TheWiggman said:
A few queries -
- There are 3 sensors pictured in the original drawing but only two used, what's the go?
- Why is there a resistor across one sensor's +ve and 'data' leg?
- How do all two/three sensors work if they're read off a single terminal?
- There are additional PCBs below the TFT, are there basically terminal boards to make screwing things easier?
The three sensors are: HLT, Mash and internal panel temperature. The software produces an on-screen alarm if the panel temperature > 50C. I've read that SSRs can produce a bit of heat if they're working hard, that said, I've never seen them get anything but warm. So far, (on ~25C days) the panel gets to about 35C. That said, they are screwed to quite a heavy metal (aluminum?) base plate.
The DS18B20 digital temperature sensors need this resistor for good readings. They suffer a bit from line noise, and this resistor helps. The capacitor there is also supposed to help with this, but it's probably a bit big. But it was the first one I grabbed, and it did seem help with failed re-reads.
The DS18B20 sensors use the "OneWire" protocol. Each sensor has a 5(6?) byte address - a bit like a MAC address of an ethernet card. When you request something from a sensor using the API, typically you include the sensor address in the call, so the sub-system knows which sensor to use.
The two extra PCBs in the photo are:
- Just a bunch of common (+) (-) screw points where power is delivered to stuff, and earths grounded.
- An attachment point for the DS18B20 sensor power lines, and the OneWire bus line. It also holds the 4.7k resistor and a hastily-added capacitor.
So yeah, you're basically correct, they just made wiring it up easier.