Note: Descriptions are shown in the official language in which they were submitted.
Docket 5982
1~4~382Z
Background of The Invention
Various forms of miniature electronic buzzers have
been proposed for use in small alarm clocks, timers, automo-
tive and aircraft warning devices, and similar installations.
There is always a variety of parameters to be considered in
the design and large scale manufacture of such buzzers. The
power and frequency requirements, audible output, sensitivity
to changes in existing voltage, and ease and cost of manufac-
ture, are all criteria that must be considered and balanced
against each other. Considerable work has been devoted to
achieving the best combination of these parameters.
In the prior art, typical miniature buzzers are
disclosed in U. S. patents Nos. 2,977,418 issued March 28, 1961,
to Haas; 3,341,842 to Breguet issued Septem~er 12, 1967, - -
3,530,463 to Spadini issued September 22, 1970; and ~rai
3,564,542 issued February 16, 1971; and small buzzers employing
piezo-electric devices are disclosed in U. S. patents Nos.
3,277,465 to Potter issued October 4, 1966; Dundon et al
3,331,970 issued July 18, 1967; Stampfli 3,341,841 issued
September 12, 1967; Mallory 3,569,963 issued March 9, 1971; and
Potter 3,697,983 issued October 10, 1972.
Summary of the Invention
The present invention relates to a high efficiency
buzzer using a transistor oscillator oscillator circuit which is
designed in conjunction with the vibrating mechanical buzzer
parts to produce maximum sound output from a device which is
relatively simple and inexpensive to manufacture; The
driving coil, for inducing a regularly fluctuating electro-
magnetic field in a core member, is connected in series
circuit with a power supply and the emitter-collector
circuit of the transistor. A shunt circuit comprising a
resistor and a diode (series connected) is also connected to
'
~ -2-
~4882Z
Docket 5982
the power supply in parallel with the transistor. A control
coil is located within the driving coil, closely inductively
coupled to the core member and the driving coil. The control
coil is connected between the juncture of the resistor and
diode and the base of the transistor.
The turns ratio of the driving coil to the control
coil varies with the design exciting voltage. The control
coil is an optimized coil which has the maximum nwmber of turns
which can still match the input impedance of the transistor.
By design the control coil has approximately 3000 turns of
No. 48 AWG copper wire, with approximately 1000 ohms impedance,
to match the type 2N2222 transistor used in a typical 12 volt
design. Preferably a diode is also connected in shunt across
the driving coil, as a so-called free-wheeling diode, to de-
crease the frequency of the electronic oscillator circuitclose to the mechanical frequency of the vibrating mechanical
` parts, e. g., the striker arm which repeatedly strikes a
sounding diaphragm. This diode also increases the feedback -
~r coupling factor between the vibrating striker arm and the
control coil. Also, the diode in the shunt circuit is type
matched to the transistor, i.e., both are silicon, or both
germanium, etc., components.
~he object of the invention, therefore, is to pro-
vide such a high output, efficient and relatively voltage -
insensitive buzzer.
Other objects and advantages of the invention will
be apparent from the following description, the accompanying
,
drawings and th~ appended claims.
- . . , ~ . .
Docket 5892 1~4882~
Brief Description of The Drawings
Fig. 1 is a longitudinal sectional view of a typical
buzzer em~odying the invention;
Fig. 2 is a sectional view taken along lines 2-2 of
~ig. 1; and
Fig. 3 is a diagram showing the novel circuit which
is a principal part of the invention.
Description of the Preferred Embodiments
The buzzer, as shown in Figs. 1 and 2 consists of a
base assembly 12 onto which a housing 10 has been secured by a
suitable cement, or by electronic welding, or some other suit-
able means. The top wall of the housing 10 consists of a
metal diaphragm 11 which is preferably molded into the
plastic side walls 14 of the housing. Other suitable
materials can be used for the diaphragm. This general
design of buzzer and housing is the subject of application
Canadian Serial No. 247,266, filed March 8, 1976, which is
assigned to the assignee of this application.
The base assembly consists of a plastic base 13
with a recess 15, such that a U-shaped core 21 of iron or
other material of high magnetic permeability fits into the
base. A circuit board 2S, with a slot 26 on one end and a
hole 27 near the other end, is fitted over the core 21 and
onto the base, such that the long cire leg 22 fits through
the ~lot 26 and the short core leg 23 fits through the
hole 27.
A set of two windings (Ll and ~2) is placed around
the short core leg and is incorporated in the circuit which
-4-
I
'.
Docket 5982 1048822
. ~
operates the buzzer. The individual windings are represented
in Fig. 3. Circuit connection pins 31, which fit through
holes in the circuit board 25 and plastic base 13, project
from the plastic base and connect the circuit to an external
power source, for example through some form of condition
responsive switching mechanism S (Fig. 3) which will apply
power to the circuit in response to the existence of some
condition as to which a warning is desired.
One end of a striker arm 35 is formed over the end
of the longer core leg 22 as shown at 36, and welded to it,
and a magnet 38 is secured to the other or free end of the
striker arm, positioned such that directly above the striker
arm and magnet is the apex of a circular recess or dimple 40
in the diaphragm 11, and directly below is the shorter core
leg 23. Thus the core and magnet form the magnetic circuit, -
and with an oscillating magnetic field induced in the core
21, the magnet secured to the striker arm is alternatively
attracted and released, or repelled and released, from the
core leg 23, and will strike diaphragm 11 in such a manner to
produce an audible sound. By locating the weld at the turned
end 36, heat from the welding operatLon does not affect the
temper of the heat treated steel spring striker arm 35 from -
its pivot point outward to the free end. Also the arm 35
may be suitably tapered in a lengthwise direction to give a
more uniform spring action and, along with other factors, to
achieve the highest Q for the oscillating arm-magnet system.
Referring to Fig. 3, the power supply is indicated
by positive and negative symbols and the switch S represents
''; ' ,' ~ ~
Docket 5982 1~48~22
a suitable switching device controlling power to the circuit.
The switching device may be closed in response to any condition,
or combination of conditions, as to which an audible indication
is desired.
The transistor T is shown as of the n-p-n type, and
the driving coil Ll is connected in series circuit with the
transistor's collector-emitter connections. In parallel with
that circuit path, across the supply, is a series circuit
comprising resistor R and diode Dl. The control or feedback
coil L2 is connected between the base of the transistor T and
the junction of R and Dl. A diode D2 is connected in shunt
across coil Ll, as shown.
Coil L2 is mounted directly around the short leg 23
of the core piece, and coil Ll is mounted directly around
coil L2. This arrangement provides a most efficient electro-
magnetic coupling between the core piece, Ll and L2. In order
to achieve the desired efficiency and control, it has been
found that the impedance of L2 should be matched to the im-
pedance of the transistor T, while still havlng the maximum
possible number of turns. Thus, L2 does not vary for different
design voltages, whereas for Ll the turns vary directly as the
design voltage and its impedance varies as the square of the
voltage. Diode Dl is selected of the same material type as
transistor T, that is, if a silicon transistor is used, diode
Dl is a silicon diode. The values of the components are chosen
to produce an oscilIator frequency which is appxoximately one
and one-half times the mechanical natural frequency of the
oscillating arm-magnet system. This allows the oscillating
-6-
Docket 5982 1048822
.
mechanical parts to synchronize readily with the electronic
oscillator for maximum efficiency. Diode D2 is u'ilized as
a free-wheeling diode to decrease the oscillator frequency
easily to the desired value, usually in the range of 500-700
Hz, and to increase the mechanical-electrical coupling factor
between the vibrating arm-magnet assembly and the electronic
circuit. -
In a typical small buzzer constructed according to
the invention, the physical size may be as small as one-fourth
the size shown in the drawings. Components actually used in
a successful embodiment of a 12 volt design are as follows:
T - type 2N2222
R - 6800 ohms
Dl - type lN914/lN4148
D2 - type lN914/lN4148
Ll - 1500 turns of No. 44 wire
L2 - 3000 turns of No. 48 wire
Power supply for this configuration is an unregulated
12 volts, which may vary _ several volts without affecting the
reliable operation of the buzzer.
Various configurations obviously are possible within
the scope of the invention. Also, instead of discrete com-
ponents, all electronic components can be provided on an
integrated circuit chip.
While the form of apparatus herein described con-
stitutes a preferred embodiment of this invention, it is to
be understood that the invention is not limited to this pre-
cise form of apparatus, and that changes may be made therein
without departing from the scope of the invention. ;
--7--
.:
