US3801799A

US3801799A - Capacitance and resistance-responsive control circuits

Info

Publication number: US3801799A
Application number: US00280219A
Authority: US
Inventors: C Atkins
Original assignee: Wagner Electric Corp
Current assignee: Cooper Industries LLC
Priority date: 1972-08-14
Filing date: 1972-08-14
Publication date: 1974-04-02
Anticipated expiration: 1991-04-02
Also published as: IT990245B; GB1411683A; FR2196492A1; DE2341161A1; CA1008145A; JPS4959277A

Abstract

Control circuits for sensing and responding to impedance changes caused by a mammal in contact with or in proximity to an antenna or other sensor. Each circuit includes at least one tuned circuit which is periodically energized to cause it to ''''ring'''' at its natural frequency, i.e., to produce damped oscillations. Each tuned circuit includes an antenna or other sensor by which an external impedance is coupled to the tuned circuit. Thus, the oscillations in the tuned circuit are attenuated to a degree determined by the capacitive and resistive components of the external impedance. In the case of mammals, the resistive component is substantial and causes a marked change in the amplitude and duration of the damped oscillations in the tuned circuit, which are then enhanced and/or detected to produce a predetermined control signal.

Description

Atkins CAPACITANCE AND RESISTANCE-RESPONSIVE CONTROL Apr. 2, 1974 Primary ExaminerL. T. Hix Attorney, Agent, or Firm-Eyre, Mann & Lucas [57] ABSTRACT Control circuits for sensing and responding to impedance changes caused by a mammal in contact with or in proximity to an antenna or other sensor. Each circuit includes at least one tuned circuit which is periodically energized to cause it to ring at its natural frequency, i.e., to produce damped oscillations. Each tuned circuit includes an antenna or other sensor by which an external impedance is coupled to the tuned circuit. Thus, the oscillations in the tuned circuit are attenuated to a degree determined by the capacitive and resistive components of the external impedance. In the case of mammals, the resistive component is substantial and causes a marked change in the amplitude and duration of the damped oscillations in the tuned circuit, which are then enhanced and/0r detected to produce a predetermined control signal.

20 Claims, 3 Drawing Figures CIRCUITS [75] Inventor: Carl E. Atkins, Montclair, NJ. [73] Assignee: Wagner Electric Corporation,

Newark, NJ.

[22] Filed: Aug. 14, 1972 [211 App]. No.: 280,219

[52] 11.8. CI. 307/116, 317/DIG. 2 [51] Int. Cl H05 [58] Field of Search 317/DIG. 2,116, 146

[56] References Cited UNITED STATES PATENTS 3,199,033 8/1965 Atkins et al. 3l7/DIG. 2 3,200,306 8/1965 Atkins et al. 317/DIG. 2 3,255,380 6/1966 Atkins et al. 3l7/DIG. 2 3,492,542 1/1970 Atkins 3l7/DIG. 2 3,201,774 8/1965 Uemura 3l7/DIG. 2 3,324,647 6/1967 Jedynak..... 317/DIG. 2

a 0 P w 7* 1' I 9/ .-.-Zl $i5 4, .J

sr/vso/e PAIENTEUAPR 2 I974 SHEET 1 (If 2 CROSS REFERENCES TO RELATED APPLICATIONS The present invention may advantageously incorporate the HIGH-DISCRIMINATION ANTENNA ARRAY FOR CAPAClTANCE-RESPONSIVE CIR- CUITS disclosed and claimed in U. S. Pat. No. 3,740,567 issued June 19, 1973 upon copending application Ser. No. 245,799 filed on Apr. 20, 1972 in the name of Carl E. Atkins. The present invention advantageously incorporates a modified form of the circuits disclosed and claimed in copending application Ser. No. 255,155 entitled ELECTRONIC TIMING CIR- CUITS filed on May 19, 1972 in the name of Paul A. Carlson.

BACKGROUND OF THE INVENTION The present invention relates to capacitance and resistance responsive control circuitry. A wide variety of such condition-responsive circuits may be found in the prior art; see, for example, the following U.S. Pat. Nos.:

U.S. Pat. No. Inventor 3,200,304 Atkins et al. 3,200,305 Atkins 3.275.897 Atkins Re 26,828 Atkins et al. 3,314,081 Atkins et al. 3,3392 I 2 Atkins et al. 3,382,408 Atkins 3,435,298 Atkins ct al. 3,492,542 Atkins 3,551,753 Atkins 3,555,368 Atkins 3,564,346 Atkins 3,568,005 Atkins 3,568,006 Atkins 3,569,728 Atkins However, in certain applications such as detecting proper and improperusageof seat belts in automobiles, it is necessary to distinguish between mammalian and inanimate seat occupants. I have found that by coupling an occupant through an antenna associated with oneor more selected seat surfaces to a tuned circuit, and detecting the degree of attenuation of oscillations in the tuned circuit by such coupling, a superior degree of discrimination 'ma y be achieved between humans andother mammalsand inanimate objects (conductive or non-conductive) coming into proximity or contact with a conductive sensor positionedadjacent aselected seat-surface. I have found further that this superior discrimination is due to the different degrees ofdegradation thathumans and other mammals. and inanimate objects have on the merit factorQ of a tuned circuit when coupled thereto. The Q of atunedcircuitrepresents the circuits ability tostore energy as compared to theamount ofenergy it-dissipates.

When the complex impedanceformed by a human or other'rnammalor. by. an inanimate object iscoupled to thetunedrcircuit, theattenuation of'theoscillations in thetuned circuit'is thereby, increased. Since, the cornplex impedance of humans. and other; mammals includesa substantial resistive ordissipative component which is not'found inmost inanimate objects, the Q of a tuned circuitwill'be more sharply reduced'by. loading from humans and othermammals than from inanimate objects. As aresult'of'the decrease-inQ'of the tuned,

circuit, there is adetectable decrease in thearnplitude and/or durationofthedampedoscillations or ringing of the tuned circuit when that circuit is loaded by humans and other mammals.

The circuits embodying the invention are readily adaptable to a wide variety of uses, e.g., in plumbing control systems. Because of the superior sensitivity of the tuned circuit to loading by humans, an antenna with an exposed surface area of only a few square inches may be used which is smaller than any previously employed antenna area.

SUMMARY OF THE INVENTION The present invention is embodied in and carried out by a condition-responsive control circuit in which at least one tuned circuit is periodically energized by pulses from a pulse generator and permitted to ring (oscillate) at its natural frequency during each interpulse null, whereby the change in the amplitude and/or duration of oscillations of the tuned circuit caused by coupling of an external impedance having a substantial capacitive or resistive component to the tuned circuit isenhanced and/or detected to generate a control signal. The change in the amplitude of the damped oscillations may be enhanced before detection by providing an oscillator circuit which is shock excited into oscillation by the positive amplitude of the damped oscillations. In this way, the oscillator circuit will provide an oscillatory output when the positive amplitude of the damped oscillations in the tuned circuit is above a predetermined threshold, and no oscillatory output when the positive amplitude is below that predetermined threshold. The oscillating or non-oscillating output may then be detected to provide a control signal.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention may be better understood by reading the written description thereof in view of the accompanying drawings in which like numerals refer to like parts and:

FIG. 1' is a schematic circuit diagram showing a first embodiment of the invention; v

FIG. 2 is a schematic circuit diagram showing a second, preferred embodiment of the present invention; and

FIG. 3 is a schematic circuit diagram showing a third embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT by sensor 16. Detector 18 is connected to tuned circuit 1410 detect v arjations in thearnplitude and/or duration of the damped oscillations which form the outpiit of the tuned circuit 14; H, W W

Theoperationof the circuit 'shown in FIG. 1 is as follows. Pulse generator 10 supplies periodic pulses of positive polarity. to bias circuit 12', preferably in the frequency rangeof 10-20 KHz with a duty cycle of50%,

although other frequencies and duty cycles are acceptable. The value of capacitance Cl is selected to present nearly a short-circuit to the positive-going pulses. During each positive-going pulse capacitance Cl is charged with the polarity shown in FIG. 1, but the net bias on diode D1 is forward. Thus, current flows through inductance L1 of tuned circuit 14 thereby supplying energy to inductance L1 which is stored in its magnetic field. The impedance presented by inductance L1 is small, and consequently loads the output of pulse generator to an extent which prevents generation of a control signal by detector 18 and also prevents charging of C2 to any significant degree. After its stored energy reaches a maximum value, Ll presents a short circuit to the remainder of the generator output to limit the pulse amplitude to approximately ground potential during the remaining portion of the positive-going pulse. The energy thus stored in L1 is not involved in the ringing of the tuned circuit 14. When the trailing edge of the positive-going pulse output of generator 10 is reached. the magnetic field of L1 collapses, thereby inducing a current pulse through L1 to ground that causes C1 to be charged to a higher level.

Because diode D1 is a stored-charge diode, it stores more charge for a longer period of time and loses that charge faster than most semiconductor diodes. D1 has a substantial diffusion capacitance at the time the trailing edge. of the positive pulse occurs, and while this capacitance existsit prevents l )l f r o n 1 isolating the bias circuit 12 from the tank circuit 14. Because Cl is charged by the positive pulse from generator 10 and the inductive pulse from L1, it acts as a small battery and causes current to flow from ground through Ll, the diffusion capacitance of D1, Cl itself, and through pulse generator 10 to ground. Thus, energy is again electromagnetically stored in L1. The reverse current through D1 causes the rapid elimination of the diffusion Capacitance of D1, which is thus enabled to isolate the bias circuit 12 and the tuned circuit 14. Because diode D1 is back-biased by the remaining charge on C1, it presents a high impedance'to the tuned circuit 14 so as not to load it. Similarly, detector 18 is designed so as not to load tuned circuit 14. With energy no longer being'supplied to inductance L1, its magnetic field collapses, thereby producing ringing in the tuned circuit 14 at the natural frequency of that circuit. During the interpulse null, i.e., the time between successive positive-going pulses when the pulse amplitude is at ,approximately ground potential, capacitance Cl partially discharges through resistance R1. The values of resistance R1 and capacitance C 1 are chosen so that diode D1 remainsback-biased during the interpulse null. Resistance R1 and capacitance Cl may be replaced by a short circuit if the output of pulse generator 10 goes negative instead of being at approximately ground po- I tential between positive-going pulses.

When no load is coupled to sensor 16, the amplitude, frequency, and duration of the damped oscillations of tuned circuit 14 are relatively fixed and detector 18 is designed not to respond. However, when a load with a complex impedance having a substantial resistive component is coupled to the tuned circuit 14 through sensor 16, the Q of tuned circuit 14 is reduced, thereby sharply reducing the amplitude and/or duration of oscillations of the tuned circuit 14. An inanimate object, which in most cases does not possess a substantial resistive component, has little or no effect upon the Q of the tuned circuit 14 and, consequently, little or no effect on the amplitude and/or duration of oscillations of the tuned circuit 14. In addition, aload having a complex impedance with a substantial capacitive component is similarly capable of attenuating oscillations in the tuned circuit 14. when coupled thereto through sensor '16.

Detector 18 is preferably designed in known manner to respond to the reduction in amplitude and/or duration of the damped oscillations in the tuned circuit to thereby provide a control signal.

The change in amplitude and/or duration of the damped oscillation may advantageously be enhanced before detection. The preferred embodiment of the invention shown in FIG. 2 includes enhancing means.

Referring now specifically to FIG; 2, the circuit shown comprises a pulse generator 20 of the type disclosed and claimed in cross-referenced application Ser.

No. 255,155,

bias circuits

22 and 24 of the type shown in FIG. 1, tuned circuits 26 and 28, enhancing circuits 30 and 32, detector circuits 34 and 36, and controlled

load circuits

38, 40 and 42. The two parallel control signal generating circuits formed by bias-

circuits

22 and 24, tuned circuits 26 and 28, enhancing circuits 30 and 32, and detecting circuits 34 and 36 are identical in structure and function. Therefore, detailed'reference will be made to only one such circuit.

Bias circuit 22 is connected to receive the output of pulse generator 20 and channels it to tuned circuit 26 which comprises antenna Al and inductance L2, with the capacitance for the tuned circuit being formed by antenna Al and by capacitances C7 and C8 of enhancing circuit 30. Enhancing circuit 30 comprises transistor Q3, feedback capacitances C7 and C8, and bias resistances R8 and R9, and is connected to receive the output of tuned circuit 26 and to respond to providing an oscillatory input to detector 34.

Detector 34 comprises limiting resistances R12 and R13 and detecting transistor Q5, and operates to provide a control signal to load

circuits

38 and 40 in response to a predetermined output signal from enhancing circuit 30.

The operation of the circuit shown in F 16. 2 is as follows. Pulse generator 20, bias circuit 22 and tuned circuit 26 operate as described in FIG. 1 wherein tuned circuit 26 periodically rings", i.e., produces damped oscillations at the natural frequency ,of the tuned circuit. When a complex impedance with a substantial resistive component is capacitively coupled to antenna Al, the Q of tuned circuit 26 is reduced to thereby reduce the amplitude and/or duration of the ringing" in the tuned circuit 26 during the interpulse nulls in the output of pulse generator 20. In addition, a load having a complex impedance with a substantial capacitive component is similarly capable of attenuating oscillations in the, tuned circuit 26 when coupled thereto through antenna A1.

Enhancing circuit 30 operates to accentuate the reduction in the amplitude of the damped oscillations of the tuned circuit 26 caused by loading. In the absence of a positive output from the pulse generator, transistor Q3 is normally non-conductive since no base-emitter forward bias is provided. Capacitances C7 and C8, in conjunction with the requirements of the tuned circuit 26, are chosen to provide sufficient regeneration to force transistor O3 to oscillate when O3 is forward-.

biased. Thus, when a positive pulse is provided to the vided by capacitances C7 and C8. Resistances R8 and R9 are chosen to permit the damped oscillations to ock transi to 9 in ossillation, he by e tin a threshold above which Q3 oscillates. Transistor Q3 i s biased so that during oscillation, a positive DC component is provided at its emitter. As noted previously, the positive pulse from pulse generator 20 is of insufficient amplitude to bias transistor Q3 into oscillation.

Therefore, the output of transistor ()3 taken at its emitter oscillates with a positive DC component when tuned circuit 26 rings at sufficient amplitude, and is at approximately ground potential when the amplitude of the ringing is insufficient to cause oscillation and when the relatively small amplitude of positive pulse from pulse generator 20 is present. The amplitude of the ringing of tuned circuit 26 will be sufficiently reduced to prevent oscillation of transistor 03 when a complex impedance load with a substantial capacitive or resistive component is capacitively coupled at antenna Al. As a result, the reduction in amplitude of the damped oscillations of tuned circuit 26 is enhanced at the emitter output of transistor O3 to provide a change going from a periodic oscillatory output combined with a DC component to a non'oscillatory output at approximately ground potential.

Detector 34, comprising resistances R12 and R13, and transistor Q5 detects the presence and absence of oscillations at the output of the enhancing circuit 30, and generates a corresponding control signal. Resistance R12 limits the base current and resistance R13 limits the collector current of transistor Q5. During each period in which transistor Q3 is oscillatory, indicating no loading of tuned circuit 26, transistor O5 is turned on by the DC component of the oscillation forcing the collector of transistor O5 to approximately ground potential. When transistor ()3 ceases to oscillate, with its emitter going to approximately ground potential, indicating loading of tuned circuit 26, transistor O5 is turned off and its collector rises to the supply voltage (approximately volts DC).

' Thus, when antenna A1 is coupled to an impedance having either a substantial capacitive component, or a substantial resistive component such as is presented by humans and other mammals, a control signal of supply voltage is generated at the collector of transistor Q5; otherwise the control signal is at approximatelytl ground potential.

The control signals thus generated may be utilized either singly or in combination to control a variety of load circuits. For example, the output of detector 34 may control load circuit 38 or load circuit 40 or both. Similarly, the output of detector 36 may be used to control load circuit 42 or load circuit 40 or both. Alternatively, both control signals from detectors 34 and 36 may be employed independently or in combination to control a single load 40 which may include logic circuitry such as an OR circuit or an AND circuit.

In the second, preferred embodiment described above, the values of various circuit components are as follows:

Resistances R2 IK ohms R4 330 ohms R5 330 ohms R6 3.3K ohms R7 3.3K ohms R8 500 ohms (max.) R9 470 ohms R10 500 ohms (max.) R11 470 ohms R12 470K ohms R13 33K ohms R14 470K ohms R15 33K ohms Capacitances C3 0.22 microfarads C4 390 picofarads C5 0.22 microfarads C6 0.22 microfarads C7 I50 picofarads C8 20 picofarads C9 picofarads C10 20 picofarads lnductances L2 39 microhenries L3 39 microhenries Transistors Diodes Q1 2N4248 Q2 2N3567 Dl 1N4l48 Q3 2N5132 D2 |N4l48 Q4-2N5132 D31N4l48 Q5 2N5l32 D4 1N4148 Q6 2N5l32 Referring now to FIG. 3, another embodiment of a detector 44 is shown, a portion of which is peak detector 46. The output of the peak detector is taken at the junction of diode D4, capacitance C12 and resistance R18. The negative portions of the damped oscillation of the tuned circuit 14 turn off transistor Q7, causing a controlled quantity of charge to be supplied to capacitance C12 through resistance R17 and diode D4. The value of resistance R18 is chosen to prevent capacitance C12 from discharging between positive portions of the damped oscillation of tuned circuit 14. Thus, when tuned circuit 14 is ringing, diode D4, capacitance C12 and resistance R18 function as a peak detector maintaining a detector output voltage approximately equal to the supply voltage. When tuned circuit 14 is being energized by the positive-going pulse from pulse generator 10, transistor O7 is biased off and its collector is at approximately the supply voltage and the detector output is again maintained at approximately the supply voltage. However, upon coupling the tuned circuit 14 through sensor 16 to an external impedance having either a substantial capacitive or resistive component the amplitude of the damped oscillations of tuned circuit 14 decreases and with it, the time that transistor 07 is turned off also decreases. Consequently, the quantity of charge supplied to capacitance C12 during ringing is reduced and with it, the peak amplitude detected by the peak detector 46. In this way, a detector output change of 2 to l is possible for smaller amplitude changes of the damped oscillations of tuned circuit 14.

The advantages of the present invention, as well as certain changes and modifications of the disclosed emcuit including a plate adapted for selective coupling with either antennae Al and A2. Such a load circuit could be incorporated into an article worn by a person, such as a ring or a watchband, and used as a key to cause a control circuit to open a lock, for example. The output of any or all of the detector circuits could be altered by the addition of a polarity inverter circuit. In the case of the detector of FIG. 3, a negative voltage could be provided by poling diode D4 in a manner opposite to that shown. Also, the resistive impedance for loading the tuned circuit may be ohmically coupled to the sensor or antenna, rather than capacitively coupled. Other detecting schemes other than those disclosed and well-known in the art such as synchronous detection and time comparison detection may .be used to detect the changes in amplitude and/or duration of the damped oscillations. Also, other pulse generator means other than that disclosed may be used to periodically energize the tuned circuit to cause it to ring. It is intended to cover all of those changes and modifications which could be made to the embodiments of the invention herein chosen for the purposes of the disclosure without departing from the spirit and scope of the invention.

What is claimed is:

1. A control circuit comprising:

1. first means operative to generate energizing pulses;

and I 2. second-means operative to receive said energizing pulses and to produce oscillations during each interpulse period. said second means being further operative to sense the coupling thereto of an external impedance having a substantial capacitive or resistive component and to attenuate said interpulse oscillations in response thereto.

2. The circuit according to claim 1 further comprising detection means operative to detect said interpulse oscillations of said second means.

3. The circuit according to claim 2 wherein said detection means comprises a peak detector.

4. The circuit according to claim 1 further comprising enhancing means operative'to enhance said interpulse oscillations of said second means.

5. The circuit according to claim 4 wherein said enhancing means comprises an oscillator circuit responsive to changes in the interpulse oscillations of said second means.

6. The circuit according to claim 5 wherein said oscillator circuit is operative to provide a oscillatory output voltage combined with a DC output voltage in response to said coupling of an external impedance having a substant'ial capacitive or resistive component to said second means. 1

7. The circuit according to claim 4 further comprising detection means operative to detect said interpulse oscillations of said second means after enhancement by said enhancing means and to generate a predetermined control signal in response thereto.

8. The circuit according to claim 7 wherein said detection means comprises a transistor circuit which is switched between conduction and non-conduction by the output of said enhancing means.

9. The circuit according to claim 1 wherein said first means comprises:

1. pulse generator means operative to generate an energizing voltage having alternating positive-going portions and approximately ground potential portions; and

2. bias means operative to generate a negative signal during said interpulse periods during which said energizing voltage is at approximately ground potential.

10. The circuit according to claim 9 wherein said bias means comprises a capacitance and resistance connected in parallel, and a blocking diode connected in series with said parallel-connected resistance and capacitance.

1 1. The circuit according to claim 1 wherein said first means comprises:

1. pulse generator means operative to generate an energizing voltage having alternating positive-going portions and negative-going portions; and

2. isolating means operative to decouple said first and second means during each negative-going portion of said energizing voltage.

12. The circuit according to claim 11 wherein said isolating means comprises a blocking diode.

13. The circuit according to claim 1 wherein said second means comprises a tuned circuit having a sensor by means of which said coupling is effected.

14. A control circuit comprising:

1. pulse generator means operative to generate an energizing voltage having a positive-going portion;

2. first signal channel means operative to receive said energizing voltage, and to sense the coupling of an external impedance having a substantial capacitive or resistive component and to generate a predetermined output signal in response thereto; and

3. second signal channel means operative to receive said energizing voltage, and to sense the coupling of an external impedance having a substantial capacitive or resistive component and to generate a predetermined output signal in response thereto.

15. The circuit according to claim 14 wherein said first and second signal channel means each comprises:

1. bias means operative to generate a negative signal between each positive-going portion of said energizing voltage;

2. a tuned circuit operative to receive the output of said bias means and including a sensor by means of which said coupling of external impedances is effected;

3. enhancing means operative to receive and amplify the output of said tuned circuit; and v 4. detection means operative to detect the output of said enhancing means and to generate said predetermined control signal in response thereto.

16. The circuit according to claim 14 further comprising a first load circuit controlled by said first signal channel means and a second load circuit controlled by said second signal channel means.

17. The circuit according to claim 16 further comprising a third load circuit controlled by both said first and second signal channel means.

18. The circuit according to claim 14 further comprising a load circuit controlled by both said first and second signal channel means.

19. The circuit according to claim 1 wherein said first means comprises:

1. pulse generator means operative to generate an energizing voltage having alternating negative-going portions and approximately ground potential portions; and

2. bias means operative to generate a positive signal during said interpulse periods during which said energizing voltage is at approximately ground potential.

20. A control circuit comprising:

1. a pulse generator;

2. a bias circuit comprising a first resistance and a first capacitance connected in parallel with one another and in series between the output of said pulse 3 ,80 1 ,799 g 9 l generator and one terminal of a stored-charge dia substantial capacitive or resistive component to said antenna by attenuating said oscillations; a tund ell-cult compnsmg an Inductance; a Second 4. a shock-excited oscillator connected to said tuned capacitance and an antenna connected in parallel circuit; and

with one another and to said bias circuit at the 5 I other terminal of Said St0red charge diode so as to 5. a detector circuit for generating a control signal in produce oscillations during the time periods beresponse to a Predetermmed output Signal from tween pulses from said pulse generator, and to said i atorsense the coupling of an external impedance having

Claims

1. A control circuit comprising: 1. first means operative to generate energizing pulses; and 2. second means operative to receive said energizing pulses and to produce oscillations during each interpulse period, said second means being further operative to sense the coupling thereto of an external impedance having a substantial capacitive or resistive component and to attenuate said interpulse oscillations in response thereto.

2. second means operative to receive said energizing pulses and to produce oscillations during each interpulse period, said second means being further operative to sense the coupling thereto of an external impedance having a substantial capacitive or resistive component and to attenuate said interpulse oscillations in response thereto.

2. a bias circuit comprising a first resistance and a first capacitance connected in parallel with one another and in series between the output of said pulse generator and one terminal of a stored-charge diode;

3. a tuned circuit comprising an inductance, a second capacitance and an antenna connected in parallel with one another and to said bias circuit at the other terminal of said stored-charge diode so as to produce oscillations during the time periods between pulses from said pulse generator, and to sense the coupling of an external impedance having a substantial capacitive or resistive component to said antenna by attenuating said oscillations;

3. enhancing means operative to receive and amplify the output of said tuned circuit; and

4. The circuit according to claim 1 further comprising enhancing means operative to enhance said interpulse oscillations of said second means.

4. detection means operative to detect the output of said enhancing means and to generate said predetermined control signal in response thereto.

4. a shock-excited oscillator connected to said tuned circuit; and

5. a detector circuit for generating a control signal in response to a predetermined output signal from said oscillator.

6. The circuit according to claim 5 wherein said oscillator circuit is oPerative to provide a oscillatory output voltage combined with a DC output voltage in response to said coupling of an external impedance having a substantial capacitive or resistive component to said second means.

9. The circuit according to claim 1 wherein said first means comprises:

11. The circuit according to claim 1 wherein said first means comprises:

14. A control circuit comprising:

19. The circuit according to claim 1 wherein said first means comprises:

20. A control circuit comprising: