U.S. patent number 3,901,368 [Application Number 05/450,088] was granted by the patent office on 1975-08-26 for coin acceptor/rejector.
Invention is credited to Lance T. Klinger.
United States Patent |
3,901,368 |
Klinger |
August 26, 1975 |
Coin acceptor/rejector
Abstract
Coins are tested by inserting them into an oscillator driven
tank circuit resonating at near or equal frequency when the proper
coin influences particularly the tank circuit. A narrow amplitude
detection band and flat spiral tank circuit coils of coin-like
diameter limit the response to particular coins. The oscillator
includes a similar tank circuit and both tank circuits are
subjected to the same environment.
Inventors: |
Klinger; Lance T. (Playa Del
Rey, CA) |
Family
ID: |
23786716 |
Appl.
No.: |
05/450,088 |
Filed: |
March 11, 1974 |
Current U.S.
Class: |
194/317; 73/163;
324/239 |
Current CPC
Class: |
G07D
5/08 (20130101); G07D 5/02 (20130101) |
Current International
Class: |
G07f 003/02 () |
Field of
Search: |
;194/1A,1R,99 ;324/34R
;73/163 ;209/81A |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Knowles; Allen N.
Attorney, Agent or Firm: Siegemund; Ralf H.
Claims
I claim:
1. A coin testing device comprising:
an oscillator producing a signal of a particular frequency at a
relatively high Q and of a narrow band;
a resonating sensing circuit including at least one coil disposed,
so that a coin to be tested will particularly change and determine
the resonance frequency of the sensing circuit, when having a
particular disposition with respect to the coil;
the sensing circuit being connected electrically to be energized by
the oscillator without inductive coupling through said coil;
the sensing circuit having a narrow response characteristic band
about said resonance frequency, when sensing a coin, the
oscillation frequency being in said band;
the sensing circuit having a frequency response band remote from
said oscillation frequency band when no coin is in its sensing
range; and
circuit means connected to said sensing circuit for detecting the
response of the circuit to the oscillations in the presence or
absence of a coin.
2. A coin testing device as in claim 1, wherein the circuit means
includes an amplitude detector/discriminator for detecting whether
the voltage of the sensing circuit has a particular amplitude
within a narrow range in representation of a particular type of
coin to be detected.
3. A coin testing device as in claim 1, wherein the sensing circuit
is a tank circuit, the circuit means connected to be responsive to
the amplitude of the voltage across the tank circuit.
4. A coin testing device as in claim 2, the circuit means including
comparator means defining a detection band for response to said
amplitude.
5. A coin testing device as in claim 1, wherein the oscillation
frequency equals the resonance frequency of the sensing circuit
when a coin is adjacent thereto.
6. A coin testing device as in claim 1, wherein the oscillation
frequency is slightly higher than the peak frequency of the
resonance band of the sensing circuit when a coin is adjacent
thereto.
7. A coin testing device as in claim 1, the sensing means including
at least one flat spiral coil having outer diameter about equal to
the diameter of a coin to be detected, the particular disposition
being a concentric one as between the coin and the coil.
8. A coin testing device comprising:
an oscillator including a first tuned circuit with at least one
coil for producing an oscillator signal at a relatively high Q and
having the frequency of the resonance frequency of the tuned
circuit;
a second tuned circuit including at least one coil and connected to
be energized by the oscillator so that a sensing voltage is
derivable from the second tuned circuit whose amplitude depends on
the frequency of the oscillator;
the respective resonance frequences of said first and second tuned
circuit having a particular difference when no coin is in the
vicinity of the coil of either of the tuned circuits;
a particular type of coin when in particular juxtaposed disposition
to one of the coils causing a change in the resonance frequency of
the tuned circuit to which the coil pertains, the change being
operative to diminish said difference so that said amplitude
assumes increased value; and
circuit means providing for a particular amplitude detection band
and connected to the second tuned circuit to determine whether or
not said increased amplitude falls into said detection band.
9. A coin testing device as in claim 8 wherein the other one of
said coils has disposition so that the coin may pass in its
vicinity where upon said difference is increased.
10. A coin testing device as in claim 9, wherein the coils of the
first and second circuits are located along a path for a coin, in
close proximity to each other but in magnetically decoupled
relationship.
11. A coin testing device as in claim 8, wherein the oscillator
includes two emitter - coupled transistors with a common current
source, and a feedback circuit from the collector of one of the
transistors to the base of the other one of the transistors, the
oscillator further including a first tank circuit in the collector
circuit of one of the transistors as the first tuned circuit second
tuned, while the circuit is a second tank circuit connected to the
collector of the respective other one of the transistors.
12. A coin testing device as in claim 10, including a
temperature-dependent bias for the transistors, the circuit means
including means for tracking the temperature dependency of the
oscillator and of the bias.
13. A coin detector as in claim 11, wherein the circuit means
includes means for amplitude tracking of the detection band.
14. A coin detector, comprising:
an oscillator producing a signal of a particular frequency;
sensing means connected to the oscillator including at least one
circular flat spiral coil having at least approximately the same
diameter as the coin to be detected, and being disposed so that the
coin will pass the coil coaxially, whereupon the coil as coupled to
the coin assumes a particular inductivity; and
circuit means connected to be responsive to the voltage across the
coil for detecting the coin.
15. A coin detector as in claim 14, wherein the oscillator includes
also at least one spiral coil, the spiral coils being mounted along
a travel path for the coin to be subjected to similar environmental
conditions, the coil of the sensing means and the coil of the
oscillator being decoupled electromagnetically.
16. A coin detector as in claim 14, wherein the sensing means
includes at least two coils, each being of spiral configuration and
of similar diameter and coaxially disposed to each other, so that
the coin passes between them.
17. A coin detector as in claim 14, wherein the circuit means
includes a capacitor connected to the coil and completely therewith
a resonance circuit, whose resonance band includes the oscillation
frequency.
18. A coin detecting device comprising:
an amplifier with double-ended output, and an input;
a first tuned circuit connected to one of said outputs, a feedback
circuit between said one output and said input for establishing an
oscillator, whose frequency is determined by the first tuned
circuit;
a second tuned circuit connected to the other one of said outputs,
one of said first and second tuned circuits located so that a coin
to be detected and when in particular disposition to the one tuned
circuit materially determines its tuned frequency, being the same
or approximately the same frequency as the frequency of the other
one of the tuned circuits; and
circuit means connected to the second tuned circuit for detecting
the amplitude of a signal developed by the second tuned circuit
upon oscillation of the oscillator.
19. A coin detector as in claim 18, wherein the circuit means
includes an amplitude band detector means, the amplitude of the
signal developed by the second tuned circuit, when sensing a proper
coin falling into that band.
20. A coin detector as in claim 18, wherein the second tuned
circuit is tuned to a frequency relative to the oscillation
frequency, so that the amplitude of the signal varies significantly
for small differences in one of the parameters of the second tuned
circuits.
21. A coin detector as in claim 18, the circuit means including a
pair of comparators biased for slightly different changes of state
to establish an amplitude detection band, the bias being provided
through circuitry from a voltage source, being the same source
operating said amplifier to obtain mutual tracking of the band and
of the signal.
22. A coin detector as in claim 18, wherein the resonance frequency
of the first tuned circuit is higher than the resonance frequency
of the second tuned circuit with no coin influencing either tuned
circuit, the particular disposition of a proper coin being in the
vicinity of the second tuned circuit chaning the resonance
frequency of the second tuned circuit to a value close to the
resonance of the first tuned circuit.
23. A coin detector as in claim 18, wherein the resonance frequency
of the second tuned circuit is higher than the resonance frequency
of the first tuned circuit with no coin influencing either tuned
circuit, the particular disposition of a proper coin being in the
vicinity of the first tuned circuit changing the resonance
frequency of the first tuned circuit to a value close to the
resonance of the second tuned circuit, so that the oscillator
frequency is altered accordingly.
24. A coin testing device comprising:
an oscillator including a first tuned circuit with at least one
coil for producing an oscillator signal at a relatively high Q and
having the frequency of the resonance frequency of the tuned
circuit;
a second tuned circuit including at least one coil and connected
galvanically to the oscillator to be energized therefrom, while the
coils of the two tuned circuits are spaced apart to be magnetically
decoupled;
each of said two circuits having a particular resonance frequency
band, said bands being spaced apart when no coin is in the vicinity
of either coil of the tuned circuits;
a particular type of coin when in particular juxtaposed disposition
to one of the coils causing the frequency bands to be shifted into
at least partially overlapping disposition;
first circuit means connected to said second tuned circuit to
derive therefrom a sensing voltage whose amplitude depends on the
relative disposition of said bands, increasing with increasing
overlap; and
second circuit means connected to the first circuit means and
providing for a particular amplitude detection band to determine
whether or not said amplitude as derived falls into said detection
band.
25. A testing device as in claim 24, wherein the bands overlap with
coinciding centers when a particular type of coin is adjacent to
the one coil.
26. A testing device as in claim 24, wherein the peak frequency of
the oscillation band is slightly higher than the peak frequency of
the band of the second tuned circuit when a particular type of coin
is adjacent to the one coil.
27. A testing device as in claim 24, the coils including at least
one flat spiral coil having outer diameter about equal to the
diameter of a coin to be detected, the particular disposition being
a concentric one as between the coin and the coil.
28. A testing device as in claim 24, wherein the oscillator
includes two emitter - coupled transistors with a common current
source, and a feedback circuit from the collector of one of the
transistors to the base of the other one of the transistors, the
oscillator further including a tank circuit as the first tuned
circuit and connected in the collector circuit of one of the
transistors, while the second tuned circuit is a second tank
circuit connected to the collector of the respective other one of
the transistors.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a coin detecting, discriminating
and testing apparatus or device and more particularly to apparatus
for testing a coin and accepting or rejecting it. Such an apparatus
is to be used in coin operated vending machines or other equipment
or mcahines, which are coin operated.
A coin operated machine is usually equipped with a device that
tests any coin which is being entered. The slot in the machine,
through which a coin in inserted, has usually particular dimensions
preventing at least larger coins for being inserted. Beyond the
slot equipment of one kind or another is provided to test the coin
so as to prevent coins of wrong denomination, foreign coins or
slugs, from operating the machine.
Many types of testing equipment are known here, trying to
discriminate the proper coin from others or slugs on the basis of
electrical and/or magnetic properties, and/or weight and/or size.
Unfortunately, close similarities between a proper coin and many
improper ones require rather delicate testing; the range of test
values of whatever characteristics is being used and defining or
establishing the criterium for acceptance or rejection is extremely
narrow. Slugs, of course, are the greatest problem, as they can be
made at will to resemble a proper coin as much as the forger wants
it to. But also foreign coins often resemble closely domestic ones.
The "quarter", for example, seems to have a size that amounts
almost to a kind of world-wide standard size for coins. Coin
discrimination is, therefore, a difficult problem, indeed. Needless
to say that methods can be devised and equipment can be designed
testing all conceivable properties of a coin to sort the proper
ones from the rest. However, little is gained in practice, if the
input structure of a coin-operated machine is converted into a
miniature laboratory.
Among the various methods tried, many are based on the principle of
electromagnetic interaction between the coin and an inductance, in
that the coin modifies the inductivity of a sensor coil. Reference
is made here to the U.S. Pat. Nos. 3,152,677; 3,373,856; 3,401,780;
3,481,443; 3,561,580; 3,506,103; 3,576,244; 3,741,363 and
3,749,220.
It was found, however, that little attention has been given in the
past to changes in the environment in which the apparatus is
operated. Changes in temperature and humidity coupled with abuse
(changing for example positional adjustments), may result in
significant changes in the operating parameters of the testing
equipment. Hence, its sensitivity must be reduced to permit
compensation, but that in turn makes inevitable that some false
coins are accepted; the equipment has to be de-sensitized to such
an extent that a change in ambient conditions will not produce
equipment changes which would place a "good" coin outside of the
accept range.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a coin accept /
reject apparatus, which is very sensitive but whose sensitivity
range adjusts with changes in ambient conditions.
It is another object of the present invention to provide circuitry
and equipment for coin discrimination, which operates on the basis
of electromagnetic interaction between a coin and an inductance,
but that interaction is processed in a manner different from
approaches taken in the past.
It is a further object of the present invention to provide a new
and improved coin discriminating circuit, which can be used upon
appropriate selection of parameters to discriminate different kinds
of coins on basis of the same principle.
In accordance with the preferred embodiment of the present
invention, it is suggested to use an oscillator with a first tuned
circuit, such as a tank circuit providing an electrical signal of a
particular frequency and at a Q as high as possible. A second tuned
circuit is connected to the oscillator for being energized
therefrom to develop a particular amplitude signal when a proper
coin has (temporily) particular disposition to the coil (or coils)
of the first or second tank circuit.
In the preferred form the second tuned circuit with adjacent proper
coin resonates at or near the frequency of the oscillator (and of
the first tuned circuit). The sensor coils as well as the
oscillator coils are placed alongside a chute or the like, through
which coins will drop, but the coils of the different, tuned
circuits are decoupled as much as possible and the first tuned
circuit does not sense the coin when sensed by the second tuned
circuit. The two tuned circuits are, therefore, placed into similar
environmental conditions as much as possible without producing
mutual coupling. Therefore, the two tank circuits will track each
other.
The discriminating circuit includes additional circuitry for
establishing a rather narrow detection band of signal amplitudes,
and the voltage across the second tuned circuit when constructed as
tank circuit must fall in that band for a coin to be recognized as
acceptable. A high Q of the sensor circuit results in large changes
in the amplitude of the voltage across the second tuned circuit, if
a tank circuit, even for small changes in the inductance of the
sensor coil, provided the oscillator frequency is rather close to
the resonance peak of the tuned sensor circuit. As a consequence,
the detection of whether or not the amplitude falls within the
narrow detection band, is very sensitive while, on the other hand,
the detected signals and the band track each other. Specifically,
the circuitry is designed so that the band tracks the environment
and/or any changes in supply voltage.
It is another feature of the invention that the coil or coils, at
least of the second tuned circuit, are constructed as a flat
spiral, printed circuit coil or coils having an outer diameter
about equal to the diameter of a coin to be accepted and having
coaxial disposition to the coin when in sensing position. Such an
arrangement provides maximum sensitivity, as eddy currents flow in
a circumferential path in the coin. The dimension of a coin passing
the sensing coil or coils will, therefore, materially influence the
effective inductance at the instant of passage through a coaxial
position with respect to such coil or coils.
The preferred form of practicing the invention provides for
particular response of the sensing tank circuit with juxtaposed
coin to a particular frequency as determined by the oscillator, and
here particularly by the tank circuit thereof. It is, however,
possible to operate with a fixed response of the sensing circuit in
all cases, with no coin in its tank circuit, while the coin
influences the oscillator tank circuit and causes a particular
frequency to be generated; the response of the sensing circuit to
that frequency is then used as indicator.
It was found that a coin discrimination circuit constructed in
accordance with the present invention is very selective and retains
that selectivity under a width range of ambient conditions.
DESCRIPTION OF THE DRAWINGS
While the specification concludes with claims particularly pointing
out and distinctly claiming the subject matter which is regarded as
the invention, it is believed that the invention, the objects and
features of the invention and further objects, features and
advantages thereof will be better understood from the following
description taken in connection with the accompanying drawings in
which:
FIG. 1 is a schematic elevation of a coin accept/reject device,
wherein the coin must pass through a particular path to be
accepted;
FIG. 1b is a section along 1b -- 1b in FIG. 1;
FIG. 2 is a circuit diagram of an example of the preferred
embodiment of the present invention;
FIG. 3 is a voltage vs. frequency characteristics of a tank circuit
included in the circuit of FIG. 2;
FIG. 4 is a voltage vs. time diagram showing in different sections
different cases and examples of tank circuit voltage of coin
sensing as carried out by the circuit of FIG. 2; and
FIG. 5 shows a characteristic, similar to FIG. 3 but with a slight
modification.
Proceeding now to the detailed description of the drawings, FIG. 1
shows a rather flat casing 10, which includes internal dividers to
define a pathway 11 for a coin, with two exit branches 11a and 11b.
A mechanical switch or gate 12 determines whether a coin is
permitted to pass through the "accepted" branch 11a or must exit
through the "rejected" branch 11b. The switch 12 is solenoid
operated, and the solenoid is operated by the coin test circuit
depicted as circuit diagram in FIG. 2.
The circuit is packaged to a large extent and contains miniaturized
circuit elements to be described in detail below. The mechanical
aspect of coin acceptance and rejection is not part of the present
invention; the patents mentioned above show various kinds of
mechanisms, which can be used. Many others are well-known.
As will become apparent shortly, the coin test circuit includes two
groups of coils 14 and 15 disposed in close proximity to each
other. The fact that coils 14 are also placed along the entrance
sections of chute or duct 11 is incidental, but the placement of
coils 15 is critical. The group of coils 15 includes four coils
each constructed as a printed circuit spiral and mounted in pairs
on both sides of two thin pc boards 16. The two boards are placed
alongside chute 11, on the outside and in such disposition that the
center axes of all four coils 15 coincide. The The coils 15 have a
common axis transverse to chute 11. The same is true for the group
of coils 14 but the groups of coils 14 and 15 are decoupled.
The spiral coils 15 each have a diameter quite accurately similar
to the diameter of a coin of the type to be accepted when placed
into the chute. Since the four coils 15 are coaxial and
concentrically mounted adjacent to and alongside of chute 11, a
coin of the proper diameter will pass the coils 15, so that the
center of the coin will, approximately at least, run through the
common axis of the coils, while the periphery of the coin is
aligned with the periphery of the coils in the same instant.
While the relation as described is basically arbitrary, it can
readily be seen that in the instant of coaxial passage of a coin of
the proper type, a very unique inductivity is established. That
inductivity depends on the size, the resistivity and permeability
of the coin material, the amount of the material and, possibly, the
distribution of the material within the coil. The spiral sensing
coils 15 when positioned parallel to the coin, cause maximum
sensitivity to those properties of a coin, which are effective in
the electromagnetic interaction. The eddy currents flow in a
circumferential path in the coin.
The particular disposition of coils 14 in relation to the
disposition of coils 15 is relevant in the following respect.
First, in the negative sense, the coin as such will not in the
least influence the inductance of coils 14 at the instant of
passing through the coaxial disposition with regard to coils 15.
Otherwise, it is important that coils 14 and 15 are subjected to
the same ambient conditions such as temperature and humidity.
Therefore, changes in inductance on account of such ambient changes
are similar as to both coils; they both track each other. Of
additional significance is here the shunt capacitance of the coils
as determined by the dielectric constant of the material of casing
10. Both coil groups are affected similarly here due to the
disposition on the outside surface of casing 10 adjacent chute 11.
Also, capacitors 26 and 27 may have the same dielectric type with
similar temperature characteristics.
The two groups of coils 14 and 15 pertain to the coin test circuit
depicted in FIG. 2. The circuit includes, basically, an oscillator
20, a sensing circuit 25, a threshold detector 40 and a timer 45
for controlling a solenoid, which in turn controls the lever, gate
or switch 12 (FIG. 1). Elements 40 and 45 together constitute an
amplitude detector/discriminator to determine whether or not a
particular signal as derived from tuned circuit 25 has amplitude
within a narrow range (amplitude detection band).
The oscillator includes two transistors 21 and 22 with
interconnected emitters, which in turn connect to ground potential
via a current source 23, which in this case is a simple resistor.
The collector circuits of the two transistors are established by
two tank circuits 24 and 25 whereby tank circuit 24 is comprised of
the coils 14 and of a capacitor 26, while tank circuit 25 is
comprised of the coils 15 and of a capacitor 27. Tank circuit 25 is
the tuned sensing circuit of the system. The coils 14 are connected
in series to each other, so are coils 15. Tank circuit 24 is an
example of the first tuned circuit, and 25 is an example of the
second tuned circuit as referred to in the introduction.
Thus far, the transistor circuits are symmetrical, however, a
feedback network 30 comprised of a capacitor 31, and of two
resistors 32, 33, together with a bias as applied to the base
circuit of transistor 21, provides for oscillator operation at a
frequency determined by tank circuit 24. A resistor-diode resistor
circuit 34, 35, 36 defines a bias voltage for transistor 21 and
that bias, together with the resistor 23, establishes the emitter
current for both transistors.
The L-C values of tank circuit 24 define a particular frequency,
and positive feedback of 31-32 produces oscillations at that
frequency W.sub.osc. Pursuant to these oscillations the flow of
current I alternates between transistors 21 and 22. It should be
noted, that the tank circuits are out of phase accordingly, and
their phase difference could be used as an indication whether or
not a proper coin is used inbetween coils 15. However in the
present case, the amplitude of the signal across tank circuit 25 is
monitored.
As a consequence of the fluctuation of current flow through
transistor 22, an oscillating current of welldefined magnitude is
driven through (load) tank circuit 25. This tank circuit contains
the sensor coils 15. The voltage across the tank circuit 25 (from
constant current I) depends only on the frequency of the oscillator
in relation to the resonance frequency and the Q of the tank
circuit. Additionally, (and most importantly), the voltage across
25 depends on absence or presence of a proper coin in the coaxial
sensing position relative to coils 15.
It should be noted that the two tank circuits are not components of
a bridge circuit, nor is tank circuit 25 included in a feedback
loop of the oscillator. Rather, the transistor with common emitter
current source can be deemed an amplifier, whose input is provided
by feedback (plus bias) and which has a double ended output
(collectors of the transistors), to which are connected the two
different tank circuits 24, 25.
FIG. 3 depicts the voltage (V.sub.25) across the tank circuit 25 in
response to frequency. The fully drawn characteristic is plotted
under the assumption of a variation of the oscillation frequency in
accordance with the values plotted on the abscissa, whereby it is
assumed further that a proper coin, i.e. a coin of the type to be
accepted, is "in" the coil in axial alignment therewith. The dotted
curve represents the case when the "true" coin is absent, and no
coin or other magnetizable metal (slug) is in the slot 11 between
the portions of the coils 15 on either side of the slot. The
resonance peak in the latter case is considerably displaced from
the resonance peak of the "proper coin case".
It shall now be assumed that the oscillation frequency W.sub.osc is
adjusted to coincide with the resonance peak frequency W.sub.s of
tank circuit 25. Therefore, the peak voltage across the tank in
accordance with the characteristics can and will develop when, but
only when, a proper coin is in the slot (or passes through). A coin
of a different metal alloy or different dimensions will result in a
higher or lower voltage across tank circuit 25 (V.sub.25).
One can see from FIG. 3 that the tuned tank circuit 25 with proper
coin has a particular sensitivity frequency band about the peak
frequency W.sub.s and the oscillator frequency W.sub.osc is not
only in that band, but in the center thereof. A high Q of circuit
25 results in a narrow band width. With no coin inbetween coils 15,
the band is shifted to a frequency range quite remote from the
oscillator frequency. W.sup.1 represents the resonance frequency of
circuit 25 with no coin or slug in coils 15.
The voltage across tank circuit 25, (but taken relative to ground)
is monitored by a circuit which includes a diode 37 and a pair of
serially connected resistors 38, 39. This circuit cooperates with
threshold detector 40, which includes two differential amplifiers
or comparators, 41 and 42.
Each comparator receives the same bias voltage V.sub.R derived from
the voltage source V by a voltage divider network 43/44, but the
signal input of each comparator is not the same, the resistor 38
represents the difference of response. Together with comparators
41, 42, resistor 38 establishes a particular response band.
By virtue of the common bias V.sub.R of the comparators, the two
comparators will change state for the same voltage when applied to
the respective signal input terminal, but due to the voltage drop
across resistor 38 comparator 42 will not change state at the same
voltage across tank circuit 25 which causes comparator 41 to change
state and vice versa. This difference in response to tank circuit
voltage V.sub.25 is the amplitude detection band of the system.
Hence a voltage peak which reaches that band will trigger one of
the comparators but not the other one, voltages outside of that
band will trigger both or neither.
The timer 45 provides an output pulse of sufficient width to drive
the solenoid given an input from comparator 42. One or more output
pulses of comparator 42, defining an unequal state, as compared
with the other will trigger timer 45 to drive the solenoid that
controls the coin gate 12 (FIG. 1).
FIG. 4 illustrates the voltage V.sub.S at the junction of resistor
38 and diode 37 (relative to ground) and as applied as signal input
to comparator 41. That voltage represents the voltage across tank
circuit 25 (modified by the voltage drop across diode 37). The
voltage V.sub.S oscillates about =V in either case, but with
different amplitudes depending upon absence or presence of a coin
(or other metal) in the slot 11 between the sensor coils 15.
Thus, FIG. 4 shows the voltage band .DELTA.V generally described
above and having the following specific significance. The upper
boundary of the band .DELTA.V is given by V.sub.R. For V.sub.S >
V.sub.R comparator 41 is in one state, for V.sub.S < V.sub.R
comparator 41 is in the other state. The band width voltage
.DELTA.V represents the voltage drop across resistor 38, and the
signal voltage effective at comparator 42 is V.sub.S - .DELTA.V
while the reference, of course, is the same, namely V.sub.R. Hence
for V.sub.S > V.sub.R + .DELTA.V comparator 42 is in one state,
for V.sub.S < V.sub.R + .DELTA.V comparator 42 is in the
opposite state.
Case (a) in FIG. 4 represents the fluctuating voltage V.sub.S when
there is no coin in or passes through the tank circuit. The small
amplitude of the fluctuation corresponds to the dotted
characteristics in FIG. 3. Case (b) represents the fluctuations
occurring, for example, when the coin does not decrease inductance
15 enough. In both cases, (a) and (b), neither of the comparators
41, 42 change state, i.e. they are in the normal state.
Case (c) in FIG. 4 represents the situation where a coin reduces
inductance 15 more than the material for a desired coin. The
voltage fluctuations are larger, so that both comparators change
state. They do not change state simultaneously, but one shortly
after the other, 42 before 41, but 41 causes the timer to be reset
before the solenoid can respond to the output initiated by
comparator 42.
The circuit is now adjusted that only a proper coin will produce
case (d). In this case, the voltage excursion minima terminate in
the band .DELTA.V. In other words, comparator 42 changes state, but
not comparator 41. A desired coin, i.e. one that is to be accepted,
will, therefore, produce a situation, in which temporarily
comparator 42 changes state and 41 does not change state.
Circuit 45 responds to the unequal state of comparators 41/42 and
produces a control signal of sufficient duration for actuating the
solenoid, which in turn triggers the switch 12. In FIG. 4 case (d),
comparator 42 changes state and triggers the timing circuit 45
which drives the solenoid for a defined duration. In FIG. 4 case
(c), comparator 42 changes state first and triggers the timing
circuit 45. In case (c), comparator 41 changes state soon after
comparator 42, and thereby resets the timing circuit 45 long before
the solenoid has received sufficient energy to actuate the coin
switch.
The operating conditions as described thus far use exact equality
of oscillation frequency and of sensor tank resonance frequency for
an inserted proper coin. Hence, the tank circuit 25 is operated on
the resonance peak of FIG. 3; the bias and detection band (FIG. 3)
are attuned accordingly. One can, however, use a slightly larger
oscillation frequency, so that the detection band is located on the
higher frequency flank of the characteristics as plotted in FIG. 5.
This mode of operation allows capacitor trimming for circuit
irregularities, as the final adjustment is the appropriate
placement of the detection band, which amounts merely to fine
trimming of the resistor 23 or capacitor 26. The dotted curve
represents again the characteristics of the tank circuit with no
coin between coils 15.
Another embodiment of this invention would have the oscillator
inductance 14 greater than inductance 15. In this case, detection
would occur at coil 15 when the coin passed through coil 14. In
this case only a proper coin will produce an oscillator frequency
so that the resulting response in sensing coil 15, but with no coin
in its proximity, will produce an amplitude V.sub.25 which is right
in the detection band. This embodiment also rejects voltage and
environment changes, but is less preferred, because the Q of the
oscillator tank circuit is slightly degraded in that mode of
operation.
The placement of the coils 14 and 15 in close proximity causes them
to track each other, i.e. the oscillation frequency and the sensor
tank vary similarly under changes in temperature and/or humidity.
The diode 35 should match transistors 21, 22, which presents no
problem in an IC implementation, so that the temperature dependancy
of the oscillation transistors is offset by a corresponding
temperature dependancy of the bias. Diode 37 in turn tracks diode
35 to render the amplitude detection independent from temperature
variations in the circuit. The voltage threshold, i.e. the
effective response level and placement of the detection band V is
independent from the voltage supply +V, at least to the first
order. The amplifier current I as derived from the current source
is dependent upon the value of voltage V and varies therewith to
the same extent. This increases the a.c. component of V.sub.S in an
amount and by a direction (sign) equal to the change in threshold
as defined by the bias circuit 43/44. Hence, the means establishing
the band track and changes in voltage and in sensing signal as
derived from tank circuit 25.
Returning briefly to FIG. 1 in conjunction with FIG. 2, one can see
that the distinction between coin accept/reject situations has
different consequences for coin guidance by operation of switch 12.
In addition, thereto, or in lieu thereof, one can provide an
indication, visual or otherwise, for example, in form of lamps
identifying whether or not a good coin has been presented for
testing.
The invention is not limited to the embodiments described above but
all changes and modifications thereof not constituting departures
from the spirit and scope of the invention are intended to be
included.
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