... but not one I designed or use with an Arduino... apologies!
Background: My weathermatic SLW10 rain sensor stopped working last year after a good season of helping contain the water bill. The sensor system consists of a hub (SL-HUB) that is placed inside the irrigation controller that communicates with the weather / wind / etc. sensor over two wires. The sensor platform is a 9V-block battery-powered unit. Based on my research, I ordered a replacement SL-HUB, and it works like a charm. Being curious re: the failure mode of the original unit, I decided to dissect the old one. Here is what I think I found and what has me scratching my head.
The SL-HUB appears to be powered via the same 24VAC bus as the rest of the weathermatic unit (have to confirm this via measurements). As you can see in the circuit I traced below, the incoming power runs through a half-wave rectifier before it gets to fill a 100uF 50V cap. Then it is run through a 12kOhm resistor before being regulated down to 5V with two de-coupling caps to keep things smooth. The LM7805CZ is made by WST and the maximum allowable voltage is 40V per its data sheet. I have measured the voltage coming into the weathermatic controller to be as high as 27.3VACrms. In other words, the peak voltage is around 38V. Doesn't this seem a bit close to the edge re: the maximum ratings mentioned on the datasheet?
Additionally, is it just me, or is this a weird way to deal with 24VAC incoming power to make 5VDC? All the circuit appears to be powering is three transistors and a DS1487 RS485 transceiver
. Perhaps even a drop-cap supply could have worked
Another nice touch is the diode that was hand-soldered from one of the interface pins to the 5VDC output pin of the 7805. I presume this is to prevent the voltage on said pin to go higher than 5VDC?