I finaly built a prototype of the opentherm interface (version 2) for Alex to test, since he dosen't have all the tools to test it I'm hoping that at least the physical interface works, the other features are powered through the interface and using a supercapacitor to hold the power during radio transmission.
The board has a JeeNode interface so Alex doesn't have to change much, there are two extra jumpers, one connects the opentherm rectified voltage to the PWR input of the jeenode, and the other connects the 3.3V output of the jeenode to a supercap. It is still a bit oversized but... Well it is a prototype... The OpAmp is the LM358 (dual version of the quad LM324), works from 3V to 33V and it is relatively low power. The specification of Opentherm states a open voltage of 40V so I had to protect the OpAmp with a parallel Zener (seen in the picture).
A lot has to be done on the jeenode side, but it is mostly software regarding the power saving modes and saving enough power before the short bursts of radio transmission. The schematics of this circuit is based on the published before, but I'll hold the final one until Alex gives me feedback.
1 month ago