Mosquito Magnet Liberty

In September, 2014, a used MM Liberty was acquired to help out a Defender on a 1 acre formerly rural suburban property. It ran during the summer, but after running out of gas, it failed to start. Here is some documentation.

Attaching an oscilloscope to the thermistor displays a repeating downward stair step, with each step lasting about 100 mS. One side of the thermistor at ground. The thermistor is bypassed with a capacitor to reduce noise.

The Liberty uses four fixed series resistors to measure the resistance of Liberty's 200 kOhm thermistor using voltage division. After applying 5 Volts to each resistor, the resulting voltage across the thermistor is measured. The cycle repeats every 400 mS.

The capacitor and thermistor are not used for timing like in the Defender, which uses variable pulse widths to continuously measure temperature. Instead, the Liberty circuit measures 3 important temperatures, plus a potential open circuit fault. This scheme is not self-calibrating.

Although the Liberty Controller circuit has not been reverse engineered, it seems evident that the thermistor voltage is measured by a comparator 4 times during a cycle. The On light is activated when the 2nd step voltage goes below ca 3.0 Volts, and the Fault light is activated when it rises above ca 3.0 Volts.

Using some logic we can speculate that the 1st step seems to be an over temperature protection scheme. It would go below 3 volts only if the unit got very hot. It never triggered during my test. The 2nd step is the desired temperature range -- below about 3 volts, the combustion is OK. Above, it is not. The 3rd step moves down quickly after ignition, to possibly indicate that the ignitor is working and the unit is not just flooding the area with unburned propane. The 4th step seems to measure that the thermistor is not open-circuit. This was not tested.

Measured Resistor Step Thermistor Voltages

Here are this Defender's events and thermistor pulse widths vs elapsed time:

time	1	2	3	4	Notes
start	4.8	4.2	3.2	0.4	Press On/Reset. Warm Up Flashes.
3:05	4.8	4.2	3.2	0.4	Fan slows for ca 5 seconds
4:05	4.8	4.1	3.1	0.4
5:05	4.7	3.8	2.5	0.3
6:05	4.6	3.2	1.8	0.1	Warm Up LED Solid
6:30					Warm Up Solid.
7:05	4.5	2.4	1.1	0
8:05	4.3	1.9	0.8	0
8:36	4.1	1.6	0.6	0	On LED Solid, Warm Up Off
10:05	4.0	1.4	0.5	0
11:00	3.9	1.4	0.5	0	We turn gas off (On LED Solid)
12:00	3.9	1.4	0.5	0
13:00	4.0	1.5	0.6	0
14:00	4.4	2.0	0.8	0
15:00	4.6	2.8	1.4	0.05
15:32	4.7	3.0	1.8	0.1	Fault Solid
16:00	4.7	3.4	2.0	0.2	Fault Solid
17:00	4.8	3.7	2.4	0.3	Fault, Warm Up Solid, On Flash
18:00	4.8	4.0	2.8	0.4
19:00	4.8	4.1	3.0	0.4
20:00	4.8	4.2	3.2	0.4
20:37	4.8	4.2	3.2	0.4	Fan Off

More Later

There are photos of the controller PCB that can be analyzed and the circuit reverse engineered. But that is for after mosquito season!