Build a steampunk like clock with old moving coil instruments, but not with the help of a microcontroller.
- Clock source: Crystal? NE555? stepper!
- Count secs, mins, hours: binary counters with reset CD4040.
- Instruments: look on ebay, found two ELIMA 10V DC instruments.
- Drive the instruments: Analog digital converter DAC0808.
- It should look nice.
The instruments needed 10V, so TTL logic was not sufficient. But CMOS.
Clock Source (the electromechanical part)
In germany our mains voltage is 230V at 50Hz. Using a transformer and a capacity it is possible to drive a stepper. A 200 step stepper makes one revolution every second when driven by that circuitry.
The clock signal is then fetched by a light barrier (ITR8102) and sent as clock input using a schmitt-trigger (CD4093).
Counter (the digital part)
The CD4040 is a simple IC: Clock in (negative edge triggered) and it counts up to 12 bits. And has a reset.
To divide/count up to e.g. 60 you have to trigger the reset at 0b111100. This can be done using diodes and a pull up resistor and a driver or, using an AND gate. I choused the AND gate because I also wanted some blinkenlights added and the tests showed that, the diodes solution was not working so well in that case.
The CD4040 circuit, with AND gate and LEDs to show the bits:
This circuit has to be built for seconds (divide by 60), minutes (divide by 60) and hours (divide by 24).
A small problem was: How can I set the time? The counter is falling edge triggered. My solution was a switch that disconnects CLK from the clock input and pulls the signal high. With a push button the CLK can then be pulled low. I also added a push button to restet the counter manually.
Digital Analog Converter (somewhere between digital and analog)
I used the DAC0808 with the default circuit from the datasheet. The not used lower bits were pulled down.
Moving Coil Instruments (the analog part)
I opened them up, cleaned them and painted them with shining black paint. Also a new scale must be designed.
Also, I replaced a resistor so that they now display 0..5V and not 0..10V. Unfortunately I did this after I ordered the PCBs, espessially the power PCB, because if I had done this modification earlier, I could drive everything with +/- 9V and not +/- 12V and that would have made the trafo smaller.
I decied to build it as a stack of PCBs on the right side of the meters.
- Clock Source
- Seconds Counter
- Minutes Counter
- Hours counter
- Power source (Base Plate)
I etched a 1st prototype of the PCBs on my own and optimized it afterwards (e.g. smaller switches and push buttons)
The new PCBs are here and all components are soldered
These parts work as desired (okay. I should have included something to debounce the switches, ...)
the DACs work as can be seen in the oscilloscope.
I am not happy with the power supply. So I will redesign it to use two separate transformers, one for the electronics and one for the stepper. *To be continued*