US 3901368 A
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United States Patent [1 1 Klinger Aug. 26, 1975  COIN ACCEPTOR/REJECTOR 3,796,295 3/1974 Montolivo et a1. 194/100 A  Inventor: Lance T. Klinger, 81 1O Redlands,
APL 307 playa Del Rey, C ]if Primary Examiner-Allen N. Knowles 2 Attorney, Agent, or Firm-Ralf H. Siegemund 9 91 I  Filed: Mar. 11, 1974 57 B A  Appl. No.: 450,088 1 A STR CT Coins are tested by inserting them into an oscillator driven tank circuit resonating at near or equal fre-  Cl 194/100 A; 324/34 gags 1 quency when the proper coin influences particularly  Int. Cl. R /9 i the tank circuit A narrow amplitude detection band  held of Search 194/100 and flat spiral tank circuit coils of coin-like diameter 324/34 73/163 209/81 A limit the response to particular coins. The oscillator includes a similar tank circuit and both tank circuits  g NT are subjected to the same environment.
UNITED TA S ATE S 3 599 771 28 Claims, 6 Drawing Figures 8/1971 Hinterstocker 194/100 A Mill/V7700; new! rue (wax/r rmv/m z-wr;
PATENTED MJEZSIQIS 3, 9 O1 ,3 6 8 sum 2 UF 2 1 COIN ACCEPTOR/REJECTOR 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 inductiv-ity of a sensor coil. Reference is made here to the US. 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 tempera ture 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 am-' tion 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, possibie 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. lb 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 1112. 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 contain 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 l4 and 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 l5 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 l5. Otherwise, it is important that coils 14 and 15 are subjected to the same ambient con ditions 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 l4 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 and a timer 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 I 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 l4 and of a capacitor 26, while tank circuit 25 is comprised of the coils l5 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 Pursuant to these oscillations the flow of current 1 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 across the tank cir cuit 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 fur ther 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 is adjusted to coincide with the resonance peak frequency W., 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)- One can see from HO. 3 that the tuned tank circuit 25 with proper coin has a particular sensitivity frequency band about the peak frequency W, and the oscillator frequency W,,,,,. 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 inbetwcen coils 15, the band is shifted to a frequency range quite remote from the oscillator frequency. W 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 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 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 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).
Thus, FIG. 4 shows the voltage band AV generally described above and having the following specific significance. The upper boundary of the band AV is given by V For V,,- V comparator 41 is in one state, for V V comparator 41 is in the other state. The band width voltage AV represents the voltage drop across resistor 38, and the signal voltage effective at comparator 42 is V A while the reference, of course, is the same, namely V Hence for V V +AV comparator 42 is in one state, for V VR+AV comparator 42 is in the opposite state.
Case (a) in FIG. 4 represents the fluctuating voltage V,,- 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 l5 enough. In both cases. (a) and (b), neither of the comparators 41, 42 change state, i.e. they are in the normal state. I
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 ((1). In this case, the voltage excursion minima terminate in the band AV. 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 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 sufiicient 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 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 should match transistors 21, 22, which presents no problem in an IC implementation, so that the temperature dependancy of the oscillation transistors is ofiset 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 ac. component of V; 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.
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 9 tuned circuit whose amplitude depends 6d 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 vicinityof. the coil of either of the tuned circuits; 7
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 sec ond tuned circuit is tuned to a frequency relative to the oscillation frequency, so that the amplitude of the signal varies significantly forsmall 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 de tection 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.