Self-balancing bridge-type proximity detector

A self-balancing capacitance bridge-type proximity detector having a probe forming a leg of the bridge for providing an output when a metallic object contacts or passes in close proximity to the probe, comprising in combination

• a. a high frequency oscillator,
• b. a capacitance bridge network having a proximity capacitance probe in one leg of said bridge network and a varactor effect tuning diode in a second leg of said bridge network, said high frequency oscillator having the output thereof coupled to said first and second legs of said bridge network,
• c. a differential amplifier means coupled to said first and second legs of said bridge network for producing an output therefrom on an imbalance of said bridge network caused by a metallic object to be detected contacting or coming into close proximity to said probe,
• d. feedback means having a low pass filter which has a time constant producing a time delay in rebalancing said bridge network, said feedback means being coupled between the output of said differential amplifier means and said varactor effect tuning diode for varying the capacitance of said varactor effect tuning diode to rebalance said bridge network, thereby conditioning said bridge network for the next approach of the metallic object to be detected, and
• e. peak detector means coupling said first and second legs of said bridge network to said differential amplifier for converting the peak amplitude of the outputs of said first and second legs of said bridge network to proportional DC voltages which are coupled to the inputs of said differential amplifiers.