Temperature Monitoring

First created: Sun Feb 25 13:02:20 EST 2001
Last updated: Thu Apr 19 00:17:14 EDT 2001

Impact Of Temperature Variation

The movement of the float, in blue, follows closely the change in ambient temperature, in red.
temperature circuit

The blue curve measures the amplified movement of the float with feedback, using the circuit described in the previous section. The red curve shows data collected from the temperature monitoring circuit described below.

In this picture the temperature signal drops when the temperature drops, and the the float rises when the magnets gain strength. The magnets gain strength when the temperature drops.

This next set of pictures was taken after the front end correction and after modifying the temperature monitioring circuit to show temperature variations in phase with the float. The signals drift out of range quickly.

Monitoring The Temperature

temperature monitor circuit
temp_monitor2.sch, the EAGLE schema file.

The 2 signals from a variable resistor (top) and a thermistor (bottom) are buffered and then compared. The amplified signal difference drives the LED and Vout. The Vout of this circuit follows the temperature: it increases with rising temperature and decreases with dropping temperature.

Adjust Vvar so that the diode barely lights. This in effect adjust Vvar to match Vtherm. Here's why. If Vvar matches Vtherm, then Vvar = Vtherm = V+ = V- and there is no current through the 3rd comparator feedback loop. If the thermistor's value is 10 K at room temperature, i.e. same as the resistor in series with it, then Vout is 6 V. In this case the diode draws 1.2 mA and barely lights.

Now if the ambient temperature rises, the thermistor's resistance will decrease, and Vtherm decreases, let's say from 6 V to 5.95 V. V+ is still at 6 V since nothing has changed in that part of the circuit, and the op amp will strive to also keep V- at 6 V. Therefore a current must flow through R1 to boost 5.95 V to 6 V, causing a 0.05 V gain across R1. The same current must flow through R2 and adds another 5 V (100x) bringing the comparator output to 11 V. This means 2.2 A through the diode, enough to make it turn bright.

A similar argument can be made to show that as the temperature drops the diode goes dark.

It is easy to modify the circuit so that Vout decreases when the temperature rises and increases when the temperature drops. The following circuit does just that.

temperature monitor circuit
temp_monitor3.sch, the EAGLE schema file.