TR099 V1.1


This is an electronic time switch for a gas central heating. The source code and the operating instructions are in German, so here are some brief notes:

Of course you can use this time switch not only for a gas central heating. It is usable whereever you need upto two on-off-periods per day right on the dot, but different times each day of the week. Think of garden lighting, electric advertisements, and so on...

The time switch is built around a PIC 16C84 and a DG409 analogue switch. You may need to adjust the source code minimally to support other PIC types. The display was ripped from a PC keyboard with programmable keys.

Downloads: Archive with source code, 12KB. Archive with schematic and instructions as PDF, 79KB.
Thumbnail of the schematic. Archive with schematic as Eagle file, 36KB.


Close view of the time switch. Total view of the temperature control unit.


1999 I lived in an appartment with a controlled gas central heating made by Junkers which had a time switch TRQ21 to select the target temperature. There were two wheels (see the picture above), one for the daytime temperature, and one for the nighttime temperature. This time switch used a wheel with notches for each 15 minutes of the day. The notches controlled a gold-plated alternating switch. The wheel was driven by a DC motor with great accuracy.

Some day I noticed heavy switching noises in the heating device. Eventually it was found that the gold-plated alternating switch was opening and closing too slowly, giving unreliable contact. Because I didn't want to fall in that trap again, I ask for an electronic replacement. Guess what, it was very expensive!

So the idea of a self-made time switch was born. It should outperform the offered one as well. The result worked really well for many years until I sold the flat. I think it is still in use...

Horizontal Bar.

Who to blame:

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