I picked up a handful very inexpensive digital thermometers the other day that had an LCD. I would like to hack them and stick a JeeNode in between the sensor and the LCD but I want the LCD to continue to display the temperature.
I figured the first step was get them onto the same power source. I went back to the handy Easy Electrons article and divided the voltage to come up with 1.5v. This worked great and now my jeenode and thermometer are running off the same power source (USB for now).
With nothing hooked up the LCD reads around 67.1 degrees F.
Sensor checks: Next without opening what looks to be a moisture resistant housing on the sensor I cut the wires and played with it and a multimeter. I don't sense and continuity hooked up in either direction. The resistence is reading about 9.8k and goes down significantly when warmed up. In diode mode I see about .461V in both directions and this lowers significantly when the sensor is warmed up.
Device without sensor checks: It looks like voltage jumps between 166mV, 396mV and 4V but the numbers change a lot and quickly. However I did find that the colored wire is positive and the white wire is negative. If I only had a scope... and knew how to use a scope... :-)
I hooked the multimeter inline next. When I check for DC voltage I see .1mv about every 2-3 seconds so I am guessing that is how often it is checking and it seems safe enough to hook my multimeter up inline so I did. When I put the multimeter in diode mode I see .543V very consistently hooked up one way although it lowers the LCD temperature on the LCD to 23.4 degrees F. When I hook the leads up backwards I it jumps between .543V and .519V and the LCD reads 48.4 degrees F.
There is no visible chip on the circuit board just a blob like on the RFM12B. I was assuming this is most likely a thermistor sensor but I expected to see continuity if so. I know that often in the temperature sensors that have a dual sensor a diode or transistor is used for a remote sensor. There are only 2 wires so I am guessing this can't be a transistor sensor. I had actually expected to see continuity through a diode one way after testing in both directions but that happens when I tested a normal diode also so... I am guessing my multimeter does not push enough juice to make it through.
I don't really care if I fry one of the sensors/LCDs but I treasue my jeenode. I think I have done all I can prior to hooking up my jeenode but I am not sure what the next step is. From what I told you would people agree this is a diode type sensor? If so I understand how to read temperature and can create a table for conversion. Is there an easy way to detect incoming voltage from the device to jeenode, replicate that out from jeenode to sensor, read incoming from sensor to jeenode, and replicate that back from jeenode to device while logging readings so the jeenode knows the temp? Would it be easier to try and hook inbetween the circuit board and the LCD?
Sorry for the book I just wrote but I bet this would be a fun project for others than enjoy a bit of hacking...