Electronic Master Clock: a huge train station clock is ticking again

Recently, I have been asked how an old slave clock could be controlled, it was saved from being scrapped, when a local train station closed down. It is really big and heavy. Eventually, I checked it out and it can be operated from 12 V, 24 V, or higher voltages by adding a resistor. We decided to operate it on 12 V, simply because one of the slides (this is two-sided slave clock) had already been set to this value.

In order to control the slave clock, we have to send a 1~2 second long voltage pulse, every minute, in alternating polarity. Then the minute hand will advance minute by minute. There is no second hand on this slave clock.

The control board is fairly simple, to test it all out, I soldered it on a piece of prototype board. Main control is by an ATMEGA8 microcontroller. This is using its own clock, from a Kyocera HC1 crystal oscillator to derive a 1 minute time interval, and and auxiliary 1 second output, for calibration purposes.

´╝┤he actual adjustment of the clock is done in software, because the oscillator is pretty stable, and this is just a test circuit. If the slave clock will be a minute off, or two, nobody will mind.

The clock drive is completely separate from the controller, isolated by optocouplers. I used a small transformer, with dual 12 V output (because it is small current only, the output voltage is high enough, but you may use a 15 V transformer if it is handy).

Seems we are about 25 ppm out. After correction, the clock was running well within 1 ppm, fractions of a second per day.

To make the contraption a safe and useful thing, I put it in a little box, with some cables and feed-throughs.
Now it is up for some long term test, let’s see if it needs any modification.

SMEG CS19ID-6 Range: another defect

It seems my SMEG kitchen range is getting older… at least it is again showing some issues. The right hand side front induction field is sometimes coming on, intermittently, only to shut off quickly again. Normal operation is no problem, but when it is switched off, it doesn’t remain fully switched off all the time. Could be dangerous if some pot is left on the induction field, and the range decides to switch it on by itself… a praise to all the old-fashioned switches that completely took power off the appliance.

First we need to find out the source of the issue, is the it controls electronics, or the switch? To test, I opened up the front panel (easy enough, it is just 8 screws…), and switched the cables going to two of the front panel switches. And, as it turns out, the issue also switched. So it is probably a defective switch rather than any issue with the electronics. That’s good news.

For about 35 EUR, I got a new switch, as it turns out, it is no switch put a potentiometer…

… and a quick exchange fixed the issue. Now let’s do some study of the old part.

Made by printing some conductive composite on a circuit board. Looking good. I can’t see any issue with it even under the microscope. Maybe just some contact issue, the contacts also don’t look clean.

So I will clean it all up thoroughly, bent the contacts a bit to give it slightly more force, and keep it as a spare.

These potentiometers aren’t cheap, at least the use good engineering plastics for it, like, glass fiber reinforced materials.