A fail-safe r-c phase shift oscillating type of level detector

Claim 2

A fail-safe level detector comprising, a transistor oscillation circuit having an input, and output, and a frequency determining network selectively connected between said input and said output, biasing means connected to said input and lead means connected to said output and a voltage breakdown device electrically connected across said input for regulating said input and also electrically connecting said frequency determining network from said output to said input for thereby completing a feedback circuit whereby regenerative feedback is present when said voltage breakdown device conducts and assumes a low impedance condition sO that said oscillating circuit produces A.C. oscillations when and only when said breakdown device is conductive and is operating properly so that it exhibits a low impedance condition which permits sufficient feedback to maintain said A.C. oscillations.

Claim 3

A fail-safe level detector as defined in claim 1, wherein said oscillating circuit includes a solid-state amplifier and an RC phase-shifting network.

Claim 4

A fail-safe level detector as defined in claim 1, wherein said oscillating circuit comprises a phase-shift type of transistor feedback amplifier.

Claim 5

A fail-safe level detector as defined in claim 1, wherein said voltage breadkown device is a Zener diode.

Claim 6

A fail-safe level detector as defined in claim 4, wherein said Zener diode is connected as a shunt regulator.

Claim 7

A fail-safe circuit arrangement comprising, an input, an output and a common terminal, a current-limiting resistor and a Zener diode electrically connected between said input and said common terminals, a transistor having an emitter, a collector and a base electrode, a first resistor coupling said collector electrode to the junction of said current-limiting resistor and said Zener diode and a capacitor coupling said collector electrode to said output terminal, a second resistor coupling said emitter electrode to said common terminal, a third resistor coupling said base electrode to the negative terminal of a supply source, and a resistance-capacity phase-shifting circuit coupled from said collector electrode through said Zener diode to said emitter electrode whereby regenerative feedback occurs and A.C. oscillations are produced on said output terminal when and only when a D.C. voltage of sufficient amplitude is present on said input terminal to cause said Zener diode to conduct and assume a low dynamic impedance condition.

Claim 8

A fail-safe circuit arrangement comprising, an input, an output and a common terminal, a current-limiting resistor and a Zener diode electrically connected between said input and said common terminals, a transistor having an emitter, a collector and a base electrode, a first resistor coupling said collector electrode to the junction of said current-limiting resistor and said Zener diode and a capacitor coupling said collector electrode to said output terminal, a second resistor coupling said emitter electrode to said common terminal, a third resistor coupling said base electrode to the negative terminal of a supply source, and a resistance-capacity phase-shifting circuit coupled from said collector electrode through said Zener diode to said emitter electrode whereby regenerative feedback occurs and A.C. oscillations are produced on said output terminal when and only when a D.C. voltage of sufficient amplitude is present on said input terminal to cause said Zener diode to conduct and assume a low dynamic impedance condition.