Reviving a Dead Mains Timer

It is that time of year when one puts electric advent stars and candlesticks in the windows. I usually use timers to control them, but this year I ran into a problem. Some of my timers were of the type “EverFlourish EMT757A”, probably bought at Clas Ohlson:

Back side of timer

Back side of timer

Despite having a notoriously bad user interface (that forces me to google the user manual each time I want to use them), the timers, that had been laying in a drawer since last season, showed an entirely blank LCD display, even when connected to a mains outlet. I guessed that an internal battery just need to be charged, so I left them connected for a day to allow them to be charged up. The display however remained blank.

Hmm. I had encountered similar timers before that needed the internal battery to be replaced, so I decided to have a look inside to determine if this was the case again. It turned out that the screws used to keep the units together had tamper proof tri-wing heads. Fortunately, I happened to have bits that fit this kind of head, so this did not deter me.

Tri-wing screw head

Tamper proof tri-wing screw head

Tri-wing bit

Tri-wing bit

I found that there was indeed a tiny chargeable NiMH battery inside the unit (the green component with white corrosion on the side, close to the center of the picture below). Despite the unwanted corrosion, I found that the battery was actually charged to 1.3 V or thereabout, which is fine for a NiMH battery. Maybe it did not need to be replaced after all?

Opened mains timer

Opened mains timer

But why then was the display dead?

I put the timer back together again and had another look at the front. Pressing any of the normal buttons did not result in any reaction, but there was also a tiny little recessed button marked RESET. I used a pen to press it and voilĂ ! The display came to life!

The timer has been brought back to life.

The timer has been brought back to life.

Stupid me to not try this before I disassembled the unit.

I suppose the reason the timer behaves like this is that the processor controlling the display probably does not have a proper power on reset circuit. So when the supply voltage is ramped very slowly, as it is when it is powered from a battery that is trickle charged from a state of being completely dead, the processor does not jump into action properly, even after the supply voltage reaches its proper level.

4 thoughts on “Reviving a Dead Mains Timer

  1. You saved my day… Like you I was going to take apart the timer, without pressing the obvious reset button first… (must be a guys thing ;-) )

    Thanks again.

  2. I have such timer ,when it is unplugged of the outlet the displays faint digits but when i plug it to the outlet ,the digits become clear and bold,are this normal?

  3. Hi Riad,

    I would think this is due to deterioration of the battery inside the timer. The battery is mostly there to keep the time and retain the memory when the timer is not plugged in (or there is a power outage), so my guess is that the timer will work fine as long as it is plugged in and perhaps still keep time and memory even without power for a while. But it does of course become hard to program it unless it is plugged in if the digits are hardly visible.

    I have two timers where I replaced the old battery by a suitable supercapacitor to fix a similar issue, but this is not something I would recommend you to attempt unless you really know what you are doing. It is easy to get an electrical shock or perhaps blow up the capacitor.

    Per

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