US2981898A

US2981898A - Electronic timer

Info

Publication number: US2981898A
Application number: US646861A
Authority: US
Inventors: John Dale E St
Original assignee: Individual
Current assignee: Individual
Priority date: 1957-03-18
Filing date: 1957-03-18
Publication date: 1961-04-25
Anticipated expiration: 1978-04-25

Description

April 1961 D. E. ST. JOHN 2,981,898

ELECTRONIC TIMER Filed March 18, 1957 w wm Cl 62 $3 t4 t5 "L6 t7 t6 t9 INVENTOA. DALE 5. 5T. IJOHN BY H/S HTTORNEYS. HARP/6, mac/15, FOSTER 6c HARRIS United States Patent M ELECTRONIC TIMER Dale E. St. John, Torrance, Calif. (25914 Chalmette Lane, Rolling Hills Estates, Calif.)

Filed Mar. 18, 1957, Ser. No. 646,861 6 Claims. (Cl. 331-112) This invention relates to electronic timers and in particular to an electronic timing circuit for providing a pulse output at a predetermined repetition rate, which output may be used for operating a relay or other indicating device.

It is an object of the invention to provide a compact and simple timing circuit which is resistant to shock and vibration and which is stable over wide limits of temperature. and humidity. A further object of the invention is to provide such a timing circuit in which the pulse interval is adjustable over a range of milliseconds to seconds and longer.

, It is another object of the invention to provide a timing circuit which utilizes a diode operated in the zener breakdown region to provide noise trigger pulses for a single swing oscillator. Another object of the invention is to provide such a timing circuit in which the breakdown voltage is applied to the zener diode through a resistancecapacitance charging circuit with breakdown occurring at very small currents, such as in the order of ten microamperes, permitting the desired time constant to be obtained with very large values of resistance and small values of capacitance. A further object of the invention is to provide such a timing circuit in which the single swing oscillator is triggered by the breakdown noise currents existing in the zener diode.

It is another object of the invention to provide a single swing oscillator which may be operated without requiring a bias voltage to prevent it from running free. A further object of the invention is to provide such an oscillator which may be biased toward conduction to increase its sensitivity.

It is an object of the invention to provide a timing circuit having more than one mode of operation. A further object of the invention is to provide such a timing circuit which, when operated in one mode, will provide an indication at the end of a predetermined time interval the beginning of which is initiated by an outside source. Another object of the invention is to provide such a timing circuit which, when operated in another mode, will provide output indications continuously at a predetermined repetition rate.

It is another object of the invention to provide a timing circuit having a high impedance resistance-capacitance charging circuit with a low impedance voltage divider for controlling the pulse interval of the circuit so that the control may be positioned remote from the remainder of the circuit without affecting the stability of the circuit. It is a further object of the invention to provide a timing circuit utilizing a zener diode for triggering the oscillator and a similar zener diode as a voltage regulator across the charging circuit so that temperature variations and the like which affect both diodes cause the voltage applied to the triggering diode and the breakdown potential of the triggering diode to vary in the same manner thereby not affecting the pulse interval.

The invention also comprises novel detailsof contruction and novel combinations and arrangements of parts,

which will more fully appear in the course of the following description. The drawing merely shows and the description merely describes a preferred embodiment of the present invention which is given by way of illustration or example.

In the drawing:

Fig. 1 is a schematic diagram of a preferred embodiment of the timing circuit of the invention; and

Figs. 2, 3 and 4 are graphs showing the voltage levels at various points within the circuit of Fig. 1 during the operation of the circuit.

In the circuit of the invention, a capacitor is charged through a large resistance until the voltage across the capacitor reaches the zener breakdown potential of a diode. Upon breakdown of the diode, noise currents are generated therein which excite a single swing oscillator. A sensitive relay in the power supply circuit to the oscillator is actuated during oscillations because of the increase in current therein. A set of contacts of the relay is connected to short out the charging capacitor. The contacts are held closed for a predetermined period by a capacitor connected across the relay coil and,'at the end of such period, the contacts are opened and the cycle is repeated. Additional sets of contacts may be provided on the relay for output indication or control purposes.

Referring now to Fig. l of the drawing, the timing circuit of the invention includes a charging circuit 10, a triggering element 11, a single swing or blocking oscillator 12, an output circuit 13 and a power source 14.

A resistor 17 and a zener diode 18 are connected in series across the power supply to form a voltage regulator circuit for maintaining a substantially constant voltage between the

points

19 and 20. The voltage drop across a silicon crystal diode at the negative breakdown point is substantially independent of current. Also at low current levels of approximately zero to two hundred microamperes, the diode current due to reverse breakdown voltage is not a steady or direct current but consists of pulses of random and short duration with a band width of nearly one hundred kilocycles. These random pulses of current are usually referred to as noise. The breakdown point is referred to as the zener point and diodes exhibiting such characteristics are called zener noise diodes. Therefore, the term zener noise diode as used herein refers to any rectifier which has such characteristics.

A voltage divider consisting of a resistor 21 and a potentiometer 22 is connected between the points 19,

20. A time constant resistor 25 and a time constantcapacitor 26 are connected in series between the positive side of the regulated voltage at point 19 and an arm 27 of the potentiometer 22. A capacitor 28 is connected between the arm 27 and the negative side of the power supply.

The triggering element 11 consists of a zener noise diode 31 with its cathode connected to the junction point 32 of the time constant resistor 25 and time constant capacitor 26. The anode of the zener noise diode 31 is connected into the single swing oscillator 12 to provide a trigger pulse therefor.

The single swing oscillator 12 includes a transistor 33, a pulse transformer 34 and a rectifier 35, the emitter of the transistor being connected to the negative supply point 20 and the collector of the transistor being connected through the primary winding 36 of the pulse transformer and the parallel combination of a capacitor 37 and a relay coil 38 to the positive side of the supply. An n-p-n transistor is shown in Fig. 1. However, it is understood that a p-n-p transistor may be used with appropriate p0 larity changes and that otherthree-element electron con- 3 Patented Apr. 25,.1961

3. trol devices having similar characteristics may be substituted for the transistor 33.

The rectifier 35, which may be a conventional germanium diode, is connected in series with they secondary winding 40 of the pulsetransformer between the negative supply point 20 and the. base of the transistor. It is-also preferred to provide a resistor 41. connected between the positive supply point 19 and the base of the transistor.

A normally open set of contacts 42 of the relay is connected across the time constant capacitor 26 in series with contact 45 and arm 46 of a mode selector switch 47 and a. current limiting resistor 43 for discharging the capacitor through the resistor when the relay is energized. Another set of contacts 44 may be provided on the relay to produce an output for indication and/or control-purposes.

Contact 48 of the mode selector switch 47 is connected through a normally open switch 49 to the side of the capacitor 26 not connected directly to the resistor 43 so that the capacitor 26 may be discharged by closing switch 49 when the mode selector switch is in the reverse position, i.e., arm 46 engaging contact 48.

When power is applied to the timing circuit, a regulated voltage is maintained across the

points

19, 20 and a portion of the regulated voltage is applied across the time constant resistor 25 and capacitor 26, the actual voltage being controlled by the setting of the potentiometer 22. The capacitor 26 is charged through the resistor 25, the voltage at point 32 rising exponentially toward the voltage of the point 19. When the voltage at the point 32 reaches the zener voltage of the diode 31, the diode breaks down, causing its back impedance to change from an extremely high value to a very low value, resulting in a pulsating current flow to the base of the transistor through the diode.

I It is preferred to make the value of the resistor 25 very large so that there will be only a very small current in the diode 31. The diode current may be in the order of ten microamperes, thereby permitting the resistor 25 to be in. the order of several megohms in a typical circuit. Following its breakdown, the zener noise diode 31 functions as a voltage regulator maintaining the potential of the point 32 substantially constant.

As previously described, the small value of breakdown current in the diode 31 produces high noise current pulses which are coupled to the base of the transistor. A low impedance path for the high frequency zener noise current exists through the base circuit of the transistor 33, the capacitor 28 and the capacitor 26.

A smallstatic current exists in the transistor through the winding 38 of the relay, the winding 36 of the transformer and thecollector and emitter of the transistor. However, this static current will not cause the oscillator to run free. The static collector current in the transistor is increased by means of the. resistor 41 connected between the positive supply point 19 and the base of the transistor, but the current is not increased to the point Where the relay will be actuated. The purpose of providing a static transistor base current is to increase the sensitivity of the, oscillator, i.e., reduce the magnitude of the pulse required to trigger the oscillator. I

When the zener noise diode 31 breaks down, the first pulse of noise current into the. base of the transistor produces an increase in transistor collector current. This increase in current at the primary winding of the pulse transformer generates a pulse inthe secondaryiof the transformer, the transformer windings and the diode 35 being connected with polarities such that the pulse in the secondary is conducted through the diode to the base of the transistor, producing positive feedback or regeneration driving the transistor into saturation. This regenerative action occurs practically instantaneously. The rectifier 35 prevents the positive noise pulse generatedby the zener noise diode 31 from being shunted to the negative supply point 20 through the secondary winding of the pulse transformer. The rectifier 35 also prevents continuous oscillation in the oscillator circuit since it presents a very high impedance to the feedback circuit on one half of an oscillation cycle.

When the transistor is driven into saturation, the current therein ceases to change and the field in the transformer collapses and a pulse of polarity opposite to that of the previous pulse is generated in the secondary of the transformer. This second pulse is blocked from the base of the transistor by the rectifier 35. The zener noise diode 31 continues to generate noise pulses which are amplified by the transistor and coupled to the pulse transformer secondary via the primary. The voltage applied to the rectifier 35 by the secondary of the pulse transformer decreases exponentially from the negative value to which it went following the decay of the transformer field and when this voltage has decreased sufficiently, the rectifier will again conduct the positive going noise pulses, providing positive feedback to the base of the transistor and another single swing oscillation cycle is initiated. The single swing oscillation cycle will be repeated continuously as long as the zener noise diode 31 is conducting. The single swing oscillation cycles are normally of very short duration, the circuit components ordinarily being selected to provide a plurality of such cycles in a fraction of a millisecond.

The collector current of the transistor passes through the parallel combination of relay coil 38 and capacitor 37, the characteristics of the components preferably being such that a single cycle of oscillation will not cause the relay to be actuated. However, a plurality of cycles of oscillation will build up a charge on the capacitor 37 and the relay will then be energized. Actuation of the relay closes the set of contacts 42 and discharges the time constant capacitor 26 through the resistor 43 which limits the peak discharge current of the capacitor. The resistance of the resistor 43 is quite small so that the discharge time of the capacitor 26 is not a factor in the operation of the timing circuit. The charge on the capacitor 37 maintains the relay energized for a short period after the transistor current has dropped to its static value, thereby insuring the complete discharge of the capacitor 26. When the relay is deenergized, the time constant capacitor 26 begins to charge through the time constant resistor 25'and another timing cycle is initiated.

The operation of the timing circuit is shown graphically in Figs. 2, 3 and 4, wherein Fig. 2 represents the voltage at the base of the transistor, indicated as A, Fig. 3 represents the voltage at the collector of the transistor, indicated as B, and Fig. 4 represents the voltage at the anode of the rectifier 35, indicated as C. Before time t1, the time constant capacitor 26 is charging exponentially. At time :1, the zener noise diode 31 breaks down and the first pulse of noise current triggers the single swing oscillator and the transistor becomes saturated practically instantaneously. The collector voltage B drops to a low value and the voltages at points A and C increase.

In the interval between t1 and t2, the transistor is in saturation and the field of the transformer begins to collapse, producing the negative spike in the voltage at C at time t2. This negative pulse is not transmitted tothe transistor base because of the rectifier 35; however, the noise pulses from the zener diode 31 continue to appear at the transistor base and are amplified by the transistor. The negative pulse at point C decays exponentially during the interval 12 to 23 until the potential at point C rises to a value at which an amplified positive going noise pulse will be conducted by the diode 35 from the pulse transformer to the base of the transistor, thereby initiating another single swing oscillation cycle.

After a number of such single swing oscillation cycles, three cycles as shown in Figs. 2, 3 and 4, the relay is ac tuated, the time constant capacitor is discharged and the charging thereof begins anew. Time t6 indicates the energization of the relay and time t7 indicates the breakdown point of the zener noise diode which initiates another oscillation cycle, the time 17 corresponding to the time 21. The elapsed time from t1 to I6 is in the order of a fraction of a millisecond while the elapsed time between t6and 17 may be varied from milliseconds to seconds and longer.

Since the single swing oscillator is driven into saturation practically instantaneously by a single noise pulse applied at the base of the transistor, the amplification characteristics of the transistor are not critical and, hence, instability thereof does not adve rsely affect the operation of thetiming circuit. The single swing oscillator of the invention is not free running and is inherently stable, not requiring any bias to keep it in the nonoscillating condition. Since it is normally not free running, its sensitivity can be increased by providing the positive bias through the resistor 41 so that extremely small noise pulses will produce oscillation.

. The time required for the capacitor 26 tocharge to a potential so that the potential at point 32 is at the breakdown potential of the zener noise diode 31, and therefore, the repetition rate of relay actuation, is controlled by the potentiometer 22. When the arm 27 is moved to a more positive value, the magnitude of the voltage applied to the time constant resistor and capacitor is reduced; however, the voltage between

points

20 and 27 is in series with the voltage across the charging capacitor. Therefore the total voltage from point 20 to point 32 reaches the breakdown voltage of diode 31 with less charge on capacitor 26. This circuit permits precise control of the pulse interval while allowing use of a relatively low impedance control component. The potentiometer 22 may have a resistance in the order of ten thousand ohms and, therefore, may be located remote from the remainder of the circuit without producing adverse effects.

For stable operation of the timing circuit, the time constant resistor 25 and capacitor 26 must be stable components. Stable resistors are relatively easy to obtain and are small in size; however, stable capacitors, particularlarly in the higher capacitance value, are not easily produced and are extremely bulky. Since it is desired to maintain the current through the diode 31 quite small, it becomes possible to make the time constant resistor quite large, such as in the order of several megohms, thereby permitting the capacitor to be relatively small while obtaining the desired time constant. This permits construction of a stable and at the same time compact timing circuit.

Of course, it is not essential that a voltage regulator be provided for the charging circuit; however, the use of a voltage regulator greatly improves the accuracy and stability of the circuit. The particular voltage regulator shown in Fig. 1, wherein the zener diode 18 is similar to the zener noise diode 31, contributes to the improved operation of the circuit in another manner. The zener breakdown potential of the diode 31 increases with in creasing temperature and this characteristic would ordinarily adversely affect the stability of the timing circuit by requiring capacitor 26 to charge to a higher potential before diode 31 breaks down. However, when a similar diode is used in the voltage regulator circuit, the regulated voltage will also increase with increasing temperature since the breakdown potential of the diode used as a regulator will also increase. The increase in regulated voltage causes capacitor 26 to charge to the zener noise breakdown point at a faster rate. Thus the repetition rate is maintained constant substantially independent of variations in temperature since the two conditions have opposite effects on the cycling time.

While the output circuit 13 has been shown as a relay which is actuated by a plurality of pulses from the single swing oscillator, it is understood that other integrating devices may be substituted for the relay and that other output circuits may be actuated by a single pulse from the single swing oscillator when desired by coupling direct to the transistor collector.

When operated as described above, the circuit will pro vide output pulses continuously at the predetermined rate.

The circuit may also be operated as an interval timer to provide a single output indication at a predetermined time interval after initiation of the time interval by an outside source. The mode selector switch 47 is moved to the reverse position with arm 46 engaging contact 48. Then the time constant capacitor 26 is discharged by momentarily closing switch 49, thereby initiating the time interval. The end of the time interval will be indicated in the usual manner, such as through the set of contacts 44 of the relay. However, the energization of the relay will not discharge the time constant capacitor and another timing interval will not start until the switch 49 is again actuated.

Although an exemplary embodiment of the invention has been disclosed and discussed, it will be understood that other applications of the invention are possible and that the embodiment disclosed may be subjected to various changes, modifications and substitutions without necessarily departing from the spirit of the invention.

I claim as my invention:

1. In a timing circuit for operation from a direct current voltage source, the combination of: a resistancecapacitance charging circuit connected across a voltage source, said charging circuit including a point at which the voltage increases with time; a single swing oscillator having a trigger input circuit, with a voltage pulse on said input circuit initiating an oscillation cycle of said oscillator; a zener noise diode having an anode and a cathode, said cathode being coupled to said point, said anode being coupled to said trigger input circuit, with said zener diode producing noise voltage pulses for triggering said oscillator when the voltage across said diode increases to the zerer breakdown potential; and means for discharging said charging circuit by shunting the capacitance of said charging circuit.

2. In a timing circuit for operation from a direct current voltage source, the combination of: a resistancecapacitance charging circuit connected across a voltage source, said charging circuit including a point at which the voltage increases with time; a single swing oscillator having a trigger input circuit, with a voltage pulse on said input circuit initiating an oscillation cycle of said oscillator; a zener noise diode having an anode and a cathode, said cathode being coupled to .said point, said anode being coupled to said trigger input circuit, with said zener diode producing noise voltage pulses for triggering said oscillator when the voltage across said diode increases the zener breakdown potential; and switch means energized by a plurality of oscillations of said oscillator for discharging said charging circuit by shun-ting the capacitance of said charging circuit.

3. In a self-triggered timing circuit for operation from a direct current voltage source, the combination of: a voltage divider connected across the voltage source, said voltage divider including a potentiometer having a movable arm for varying the voltage appearing at said arm; resistor and capacitor means connected in series between a positive end of said voltage divider and said arm with the resistance of said resistor means being at least an order of magnitude greater than the resistance of said potentiometer, the voltage at the junction of said resistor and capacitor means increasing with time; a singleswing oscillator having a control element, with a voltage pulse on said control element initiating an oscillation cycle of said oscillator; and a zener noise diode having an anode and a cathode, said cathode being coupled to said junction and said anode being coupled to said control element, with said zener diode producing noise voltage pulses for triggering said oscillator when the voltage across said diode increases to the zener breakdown potential.

4. In a timing circuit, the combination of: a voltage source having a point at which the voltage increases with time; a single swing oscillator having a control element,

with a-voltage pulse on said control element initiating an oscillation cycle of said oscillator; and a IZBHGI' noise diode having an anode and a cathode, said :cathode being coupled to said point and said anode being coupled to said control element, with said zener diode producing noise voltage pulses for triggering said :oscillator when the voltage across said diode increases to'the zener breakdown potential.

5. A timing circuit asdefined in claim 1 in which said single swing oscillator includes a transistor having a base, emitter and collector, a pulse transformer, first circuit means for coupling one winding of said pulse transformer in series with said transistor through said collector and emitter across a voltage source,'and second circuit means for coupling the other winding of said pulse transformer to said base in positive feedback relation,;said second circuit means including a rectifier connected in series with said other winding, and with said trigger input circuit connected to said base.

6. A timing circuit as defined in claim 5 including positive bias means connected to said transistor base for biasing said transistor toward increased conduction.

References Cited in the file of this patent UNITED STATES PATENTS 2,787,707 Cockburn Apr. 2, 1957 2,846,580 Light Aug. 5, 1958 2,810,080 Trousdale Oct. 15, 1957 2,841,712 Hoge et a1. July 1, 1958 2,846,580 Light Aug. 5, 1958 2,850,648 Elliott Sept. 2, 1958 2,877,399 Shaull Mar. 10. 1959 OTHER REFERENCES Article: Transistor Blocking "scillator Counter, by Painter, pp. 152-156 of The Application of Transistors to Military Electronics Equipment," dated Nov. 24, 1953.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 2;,98lq898 April 25 1961 Dale E. St., John It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent? should read as "corrected below.

Column 5 line 8 for "at" read to lines 38 and 39 for "particularlarly" read particularly column 6 line 49, after "increases" insert to Signed and sealed this 1911; day of September 19610 (SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer I Commissioner of Patents USCOMM-DC UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2, 981q898 April 25 1961 Dale E'. St John It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent? should read as corrected below.

Column 5 line 8 for "at" read to -f-; lines 38 and 39 for "particularlarly" read particularly vcolumn 6 line 49, after "increases" insert to e.

Signed and-sealed this 19th day of September 1961o (SEAL) Attest:

ERNEST W. SWIDER Attesting Officer DAVID L. LADD Commissioner of Patents USCOMM-DC