Russ's Mercedes Benz 300D Turbo Temperature Sensor Network
I have designed and constructed a temperature monitoring system for my 1983 300D turbo model 123.133. It uses four DS18B20's digital temperature sensors from Dallas Semiconductor / Maxim. A link is provided to the Dallas Semiconductor Web site and the data sheets about the DS18B20 1 Wire network sensors. http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2812 The Sensors cost less about $5.00 each from Digi-Key Co. Each sensor is in a 3 lead TO-92 package. It has the size and form factor of a standard plastic cased transistor you get at Radio Shack. There are two sensors mounted under the hood. One measures the coolant temperature at the outlet of the thermostat, the other is mounted to one of the cooling refrigerant lines to keep a check on the AC. The foil tape over the radiator hose holds the DS18B20 in contact with the radiator hose just above the thermostat housing. All four sensors are wired in parallel on a three wire cable. The cable provides 5 volts DC, ground and a data line. The Basic Stamp 2P doesn't provide true 1-Wire support. So I provide the 5 volts DC to power the sensors instead of using parasitic power from the data line.
I also have an outside air temperature sensor and an inside air temperature sensor. The placement of the outside temperature sensor while not very elegant it is functional. Return to warmer weather will let me find a less conspicuous location for the sensor.
The inside sensor is located inside that foam tube that runs from the vent on the dash back to the blower fan. This is where the thermistor for the climate control system is located . The DS18B20's all talk to a Basic Stamp BS2-P single chip computer It is available from Parallax Co. .http://www.parallax.com/html_pages/products/basicstamps/basic_stamps.asp The stamp drives an 4 line by 20 character line LCD display. The basic stamp is mounted inside the unused ashtray, power is picked up from the connections from the cigarette lighter power connections. The LCD display is mounted on the front of the ashtray. The system reads each sensor and updates the display about every 600 milliseconds.
LCD display is secured to ashtray with
Velcro
Close up of Blue LCD with actual temperatures on a cold morning
Three shots of display mounted on ashtray face. The shot to the immediate right shows my dual band 144/440 MHz Ham transceiver and a Radio Shack digital thermometer I used when troubleshooting the climate control system. I may eventually router out a rectangular hole in the ashtray front face to hold the LCD. It would give it a more finished look, but I need to find another ashtray to experiment with. On the left you can see the Turbo boost monitoring gauge ==================>
The system has been in use for about 5 months. I built it to try and figure out if the temperature sensor on the dash was lying to me or the car actually runs hot. I believe it actually runs HOT. When the car is cold and starts to warm up you can see when the thermostat actually starts to open. I realize that reading the coolant or engine temperature thru the radiator hose is not accurate. The readings are affected by air blowing through the engine compartment, however the system does let me keep track of what is going on under the hood.
Here is a basic schematic of how the sensor
network is wired. Some details have been omitted to keep the diagram simple.
Click Here
The program that runs the whole thing is written in PBASIC is only 95 lines of code including comments. The only thing that required a lot of thought was how to deal with the 16 bit 2 's complement output of the DS18B20 when temperatures drop below 0° C. The display is back lighted so it can be easily read at night. I can E-mail the PBASIC code to anyone who is interested in duplicating or building a similar system. My E-mail address is Russ@BEESCOLLC.COM. It has been a fun project !!!!