FAIL-SAFE CIRCUIT ARRANGEMENT

• US 3,553,488 A
• Filed: 02/06/1968
• Issued: 01/05/1971
• Est. Priority Date: 02/06/1968
• Status: Expired due to Term

• Current Assignee: Darrow John O. G., Raymond C. Franke
• Original Assignee: Darrow John O. G., Raymond C. Franke
• Inventors: DARROW JOHN O G, Franke, Raymond C.
• Application Number:
  US04/707,363
• Time in Patent Office: 1,064 Days
• US Class Current: 327/1
• CPC Class Codes: F16P 3/08 in connection with the locking of doors, covers, guards, or like members giving access to moving machine parts
• CPC Class Codes: H03K 17/9502 Measures for increasing reliability
• CPC Class Codes: H03K 17/9547 with variable amplitude

Abstract

A fail-safe signal absence detection circuit conditioned by the presence of input signals for establishing one of two conductive states of a trigger and a switching circuit for maintaining an oscillating circuit in a nonoscillating condition wherein an output signal is not produced and conditioned by the absence of input signals for alternately establishing the two conductive states of the trigger and the switching circuit for permitting the oscillating circuit to assume an oscillating and a nonoscillating condition wherein an output signal is produced.

Claims

1. A fail-safe circuit for detecting the presence and absence of an input signal comprising:

an oscillating circuit;

first means coupled to said oscillating circuit for rectifying and storing a predetermined voltage charge when said input signal is present, and when said oscillating circuit is oscillating but said input signal is absent;

an electronic regenerative trigger circuit coupled to said first means which assumes a first stable current conducting condition whenever said predetermined voltage charge is developed and which assumes a second stable current conducting condition whenever said predetermined voltage charge is sufficiently decreased;

second means coupled to said trigger circuit for maintaining said oscillating circuit in a nonoscillating condition whenever said trigger circuit is in said first stable current conducting condition and for causing said oscillating circuit to oScillate whenever said trigger circuit is in said second stable current conducting condition; and

third means coupled to said trigger circuit for only providing an output signal whenever said trigger circuit alternately assumes said second and said first stable current conducting conditions due to the absence of said input signal.

2. A fail-safe circuit as defined in claim 1, wherein said first means includes a transistor amplifier, a diode rectifier and a four-terminal capacitor.

3. A fail-safe circuit as defined in claim 1, wherein said trigger circuit comprises a transistorized Schmidt trigger.

4. A fail-safe circuit as defined in claim 1, wherein said oscillating circuit comprises a Colpitts-type oscillator.

5. A fail-safe circuit as defined in claim 1, wherein said oscillating circuit includes a tuned circuit having an inductor coil and split capacitors and having the input signal induced into said inductor coil.

6. A fail-safe circuit as defined in claim 1, wherein said second means comprises a switching circuit having a first and a second conductive condition.

7. A fail-safe circuit as defined in claim 6, wherein said switching circuit controls the oscillating condition of said oscillating circuit by controlling the level of operating potential applied to said oscillating circuit.

8. A fail-safe circuit as defined in claim 1, wherein said third means includes an amplifier and a rectifier circuit.

9. A fail-safe circuit as defined in claim 8, wherein said rectifier circuit comprises a voltage doubler.

10. A fail-safe circuit as defined in claim 8, wherein said amplifier comprises a first and a second transistor in which a diode interconnects the emitter and base electrodes of said second transistor.

11. A fail-safe circuit arrangement comprising, a source of input signals, a first amplifier circuit coupled to said input source for amplifying said input signals, a rectifier-filter circuit coupled to said first amplifier circuit for rectifying and filtering said amplified signals, a bistable trigger circuit having a first and a second conductive condition coupled to said rectifier-filter circuit, a switching circuit coupled to said bistable trigger circuit and assuming a first and a second conductive condition in accordance with the conductive conditions of said trigger circuits, an oscillating circuit intercoupled from said switching circuit to said first amplifier and controlled by the conductive condition of said switching circuit, a second amplifier circuit coupled to said bistable trigger circuit, and a rectifier circuit coupled to said second amplifier circuit for providing an output when and only when the signals from said input source are absent.

12. A fail-safe circuit as defined in claim 11, wherein said second amplifier circuit comprises a first and a second transistor and includes a diode for interconnecting the emitter electrode to the base electrode of said second transistor.

13. A fail-safe circuit as defined in claim 11, wherein said rectifier circuit comprises a voltage doubler.

14. A fail-safe circuit as defined in claim 11, wherein said bistable trigger circuit comprises a Schmidt trigger which assumes and remains in a first conductive condition during the presence of said input signals.

15. A fail-safe circuit as defined in claim 14, wherein said rectifier-filter circuit includes a four-terminal capacitor which maintains a sufficient voltage charge for holding said Schmidt trigger in its first conductive condition during the presence of said input signals.

16. A fail-safe circuit as defined in claim 15, wherein said switching circuit controls the oscillating condition of said oscillator circuit by controlling the level of operating potential applied to said oscillator circuit.

17. A fail-safe circuit as defined in claim 11, wherein said source of input signals comprises an electromagnetic pickup device.

18. A fail-safe circuit as defined in claim 17, wherein said oscillator circuit includes inductance-capacitance tuning.

19. A fail-safe circuit as defined in claim 18, wherein said electromagnetic pickup device includes an inductor forming the inductance of said inductance-capacitance tuning.