Automatic turnoff system for receiver with fm demodulator

Claim 2

In an electromagnetic wave receiver; a relay having an actuating coil and operable when the coil is energized to turn the receiver off, and means responsive to the absence of an intermediate frequency FM sound carrier in the receiver for energizing the said coil of said relay, said means comprising a capacitor in circuit with said coil and adapted when charged to a predetermined potential to discharge through said coil and thereby energize said coil, first means connected to said capacitor to supply a charge thereto, and second means for diverting the charge from said capacitor only when an intermediate frequency FM sound carrier is present.

Claim 3

In a receiver according to claim 1 in which said capacitor is an electrolytic capacitor, said first means is a source of direct current and a resistor connecting the source to one side of said capacitor, and said second means comprises a transistor having its collector-emitter path connected between the said one side of said capacitor and a point to which said source will drain, said transistor having its base connected to a predetermined point in the audio channel of said receiver, said transistor base-emitter voltage biasing said transistor to conduction when an intermediate frequency FM sound carrier signal is present and to nonconduction when no intermediate frequency FM signal carrier is present.

Claim 4

In a receiver according to claim 2 in which said audio channel includes demodulator means having a point which is at one potential when an audio signal is present and at another potential when an intermediate frequency FM signal is absent, said point in said demodulator means being the said predetermined point which is connected to said base of said transistor.

Claim 5

In a receiver according to claim 3 in which said demodulator means comprises a demodulator tube having a cathode resistor connected between the cathode of the tube and ground, said predetermined point being the cathode end of said cathode resistor.

Claim 6

In a receiver according to claim 4 in which said transistor is an NPN transistor and has its collector connected to said one side of said electrolytic capacitor and resistance means connecting the emitter of said transistor to ground.

Claim 7

In a receiver according to claim 5 in which said resistance means comprises a resistor element connected between a direct current source and ground and having an adjustable tap connected to said emitter.

Claim 8

In a receiver according to claim 6 which includes a second electrolytic capacitor connected between ground and the end of said resistor element opposite ground.

Claim 9

In a receiver according to claim 7 which includes a second resistor element connected between the cathode end of said cathode resistor and the base of said transistor, and a capacitor connected between said base and ground.

Claim 10

In a receiver according to claim 3 in which said transistor is an NPN transistor and said demodulator means comprises a ratio detector circuit having a coil, a first diode having its anode connected to one end of said coil and its cathode to the base of said transistor, and a second diode having its cathode connected to the other end of said coil and its anode connected to the emitter of said transistor, the collector of said transistor being connected to said one side of said electrolytic capacitor.

Claim 11

In a receiver according to claim 3 in which said transistor is an NPN transistor and said demodulator means comprises a ratio detector circuit having a coil, a first diode having its anode connected to one end of said coil and a second diode having its cathode connected to the other end of said coil, a pair of serially connected capacitors connecting the cathode of the first diode with the anode of the second diode and the juncture of said capacitors being grounded, a pair of serially arranged resistors arranged in parallel with said capacitors with the juncture thereof forming the audio output connection, a second pair of serially arranged resistors connected between the cathode of said first diode and ground, and the juncture thereof being connected to the base of said transistor, the emitter of said transistor being connected to ground.