MAY5

Master Clock driver for antique slave clocks

Big old wall clocks that were used in facilities such as schools, factories and hospitals are very appealing and will probably never go out of style. They are very durable and almost never break down due to very simple mechanism they have. This mechanism is actually pretty simple, it is just an impulse motor (two coils) and a couple of gears - no further complications. This is a blessing and also a curse for these antique clocks because for their operation they now require something called a master clock. Master clock is an electronic device that sends pulses to these slave clocks so that they can work.

Note: Firmware, schematics and everything can be downloaded by clicking on the red download button under the article.

Here we present a master clock solution made with an (antique?) PIC16F84 microcontroller. PIC has a task to send a DC pulse every ~100ms to this clock. But there is a caveat - this pulse has to change polarity every time it is sent out. Therefore this circuit incorporates an H-bridge with four transistors as we can see in the schematics bellow.

Microcontroller uses a 131.072 kHz crystal for a practical reason - it can be nicely prescaled so that we get a 1 second interrupt. For this idea, Timer0 is prescaled by 128 which means means that Timer0 will overflow exactly once every second: 131.072 / 128 / 256 / 4 = 1s. Here we divide by 256 because Timer0 overflows when it counts to 256, and further on we divide by 4 because PIC instructions take 4 internal cycles to complete. In the firmware, we simply count 60 seconds and issue a pulse to the slave clock. This basically advances the minute-hand, and that's about it.

Voltage that these slave clocks use is a bit higher than PIC's voltage so some kind of voltage boosting is required. For this, a simple DC-DC step-up converted bought from eBay does a great job. It is experimentally determined that this particular ISKRA slave clock loves 24V DC pulses after the internal resistor is shorted out - as seen on the photo bellow.

Because these old clocks were used in large facilities, mane of them were connected to a single master clock. They were connected in parallel and because of the length of that installation voltage must have been around 100V DC!

Time adjustment is done simply by clicking the S1 button. This advances the minute-hand by 1 minute.

Source code is written in PICBasic and is available for download by clicking on the red download button bellow. It can also be improved by adding daylight savings, moving minute-hand +1 and even -1 minute, adding IR remote for remote time adjustment and so on.

Author of this master clock electronics is our good friend Edo Lelic. Be sure to check out some of his other great work:

- Smart battery tester and charger

- RDS decoder with ATtiny2313

- Biennophone radio from 1953 with mp3

- Housing for Nixie Tube Clock - Warmtube clock

- Phase loss protection - protect your motors

[ Source: Edo Lelic ]

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