DR. DAVID LEE HALL, MD
Marriage and Family Therapists at Taub Loop, Houston, TX

4169543, Oct 2, 1979

Oct 20, 1977

5/844040

David L. Hall - Houston TX

Keystone International, Inc. - Houston TX

B67D 508

222 56

Disclosed are apparatus and method for detecting the presence, or absence, of material at a predetermined location. A crystal controlled oscillator generates a constant-frequency, constant-amplitude signal which is fed through a load resistor to an antenna sensor circuit, including an antenna probe positioned at the sensing location. The amplitude of the signal fed to the sensor circuit varies as the impedence of the antenna probe is affected by material in its environment, or the lack thereof. The signal amplitude is converted to a d-c voltage level, which is added to a reference d-c voltage level preset with the antenna probe free of impedance-affecting material. The difference voltage level value thus obtained, and which reflects the environment of the antenna probe, is used to control a switching circuit whose output assumes one value or another, depending ultimately on the antenna probe's environment. The output from the switching circuit may be used to operate one or more devices indicating the environment of the antenna probe, and/or affecting the environment by adding to, and/or removing therefrom, material affecting the impedance of the antenna probe.