US2878708A - Capacitative switching apparatus - Google Patents

Description

March 24, 1959 J. M.IHANERT 2,878,768

CAPACITATIVE SWITCHING APPARATUS Filed Oct. 3, 1956 United States Patent 2,878,708 CAPACITATIVE SWITCHING APPARATUS John M. Hanert, Des Plaines, Ill., assignor to Hammond Organ Company, Chicago, 111., a corporation of Delaware Application October 3, 1956, Serial No. 613,700 1 Claim. (Cl. 841.08)

The invention relates generally to capacitative switching apparatus and more particularly to an improved capacity switch mechanism which is thoroughly shielded when the switch is in open position. The invention is more particularly concerned with an improvement in capacitative switching means for electrical musical instruments in which the output of a signal generator is transmitted to the output system of the instrument through such capacitative switch means.

It is an object of the invention to provide a capacitative switching means in which the switching parts are thoroughly shielded when the switch is open and in which depression of a playing key causes a capacitative coupling between the source of tone signals and the output system of the instrument.

A further object is to provide a capacitative switching means for electrical musical instruments in which the signals from generator of the unison pitch together with those from the generators of octavely related pitch are successively transmitted to the output system of the instrument as the playing key is being depressed.

Other objects will appear with the following description, reference being had to the accompanying drawing in which:

Fig. 1 is a partially diagrammatic and partially schematic wiring diagram of a typical portion of an electrical musical instrument;

Fig. 2 is a broken vertical on the line 22 of Fig. 1;

Fig. 3 is an enlarged plan view of the movable element of the capacity switch showing schematically its connection to the tone signal generators;

Fig. 4 is a sectional view of the capacitative switch element taken on the line 4-4 of Fig. 3; and

Fig. 5 is a bottom plan view of the capacity switch plate element shown in Figs. 3 and 4.

In electrical musical instruments it has been endeavored frequently to couple the output of the signal generator to the output system of the instrument through a key operated capacity switch. Such proposals have not met with acceptance primarily because of the existence of a certain amount of coupling between the signal source and the output system due to coupling between the respective plates of the capacitor switch when the switch was in open position. In the present invention such undesirable residual coupling is avoided by providing a substantially complete ground-connected electrostatic shielding for the switch when it is in open position, thus preventing any substantial coupling between the capacitor plates of the switch when the switch is in unoperated position.

In Fig. l the apparatus is diagrammatically illustrated as including a playing key representative of the sixtyone, more or less, keys of an electrical musical instrument such as an organ. Each of the playing keys 10 is secured to a key shank or channel 12 which at its rearward end is secured to the upper end of a flexible leaf spring 14. The spring 14 is fixed at its lower end to a stationary part 16 of the keyboard frame and at its upper end to a sectional view thereof taken 2,878,708 Patented Mar. 24, 1959 deformable angle clip 18, which in turn is secured to the shank or channel 12. The spring 14 is stressed so as normally to maintain the forward end of the key 10 in engagement with a stationary felt upstop 20. By bending the angle clips 18 the effectiveness of the springs 14 may be readily adjusted to provide the desirable touch or resilient resistance to depression of thekey 10. Each of the key stems or channels 12 has secured to it at its rearward end a vertically extending arm 22. At its upper end the arm 22 has secured thereto a condenser plate 24, which upon operation of the key 10, is adapted to project through a slot, or opening, 26 formed in a shielding housing 28 and into a slot 30 formed in a condenser plate 32 which is supported in the housing 28 by insulating means 34. The slotted plate 32 extends the full length of the keyboard. The shielding housing 28 is supported by any suitable part 36 of the keyboard frame and has its capacitor plate connected to a conductor 38, the housing 28 and a shield 40 for the conductor 38 are respectively connected to ground.

The conductor 38 is connected to ground through a capacitor C42 and grid resistor R44 which provide sink and grid return circuits for the input signals which are impressed upon the grid 46 of a preamplifier triode 48. The cathode of triode 48 is connected to ground through a suitable self-bias mesh comprising capacitors C50 and resistor R52, while the anode or plate thereof is connected through a load resistor R54 to a suitable source of plate current supply indicated as a terminal B+ of the customary power supply. The plate is also connected through a blocking capacitor CS6 to the input of a power amplifier 58, the output of which is connected to a speaker 60 The mounting for the keys 10 is such that after the keys have been depressed to a certain limited extent a spring 66 secured beneath each of the keys comes into contact with a stop pad 68, suitably mounted on a fixed frame member 70, so that further depression of the key will require the application of additional force so as to overcome not only the force of the spring 14 but also that of the spring 66. The relative strength of the springs 14 and 66 should be such that when the key has been sufficiently depressed to cause the spring 66 to become effective, the player will readily sense the ditference in force required to depress the key against the force of both springs, and thus readily play the instrument using the technique of second touch.

The body of the capacitor plate 24 is made of a suitable dielectric material and has applied thereto suitable separated areas, or islands, of conducting material as shown in Figs. 3, 4, are electrically connected to ground being joined along the edges of the plate 24. When the key 10 is in normal position, as shown in Fig. l, the conductive islands thus form, together with the housing 28, a substantial complete electrostatic shield, preventing capacitative coupling between other conductive areas of the plate 28 and the slotted plate 32. Conductive areas 74 and 74a, connected together along one edge of the dielectric plate 24, are suitably connected to a generator 1 preferably providing complex musical tone signals of the pitch of the unison or fundamental frequency of the tone to be controlled by each particular key. Similarly, conductive areas 76 and 76a, which are likewise electrically connected, are connected to a tone signal source, or generator, 2 which provides the octave tone signals. Also, if desired, conductive areas 78 and 78a may be similarly connected to the generator 4 of tone signals two octaves higher in pitch than the unison.

The slot 30 in the condenser plate 32 is sufficiently wide to provide adequate clearance for the entry of the capacitor plate 24 and to permit the slight relative tilting 3 action of the plate 24 with respect to the horizontal walls of the slot 30. The degree of capacitative coupling between the conductive surfaces on the plate 24 and the respective surfaces of the slot 30 of the condenser plate 32 may vary somewhat due to the pivotal mounting of the plate carrying arm 2t) and due to unavoidable inaccuracies in construction, but the total coupling will not be affected by such variations. The degree of capacitative coupling will be afiected only by the extent to which the capacitor plate 24is inserted in the slot 30.

As previously indicated, the capacity switch is completely shielded by the grounded housing 28, and the capacitor plate 24 has grounded conducting portions which form, with the edges of the slot 26 in this housing, a substantially complete electrostatic shielding means, to prevent any capacitative coupling between the signal carrying the conductive areas on the dielectric platea24 and the slotted plate 32. However, when a key is partially depressed the conducting portions 74 on the plate 24 gradually are moved to a position to provide electrostatic coupling between the signals provided by the source 1 and the plate 32 so as to impress signals of unison pitch upon the control grid of the triode 38 which, being amplified, is translated into sound by the speaker 60.

When substantially all of the conducting area 74, 74a lie within the slot 36 of the plate 32 the spring 66 of the depressed key will have moved downward at least a sufficient distance for its lower end to engage the stop 68, and the force required for further depression of the key will thus be increased. Thereafter, further depression of the key against additional resilient force of the spring 68 will cause the conducting portions '75, 76a of the plate 24 to be moved into capacitative coupling relation with the plate 32, and upon complete depression of the key the conducting area 78, '78:: will be brought into capacitative relation with plate 32. Thus, signals from the generator 2 and the generator 4 will be combined successively with the output of the generator f.

It will be noted that the conducting areas 72, 74, 76, and 78, and their obverse parts are spaced so as to be adequately insulated from one another. These conducting areas may be formed on the dielectric plate 24 in the form of sheet metal foil applied to the plate with suitable adhesive, or may be produced by any of the well-known printed circuit techniques.

This capacitative switch means differs from others known to applicant in that the movable part of the switch has a grounded portion normally providing a shielding bridge across the opening through which the movable part projects so that, together with the housing of the switch, the parts thereof are thoroughly shielded. Another feature of the invention is that upon operation of the key to the extent necessary to provide capacitative coupling of the source of the signal of unison pitch, the resilient means becomes effective to require noticeably increased force to effect continuation of key depression, and at the same time, the switch becomes effective to couple capacitatively the output of additional tone signal sources which are harmonically related in pitch to that of the generator first coupled to the output system by the capacitative switch. Of course, if desired, additional harmonically related sources of electrical tone signals might be coupled to the output system by adding conductive areas such as 76 and 78 to the movable plate of the capacity switch.

It will be clear to those skilled in the art that the shape of the movable plate 24 is not necessarily rectangular in cross section but that it should be of such conformation as to provide a shield for the opening in the housing 28 by virtue of its grounded end conducting portion. While the slotted shape of the fixed plate 32 is of advantage in providing compensations for inaccuracies in construction and for the arcuate movement of the plate 24, this conformation of the plate 32 is not essential to the operation of the switch. The more fundamental features of the invention are that the movable plate 24 has at its forward end a grounded means for shielding the opening in the housing 28 or, more particularly, for providing a shield between the fixed condenser plate 32 and those conducting portions on the movable plate 24 which are connected to signal sources.

While I have shown and described a particular embodiment of my invention, it will be apparent to those skilled in the art that numerous modifications and variations may be made in the form and construction thereof, without departing from the more fundamental principles of the invention. I therefore desire, by the following claim, to include within the scope of my invention all such similar and modified forms of the apparatus disclosed, by which substantially the results of the invention may be obtained by substantially the same or equivalent means.

I claim:

For use in an electrical musical instrument having a plurality of musical tone signal sources, each source having one of its output terminals grounded, a pivotally mounted playing key, and an amplifying output system having one of its input terminals grounded, the improvement which comprises a capacitative switching apparatus coupled between the ungrounded output terminal of each signal source and the ungrounded input terminal of the amplifying output system and comprising in combination, a grounded shielding housing having an opening therein, a capacitor plate mounted within the housing adjacent the opening therein and insulated from the housing and coupled to the ungrounded input terminal of the amplifying output system, a dielectric element mechanically connected to the key and movable a substantial distance into the housing through the opening therein upon full depression of the key, the portion of the element which penetrates the housing having means forming a plurality of separate conductive surfaces arranged successively to come into capacitative coupling relationship with the capacitator plate as the element is moved into the housing, and meansconnecting the first of the conductive surfaces which successively move toward the capacitator plate to ground and the rest, respectively, to the ungrounded output terminals of the signal sources.

References Cited in the file of this patent UNITED STATES PATENTS 2 ,137,435 Yolles Nov. 22, 1938 2,215,809 Miessner Sept. 24, 1940 2,329,544 Larsen Sept. 14, 1943

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