Slave Dials are clock movements without the actual time keeping circuit. All they contain is the mechanism to drive the hour and minute hands, which can be advanced by an electric pulse. This pulse is sent by a centralised master time-keeping mechanism. They were often used railway stations or large office building to ensure that all the clocks read the same time. You will still occasionally find such dials at markets, car-boot-sales and disposals.
I like the idea of bringing new life to old technology, and so conceived of the idea of melding the slave dial with a micro-controller and internet connection to create a old style slave dial with atomic clock accuracy.
These steps describe the process I went through, and includes all the schematics and source code. This same technique might also be applied to modern clock movements by bypassing the existing pulse-generation circuit.
- Arduino with WiFi (I used a YellowJacket board from Async Labs, but you could use any Arduino with WiFi or Ethernet enabled or a suitable shield).
- LM317 Regulator.
- TIP31 Transistor.
- 1N4004 Diode.
- Red LED.
- 9V Power Supply.
- DC Power Socket (to suit power supply).
- 0.1uF Capacitor.
- 1uF Electrolytic Capacitor.
- 22R Resistor.
- 3 x 1K Resistor.
- 10K Resistor.
- 100R Resistor.
- Self-adhesive Velcro.
- Electrical Tape.
- Foam (for sound insulation).
Step 1: Identify Slave Dial Characteristics
In order to drive the mechanism, I needed to know how much voltage had to be applied to the coil to reliably advance the hands, for how long, and how many seconds the hands would advance for each pulse. I was concerned about overdriving (and hence damaging) the coil, but also concerned about under-driving it, resulting in an unreliable movement.
I was able to find some general information on these units on the web (thanks to Google). My thanks to the people who put together the English Clock Systems website. It turns out that most such clocks would advance either 30 or 60 seconds depending on the model. The mechanism was specified as a combined resistance of around 3.5 ohms, and a current rating of 0.3 amps, for an operating voltage of 1.05 volts. IN reality, most such units were driven by banks of old dry-cell batteries with a voltage of 1.5 volts, so that is what I aimed for.
Note that slave dials such as this typically had a "shunt" resister is parallel with the coil. The reason for this was so because these clocks were often wired in series. Without the resister, if the coil on one clock went open circuit, all the clocks would stop working.