At work I test heart monitors and I needed a good temperature simulator since the Fluke MedSim 300B only offers a narrow band of simulated temperatures.
Temperature is a parameter most often measured using the resistance of a thermistor. As the temperature increases, the thermistor's resistance goes down (strangely enough) and vice versa. With this in mind, the easiest way to simulate temperatures would be to use fixed resistors. To find the proper resistance range, I tested several resistors by plugging them into the heart monitor's temperature module.
I decided to go with the following resistances (labeled with their correlating temperature on the heart monitor):
470 Ohms - Too High
1k Ohms - 45C
3k Ohms - 16C
5k Ohms - 8C
8k Ohms - Too Low
I then gathered up the resistors I would need, some toggle switches, and a plastic project box to house them. A power drill was used to put holes in the case and once that was done I mounted the switches. After about 30 minutes of soldering, my project was completed.
The circuitry is pretty simple, it is essentially just a bunch of parallel resistors with switches in front of them. You use the toggle switches to determine which resistors will be placed across the leads and you can select more than one at a time to get extra values.
I initially planned to use alligator clips mounted to two metal posts for leads and I added those in there. However, I later found a 1/4" phono cord, which is the standard cable used for temperatures in medical devices, and mounted that in there as well. Note how I knotted the cable before the hole to avoid placing pressure on the solder joints. The last step was to add labels to the top so I knew what was what.
Now that the project was done, it was time to give it a test run. I tested it on multiple heart monitors from different brands, including the Propaq Encore, HP Omnicare, and the GE Dash 4000. The simulated temperatures worked perfectly, as you can see in the shot I took of the Omnicare.
In conclusion, it was a nice project and took me only about 6 hours to finish from planning to testing. This device would be good for simulating a thermistor if you are making any system that is controlled by temperature or measures it.
Return to the Projects Section
Other Articles That You May Be Interested In: