Hovilink

Homebrew viessmann optolink is an optical to serial interface built to connect a Viessmann Vitotronic heating controller to an RS232 host. This design is based on some other designs found on the wiki. This interface translates the optical signal to an RS232 signal. The board is designed with Eagle 7.7 free edition. I ordered it from OSH park.

Optical interface

The vitotronic has a green LED which also functios as an IR Led: SFH 7222 SMT Multi TOPLED® with IR Emitter and LED (Emission wavelength 880nm). I paired this with a TEMT1020 which has a peak sensitivity at 880 nm. I used the images from the datasheets and the pairing is perfect.

The vitotronic's red LED is an IR receiver: SFH 7226 SMT Multi TOPLED® with visible LED and Si-Phototransistor with maximum sensitivity at 860nm. I paired this one with a TSMF1020 which has a peak emission at 850 nm.

Schematic

The schematic is not too complicated. On the left side I have put the optical interfaces. I also added two LEDs to give a visual feedback of the IR signals for easier diagnosing. (And it looks extra geeky) In the middle there is the MAXRS232 with it's buffer capacitors and on the right are the connection pads.

Board design

I set myself the goal to make this fit in the small lowered space on my heater. The board is tiny and two sided. On the bottom side I tried to keep as much free space as possible for the 'V'-sign.

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