DIY clock advice

[kuro]

Hi there. Sorry if this isn't the right place to ask, but I spent a while searching and this forum was the best option that came up so I hope you guys can help me out with some advice.

I want to make some wall clocks. The original idea was to have three clocks, one each for Tokyo, London and New York time, all synchronized. They would have smooth sweep second hands that move in perfect unison, and be battery powered. It's possible to do it with low power radio modules, but the problem is finding a suitable clock movement.

There are movements with sensors to detect when the hands are at the 12 o'clock position, but I had a look at a couple and they are very highly integrated and difficult to modify for radio control. I'd be using a microcontroller like an Arduino (but not an actual Arduino). The closest thing I found was the Juken X10, which doesn't have a second hand and seems to need 5V to drive it, not ideal for battery power.

Also, smooth movement of the second hand requires a complex drive signal. Chips are available to do it, but none of them are low power enough for battery use that I can see.

So then I started looking at slave clocks. Most don't have a second hand either, and would need to be wired rather than wireless because they usually need fairly high voltage to step them. The main issue is that you can't seem to buy them new, and the used ones on Ebay are very expensive and rarely available in lots of 3 identical ones.

Maybe I can do without the position sensor, by detecting when power is removed and saving the position using a capacitor to hold the supply up.

So here I am, asking for advice and ideas. It seems like the only way to build a DIY clock is to retrofit your own movement to an existing clock. Simple movements without any position sensor are not too hard to modify if you don't want smooth sweep, but is there a better option?

[Tmuir]

Wanting them to be battery powered is the hard part. If mains powered and battery backed up for short power outs was an option you could go fully digital.

Use a real time clock module and have 3 LED display boards for the time and just send the time from the one RTC clock module to all 3 with the offsets of the timezones.

Synchronome slave dials would work with batteries, but as you say find 3 the same without breaking the bank may take some time.

The current passing through the coils on these clocks is more important than voltage and once set up should be around 330mA and with the coils only having resistance of 2.4 to 4 ohms with the nominal being around 2.7 ohms you only need about 0.8 volts per dial, so about 2.4V for 3 slave dials.

So you could in theory run them off batteries although I don't know how long even D cells batteries would last running the clocks

[kuro]

Thanks Tmuir.

I built a lot of battery powered devices for work so I did have a plan. Texas make the CC110x family of radio modules, so my intention was to run the clock on battery alone with the microcontroller in ultra low power mode for 58.9 seconds, power up the radio to get a synchronized tick signal from a transmitter that was plugged in nearby, and then power it off again.

The micro would use a basic crystal in-between synchronizations, should be accurate enough over such a short period of time and everyone will be looking for the 12 o'clock crossing to be spot on anyway.

 

Sounds like maybe I should get one of the Juken modules to test out. They are designed for automotive use though so I'm guessing that they won't be so good on power consumption.

 

If I go for synchronome clocks I'd just use mains power I think. Speaking of power, I find it interesting that there are relatively few solar powered clocks, and they are relatively large panels in them. I guess the torque needed for large hands is actually considerable.

[Tmuir]

I would say the reason why there are very few solar clocks is most clocks are for indoor use and the light inside is not enough for charging, also its easy to change an AA battery in a clock, pretty much anyone can do that, but changing the cell in a watch there are many people that should definitely not do that and by having a solar watch it extended the time between when it needs to be opened to replace the cell which is something most people cant do.