US3045149A

US3045149A - Command circuit

Info

Publication number: US3045149A
Application number: US43254A
Authority: US
Inventors: David S Willard
Original assignee: Individual
Current assignee: Individual
Priority date: 1960-07-15
Filing date: 1960-07-15
Publication date: 1962-07-17
Anticipated expiration: 1979-07-17

Description

y 1962 D. s. WILLARD 3,045,149

COMMAND CIRCUIT Filed July 15, 1960 THERMAL SWITCH INVENTOR. DAVID S. WIL ARD 14 ATTORNEY United States Patent 3,045,149 COMMAND CIRCUIT David S. Willard,'P.0. Box 5, High Rolls- Mountain Park, N. Mex. Filed July 15, 1960, Ser. No. 43,254 2 Claims. (Cl. 317-147) (Granted under Title 35, US. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to me of any royalty thereon.

The purpose of this invention is to provide a command circuit that will respond only to a command signal meeting the following conditions: (1) An alternating signal of predetermined frequency pulsed on and off periodically; (2) A ratio of on time to off time exceeding a predetermined value; (3) An on time less than a predetermined amount; (4) An ofli time less than a predetermined amount; and (5) An off time greater than a predetermined amount. The necessity for satisfying all of these conditions insures a high degree of protection against false operation of the command circuit.

The invention will be explained in detail with reference to the specific embodiment thereof shown in the accomp anying drawing in which FIG. 1 shows the general form of the command signal, and

FIG. 2 is a schematic diagram of a command circuit in accordance with the invention.

Referring to the drawing, when an alternating current command signal of the type shown in FIG. 1, and meeting the five requirements defined above, is applied to terminal 1, switch 2 is actuated to its closed position. This switch may be included in any external circuit which it is desired to control by the command signal.

The circuit of FIG. 2 is designed for use with command signals in the audio frequency range. Relay K, may be of the vibrating reed type having an actuating coil 3, a stationary contact 4 and a movable contact 5 in the form of a vibrating reed. An alternating current in coil 3 causes the reed to vibrate. The amplitude of the vibration, however, will be sutficient for reed 5 to touch contact 4 only when the frequency of the energizing current equals, or very nearly equals, the resonant frequency of the reed. Consequently, a command signal, having substantially the same frequency as the natural frequency of the reed, will cause contacts 4-5 to make and break at an audio rate, resulting in a pulsed direct current through the coil of.K of sufficient average value to actuate this relay.

Closing the contacts of K connects C which has previously charged through R to the potential of terminal 6, across the coil'of K allowing this condenser to discharge through the coil and actuate K The charge in C is only enough for a momentary actuation of the relay and the resistance of R is high enough that the current flow to the coil of K is below that required to hold the relay in the energized state. However, when K is energized by C a holding circuit is established from the coil of K through the contacts of K and contacts 1-2 of K to condenser C This condenser is larger than C, and was previously charged through contacts 1--2 to the voltage of terminal 6. The discharge of C through the coil of K holds this relay energized either until K opens or until the discharge current has fallen to the dropout level of the relay. It is therefore seen that K can not remain energized for longer than a fixed interval determined principally by the size of C The R C circuit is designed to have a very short time constant so that C recharges almost immediately upon release of K and closure of contacts 3-2. For

3,045,149 Patented July 17, 1962 ICC? example, R may have a resistance of 47 ohms and C; a capacity of microfarads, giving a time constant of about 0.5 millisecond.

The R C circuit is designed to have a relatively long time constant as compared to the R --C circuit. For example, R may have a value of 68000 ohms and C a capacity of 0.5 microfarad, giving a time constant of 34 milliseconds. Since C can not charge while the contacts of K are closed, the time constant of this circuit determines the minimum ofi period of the command signal. It the command signal has an o period less than this minimum, including a zero off period (continuous signal), only a single actuation of K can occur.

During the time that K is actuated, heating element 7 of thermal switch 2 is energized through contacts 45.

. This switch is designed so that, when pulses of current of a constant specified amplitude are periodically applied to it, the ratio of the pulse duration to the interval between pulses, i.e., the ratio of the on time of the energization to the olf time, must exceed a predetermined value to produce enough heating to cause the switch contacts to close. For example, the predetermined value of this ratio may be unity. Stated in other words, the average value of the pulsed current flow in element 7 must exceed a predetermined amount in order to close the switch contacts.

Returning to the five previously listed conditions that must be met by the command signal before it is capable of causing actuation of switch 2, the features of the circuit responsible for these requirements are as follows:

Condition 1: The signal must be an alternating signal of the required frequency to actuate relay K The signal must be pulsed in order to secure energization of thermal switch 2 over a suificient period of time to cause its contacts to close. If the signal were continuous, the contacts of K would remain closed and only a single actuation of K applying only a single pulse of energy to switch 2, would occur.

Condition 2: Since, for switch 2, the ratio of the heating current pulse duration to the interval between current pulses must exceed a predetermined value to operate the switch and since the characteristics of the K circuit prevent this ratio from ever exceeding the on time to otf time ratio of the command signal, it follows that the latter ratio must exceed the predetermined ratio for the switch 2 in order to effect operation of the switch.

Condition 3: As already explained, relay K will not remain energized longer than a fixed interval determined mainly by the size of C The maximum duration of energizing pulse for switch 2 equals this fixed interval. For this maximum pulse duration, there is a corresponding maximum interval between pulses above which the required energization of switch 2 will not be achieved. If the on period of the command signal exceeds the K fixed interval plus the above defined maximum interval between pulses, switch 2 will not be actuated.

Condition 4: If the command signal off time exceeds the above defined maximum interval between pulses, actuation of switch 2 will not occur.

Condition 5: If the command signal off time is less than the time required for C to receive sufiicient charge to actuate K only a single actuation of K will occur and, consequently, switch 2 will not be actuated.

I claim:

1. A command circuit comprising: frequency sensitive means, having a set of normally open contacts, for receiving a command signal and closing said contacts when said signal has a predetermined frequency; a relay having two sets of normally open contacts, a set of normally closed contacts and an actuating coil therefor; first and second capacitors; a source of direct current; means including a resistor for connecting said first capacitor across a said source; means including the contacts of said frequency sensitive means for connecting the first capacitor across the coil of said relay; means including the normally closed contacts of said relay for connecting the second capacitor across said source; means including one set of normally open contacts of said relay and the contacts of said frequency sensitive means in series for connecting the second capacitor across the coil of said relay; a current actuated switch responsive only to an energizing current having an average value in excess of a predetermined amount; and means for applying an energizing (5. current to said switch through the other set of normally open contacts of said relay.

2. Apparatus as claimed in claim 1 in which said switch is a thermal switch having a heating element through which said energizing current passes.

References Cited in the file of this patent UNITED STATES PATENTS 2,347,194 Holliday Apr. 25, 1944 2,724,074 Welker Nov. 15, 1955 2,914,709 Rabinow Nov. 24, 1959