Hi All,
The ice detector system I am familiar with is fitted to all Lockheed P3s.
It's an aerodynamic probe with a line of 5 or 6 small holes and one larger hole, all subject to normal airflow, the probe is mounted roughly in the same vicinity as your pitot probes, near the nose.
These small/large holes in the probe are cleverly designed to be of the same total area as each other, (the total of all the smaller ones equals the size of the larger), allowing the diaphram inside, sensing between them, to remain centralized.
As soon as ice is detected, the smaller holes ice up quite quickly, forcing the inner diaphram to be displaced, doing two things:
1/ illuminates the 'ICING' light in the flight deck,
2/ turns on a small heater in the probe to melt the ice.
When the ice melts, both the light and heater turn off.
It works brilliantly, and the severity of the icing can be gauged roughly by the rate of the indicator light flashing.
Cheers,
FD