Carbon / Lace Sensor
A possible alternative to the discreet resistor sensor above is the sensor pictured below.
This sensor is constructed from fabric lace and 0.4mm wire solder.
The link pitch is about 1.8mm and the per-link resistance is around 420 Ohms.
Since the test motor's wax wall is approximately 3/4" thick, there would be 10 links in the wall giving an initial resistance of 42 Ohms.
One possible readout method for this sensor would be to use a regulated 5V power supply and a series 133 Ohm resistor. This scheme draws an initial current of 28.6mA and produces the estimate voltage to Burned-Link relationship graphed below.
- Estimated Voltage vs Burned-Link count for Carbon / Lace sensor:
The Carbon / Lace sensor is constructed rather simply. Two lengths of solder woven into the lace, then the lace and solder are sprayed with a commercially available RFI shield coating. (We used GC Electronics "EMI-RFI Shield" No. 10-4807 16.25$ for the 12oz size.) This stuff is basically a spray-on carbon film similar to carbon resistor material.
The only tricks are to secure the lace against the blast from the spray can, and not to spray so heavily that the lace gets clogged with carbon.