SRT-100 Environmental Sensor repair.
At $dayjob, we have an external compound securing a generator and condensors for air conditioning, these were recently replaced and the Geist temperature sensor cable we had was damaged. Due to the way our purchasing works it cost £180, although I’ve found direct purchase from resellers for about £100. The external contractor has replaced the sensor now and I’ve grabbed the one with the damaged cable for reverse engineering.
OK I couldn’t grab all of it, I’ve the sensor side and a few centimeters of cable, I managed to grab a few meters of cable from the other end too. I couldn’t pull the cable back as it seemed to have snagged somewhere, considering the room is meant to be sealed for fire suppression (halon/fm200/inergen) it’s probably snagged where the cable goes through the wall. After cutting away the heat shrink/weather proof tubing the end cap falls away presenting a transister type thing soldered to some unshielded twisted pairs. As you can see below the semiconductor is a Dallas 18B02 in a TO-92 package.
Additionally we can see that the blue, white with blue stripe and white with orange stripe cables are used.
It looks like the endcap that houses this sensor is just over 30mm long with an internal diameter(ID) of 5mm and a 6mm outer diameter (OD)
Examining the RJ11 pinout we find the pinout to be
- Orange - unused
- White + Orange - VCC
- Purple - DATA
- White + Purple - GND
- Green - unused
- White + Green - unused
Reading the data sheet for this temperature sensor, there’s some internal magic that allows for multiple sensor data connected to a single cable. I’m not sure how off the shelf parts would work with the Geist device. Checking on Amazon, I could pick up 10 10B20s for £3.98 and 10 RJ11 plugs for £1.89, the pins could easily be crimped into the RJ11 socket for a proof of concept. You could have 10 makeshift sensors, for 58.7p each, ready to go on the end of RJ11 extensions meant for phone lines as they only use pins 2-5 but we use pins 2-4.
Amazon list(no referral)
- 18B20 TO-92 10 for 3.98
- RJ11 Connectors 10 for 1.89
- 50m External cable 50M for £12.99 (305M for £54.99)
- Temperature Probe Housing 10 for £5.39
Using the 305M cable above to provide 10x 30M cables like the stm-100 we’re replacing. The cost of parts for 10 cables is £66.25, which equates to £6.63 per cable. There’s a bit of heat shrink tubing, perhaps some epoxy or glue to waterproof the probe/cable joint so let’s round that up to £7 per cable and add enough for 1 hour work per cable at minimum wage we’re still looking at less than £20 to manufacture each cable.