Countdown Timer

Home Metalwork Radio Control

I like automation.

Rechargable batteries are expensive, and overcharing them shortens the life considerably. Since my existing constant current charger was rather rudimentary I decided to upgrade it a bit.

The prime consideration is that it not run too long. I can easily set a reminder on my cellphone, but I may not be home to unplug it when it goes off. A bit too unpredictable.

Somewhere in the house is a mechanical timer, but I just cannot find it. We do a rather occasional 'clean up' since we are both hoarders, but this means that things get put into boxes by the other party, and later cannot be found without a defence budget sized search operation. So I purchased a cheap timer.

This timer is a simple plug in type, with an integral 3 pin socket for the switched load (we use 3 round pin sockets rated at 15 amps on 240v AC here in South Africa). The timer switch is a microswitch rated at 10 amps, much more than enough for the charger. It is graduated in 15 minute intervals, effected by small plastic doofii (the black bits round the edge) that are pressed down on the circumference of the large plastic timing knob. A pressed down tag gives 5 minutes of on time.

Two issues arise:
  1. I want a simple operation where I can set it going for the required time period and it will switch off at the end of it. I do not want to have to calculate the run period based on the current time.
  2. Once it switches off, the battery must be disconnected from the charger so the feedback current does not run it flat.

For (1) I decided to modify the timer. For (2) I fitted a relay to the charger (more on that later).

Back to the timer. What is required is that the timers switch operates the timers time clock motor such that when the timer turns off the timer stops. Thus one can press down tags from midnight to however long one wants it to run, say 10 hours, then turn the dial till the load turns on. It will now run until the set time expires, at which point the timer itself will switch off and therefore not turn itself back on again.

This turned out to be very simple in this model timer, involving moving one wire end about 1cm to the right. Slight complication in that the existing wire was too short, so I replaced it.

As you can see in the image at left, the interior is very straightforward.

A satisfyingly simple operation now lets me set the charger going for any desired time period up to 23 hours 45 minutes, and should result in longer life from my 3 NiMH battery packs.

Next, I need to rig up a discharger unit, so that I can be sure the packs are properly cycled, since I doubt I'll run them really flat during flying time. Tempting to purchase some sort of sensor board and hook it to a computer, to automate the process a little more. Did I mention I like automation?

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Last modified: February 18 2013 16:00:44.