motor on 5V was likely to exceed that, so I’d been lucky it had lasted so long. I’ve now replaced it with a more suitable device, in an even bigger package. Hoping that will be the last time I need to do this but a good reminder to read the specs when selecting components.

Watering Timer

The top was glued in place but came away with a little light persuasion

Watering Timer replacement transistor

Replacement transistor, somewhat larger than the original.

Last Updated on 3 June 2015 by Mark