Metal detector protected from malfunction caused by radio waves

A sensor according to this invention comprises an oscillation circuit including an LC resonant circuit, wherein a detection signal is prohibited upon arrival of an external radio wave.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
This invention relates to a sensor which detects approaching metal without 
actually contacting the metal. More specifically, this invention relates 
to an oscillation circuit including an LC resonant circuit that serves as 
a detection circuit. 
2. Related Art 
The sensor of this invention can be installed, for example, in a pachinko 
machine. When a pachinko ball goes through a payoff hole, the sensor 
detects the passage and, accordingly, the machine pays off a certain 
amount of balls to an operator. A conventional sensor is an induction type 
sensor. When metal approaches an induction coil of a conventional sensor, 
an eddy current is induced in the metal and resistance is raised in the 
coil. Consequently, the metal is detected. 
Other sensors used for detecting the existence of pachinko balls are a 
contact type and an optical type. The contact type consists of a movable 
lever which is movably displaced by the passage of the pachinko ball. When 
the lever is fully displaced it contacts a contact and the machine is 
signaled that a ball has passed. The lever is biased in an off, or closed, 
position and only makes contact, assuming an on state, when a ball passes. 
However, such contact type sensors have drawbacks in that chattering 
occurs, potentially causing miscount of the actual number of balls passing 
through the opening. The optical sensor detects the pachinko ball when it 
passes through a beam emitted from a light emitting device. When the ball 
passes through the beam, a phototransistor does not receive the emitted 
light and detection is indicated. However, optical sensors have drawbacks 
in that "noise" interferes with the count. 
Thus, induction type sensors are usually implemented in pachinko machines, 
avoiding the drawbacks of contact and optical sensors. Specifically, 
induction sensors are subjected to a reduced effect from outside "noise" 
and have a prolonged life because they can detect the ball without 
actually contacting it. 
FIG. 9 illustrates the general appearance of an induction type pachinko 
ball sensor 11. When a pachinko ball 14 goes through a detection hole 13 
in casing 12, which is made from synthetic resin, the energy of a coil L 
incorporated in the casing 12 and surrounding the hole 13 is changed and a 
detection circuit 19, also inside the casing 12, detects the passage of 
the ball 14. 
FIGS. 10 and 11 show the principle of a sensor. When a metal piece 15 is 
far away (at a distance 11) from the coil L, an oscillation circuit's 16 
output (V out) oscillates. However, when the metal 15 approaches (to a 
distance 12), the oscillation circuit's 16 output (V out) no longer 
oscillates because the metal 15 is subjected to magnetic flux from the 
coil L. Consequently, the metal 15 produces an eddy current due to 
electron conduction. The electron conduction increases resistance in the 
oscillation coil L and oscillation stops, as shown in FIG. 11 (2). 
FIG. 12 shows a Hartly type oscillation circuit used as the oscillation 
circuit 16 in FIG. 11. This oscillation circuit 16 comprises a transistor 
TR11 for amplification, an LC resonant circuit 17, and resistor R12. The 
operation point of transistor TR11 is determined by arranging a constant 
current source 21 and bias voltage Vs. Transistors TR12 and TR13 comprise 
a mirror circuit. The collector circuit of TR13, which has the same bias 
as the collector current of TR11, is fed back to the LC resonant circuit 
17. Additionally, power source voltage is Vcc, output of the oscillation 
circuit is Vout, and G is the ground. 
As shown in FIG. 13, a metal body 15 is approaching or going away from the 
sensor containing the above-mentioned oscillation circuit 16. The output 
signal Vout, shown in FIG. 14, changes with respect to metal's proximity 
to the sensor. FIG. 14 (1) shows the oscillation coil output while FIG. 14 
(2) represents binarization of the coil's output. The Vout is binarized 
according to a predetermined threshold level; binary signal S corresponds 
to the periods of reduced oscillation from the coil that occur when metal 
is in close proximity to the sensor, i.e. when the pachinko ball 14 goes 
through the detection hole 13. 
In addition to the Hartly type oscillation circuit, a Corbitt type 
oscillation circuit, as shown in FIG. 15, may be used to detect pachinko 
balls. In a Corbitt type oscillation circuit, a pair of capacitors C1 and 
C2 and oscillation coil L comprise LC resonant circuit 20. Additionally, 
LC resonant circuit 20 and a plurality of resistors R13, R14, R15 are 
electronically connected to a pair of transistors T14, T15. 
SUMMARY 
The problem with conventional sensors is that some people are able to 
intentionally deceive the sensors to falsely indicate proximity of metal. 
Thus, such people can unlawfully cause a machine to indicate pachinko ball 
passage into a prize hole and reap the benefits accorded to such passage. 
Specifically, some people illegally direct a transceiver emitting radio 
waves toward the sensor located in the prize hole to generate a detection 
signal even when no ball goes through the hole. Such transceivers are 
small and thus easily concealed from pachinko parlor personnel, presenting 
a problem of reduced revenue for the parlor. 
Referring to FIG. 12, this intentionally caused malfunction seems to be 
caused by internal capacitance circuit 18 oscillation. More specifically, 
a radio wave induced from oscillation coil L to oscillation circuit 16 
reaches a base of transistor TR11 and decreases a difference of voltage 
from ends of internal capacitance 18. Consequently, sufficient voltage is 
not applied between base and emitter and transistor TR11 is shut off. When 
the transistor TR11 is shut off, the detection signal S is generated and 
the pachinko machine will mistakenly dispense winnings. 
Accordingly, this invention has a primary object of preventing the 
occurrence of a malfunction caused by a radio wave by having a function 
detecting an existence of such a radio wave being directed towards the 
oscillation coil of the sensor. 
Therefore, a sensor according to this invention comprises an oscillation 
circuit including an LC resonant circuit as a detector circuit, wherein 
the sensor features a radio wave detection circuit which produces a 
prohibition signal when it detects the presence of radio waves. When an 
object to be detected is far away, the oscillation circuit's output 
oscillates. When the object is near, oscillation stops and the detection 
circuit produces a detection signal S. However, if radio waves are 
directed towards the sensor, the radio wave detection circuit detects such 
waves and produces a prohibition signal so that any detection signal 
produced is ignored. 
Other objects and advantages of this invention will become apparent from 
the following description and accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a detection circuit of a pachinko ball sensor according to 
this invention. However, this invention shall be applied to other fields 
and is not restricted to this application, namely that of a pachinko ball 
sensor. Moreover, since the appearance of the sensor according to this 
invention is the same as that of FIG. 9, additional drawings and 
explanations are omitted. The detection circuit 1 shown in FIG. 1 
comprises an oscillation circuit 2 and an integration and oscillation 
comparison circuit 3 and an output circuit 4, wherein the above circuits 
are electrically connected to each other. The oscillation circuit 2 
includes a radio wave detection circuit 7 in addition to a conventional 
object detection circuit 5. As previously described, the oscillation 
circuit ceases to oscillate when the pachinko ball passes through the 
detection hole 13. 
FIG. 2 (1) shows an oscillation wave Vout-1 corresponding to the proximity 
of metal to the object detector circuit 5. The radio wave detector circuit 
7 binarized output, shown in FIG. 2 (2), is a prohibition signal K which 
is produced upon detection of radio waves. As can be seen in FIG. 2 (3, 
4), when oscillation ceases due to emitted radio waves, no detection 
signal S is produced because such a signal is prohibited by prohibition 
signal K (case b). Furthermore, even if the pachinko ball goes through 
detection hole 13 when radio waves are directed to the sensor, prohibition 
signal K precludes the detection signal from being produced (case a). 
Accordingly, detection signal S is only produced when the ball passes 
through the detection hole 13 while no radio waves are being directed to 
the apparatus. Consequently, pachinko machines incorporating the induction 
type sensor of this invention will no longer be fooled and cheated by 
people using a transceiver to emit radio waves. 
FIG. 3 shows an embodiment of oscillation circuit 2 using a Hartly type 
circuit, wherein object detection circuit 5 and radio wave detection 
circuit 7 are included therein. The object detector circuit 5 comprises 
transistors for amplification TR1, TR2, LC resonant circuit 6, resistor 
R1, and adjustable resistor VR, wherein amplification transistor TR1 has 
its base and collector short-circuited. A constant current source 8 
supplies amplification transistors TR1, TR2 with bias voltage. Transistors 
TR3,TR4,TR5 constitute a feedback circuit. More specifically, transistors 
TR3, TR4 constitute a mirror circuit which feeds back the same current to 
the LC resonant circuit 6 as the collector current fed back from the 
transistor TR2. 
The radio wave detection circuit 7 comprises a pair of transistors TR6, TR7 
and a plurality of resistors R2, R3, R4, R5, wherein the base of 
transistor TR6 is connected through resistor R2 to the base of transistor 
TR1 of the object detection circuit 5. Furthermore, the transistor TR6 is 
an active circuit. The voltage between the base and emitter of transistor 
TR1 is equal to the voltage between the base and the emitter of TR6 plus 
the voltage of both ends of the resistor R2, as shown by the equation: 
Vbe1=Vbe6+Vr2. 
The relationship between constant current Io of transistor TR1 and constant 
current Ic6 of transistor TR6 is such that Io is much larger than Ic6 
(Io&gt;&gt;Ic6) during periods of normal operation. Accordingly, a comparison of 
the two transistors' TR1, TR6 operation points shows that the operation 
point of transistor TR6 is closer to the shut off domain. As a result, 
when voltage Vbe1 of transistor TR1 drops for some reason, transistor TR6 
will shut off before transistor TR1 shuts off. 
FIGS. 6A and 6B show the respective voltages V1, V5 between the base and 
emitter of transistor TR1 and the output voltage Vout of the object 
detector circuit 5. FIG. 6A shows a voltage wave produced when the 
oscillation operation ceases due to an approach of a metal object. FIG. 6B 
shows the oscillating voltage wave that occurs when no metal object is in 
close proximity to the sensor. According to FIG. 6B the base voltage V1 of 
transistor TR1 is the center line, wherein the voltage represents the 
equation: (R1.times.Io)+Vbe1. The potential voltage Vbe1 is oscillating 
around center voltage R1.times.Io. The output Vout-1 is around zero volts; 
each voltage oscillates around one of the three center lines. 
Transistor TR6 of the radio wave detection circuit 7 has voltages Vb1 and 
Vb5 applied thereto so that its voltage Vbe6 stays constant and its 
connector current Ic also theoretically stays constant (actually, a slight 
oscillation impulse current appears). As a result, the voltage difference 
between both ends of resistor R4 is represented as R4.times.Ic6. If 
R4.times.Ic6 is greater than 0.6 volts (for example, around 1 volt), then 
transistor TR7 stays on whether the object detection circuit 5 is 
oscillating or not. 
FIGS. 7A and 7B show base voltage V4 of transistor TR7 and collector 
voltage V3 of transistor TR6 respectively. FIG. 7A shows the voltage wave 
in the case of oscillation termination while FIG. 7B shows the voltage 
wave in the case of oscillation. According to FIG. 7B collector voltage V3 
of transistor TR6 has a slight oscillation. 
The operation of the oscillation circuit 2 will now be described, referring 
to FIG. 4. A metal, for example, a pachinko ball, is away from the 
oscillation coil L. The oscillation circuit 2 initiates oscillation and 
each part of the circuit has a voltage level shown in FIG. 4 (1) (2) (3). 
FIG. 4 (1) shows the base voltage V1 of transistor TR1 of the object 
detection circuit 5. FIG. 4 (2) shows the base voltage V2 of transistor 
TR6 of the radio wave detector 7. FIG. 4 (3) shows the collector voltage 
V3 of transistor TR6. 
Output Vout-1 of the object detector 5 is shown in FIG. 2 (1). Outputs 
Vout-3 and Vout-4 of the integration and oscillation comparison circuit 3 
and the output circuit 4 are shown in FIG. 2 (3) and FIG. 2 (4) 
respectively. Output Vout-2 of the radio wave detection circuit 7 is shown 
in FIG. 4 (4) wherein transistor TR7 is in an ON state. Output Vout-2 of 
the radio wave detection circuit 7 is input to a comparator (not shown) 
and compared with a standard reference value TH, producing the output 
Vout-2 shown in FIG. 2 (2). 
Operation of the apparatus will now be described. When a metal object 
approaches the oscillation coil L, the oscillation circuit 2 ceases its 
oscillation operation. Each part of the circuit has a voltage as shown in 
FIG. 4 (2) (3). Accordingly, the oscillation waves disappear. As a result, 
output Vout-3 of the integration and oscillation comparison circuit 3 
drops as shown in FIG. 2 (3). However, output Vout-2 of the radio wave 
detection circuit 7 will not change as shown in FIG. 4 (4) because 
transistor TR7 is still ON. 
When a radio wave is directed towards the apparatus, it comes in from the 
oscillation coil L to the oscillation circuit 2, arriving at the base of 
transistor TR1 through the internal capacitance 18 of transistor TR1 (as 
shown by the broken line in FIG. 5). Both ends of internal capacitance 18 
drop and the base-emitter voltage Vbe1 of transistor TR1 becomes small, 
turning transistor TR1 off. As a result, base voltages V1 and V2 of 
transistors TR1 and TR6 drop as shown in FIG. 4 (1) (2). Output Vout-1 of 
the object detection circuit 5 is shown in FIG. 2 (1) wherein the output 
Vout-2 of the integration and oscillation comparison circuit 3 is raised. 
However, the operating point of transistor TR6 of the radio wave detection 
circuit 7 is set to be closer to the shut off domain as compared to 
transistor TR1 of the object detection circuit 5. Accordingly, transistor 
TR6 shuts off before transistor TR1. As a result, current Ic6 of 
transistor TR6 becomes small and collector voltage V3 is raised as shown 
in FIG. 4 (3). Furthermore, base emitter voltage Vbe7 becomes small and 
transistor TR7 turns off and output Vout-2 of the radio wave detection 
circuit 7 drops as low as 0 volts as shown in FIG. 4 (4). In this way, the 
output Vout-2 drops below the reference value TH of the comparator. Output 
Vout-2 is raised, as shown in FIG. 2 (2), producing prohibition signal K. 
Thus, output circuit 4 regulates the passing of output Vout-3 of the 
integration and oscillation comparison circuit 3, based on the arrival of 
a radio wave, detected by the means of the prohibition signal K. 
The above embodiment employs a Hartly type oscillation circuit 2. However, 
a Corbitt type could be used, as shown in FIG. 8. The Corbitt type 
oscillation circuit 2 includes an oscillation coil L, an LC resonant 
circuit 6, and a pair of capacitors C1 C2, wherein resistors R6 R7 R8 of 
the LC resonant circuit 6 are connected to transistors TR7 TR8, 
constituting an object detection circuit 5. The base of transistor TR8 is 
connected to transistor TR9 through resistor R9. Further, transistor TR9 
is connected to transistor TR10 through resistors R10 R11, constituting a 
radio wave detection circuit 7. Finally, the oscillation circuit 2 of FIG. 
8 has a function of object detection and radio wave detection similar to 
that of the Hartly circuit. Accordingly, specific explanation of the 
operation is omitted. 
FIG. 16 shows a block diagram illustrating another embodiment of sensor 
according to this invention. The sensor has an oscillatin circuit 20 
connected to an LC resonant circuit 10. LC resonant circuit 10 has a 
detection coil L disclosed close to a detection position of a metal ball. 
The oscillation circuit 20, e.g. a Hartly type, is supplied with voltage 
and oscillates during periods of normal operation, i.e. when no metal is 
present. An oscillation output signal is fed to a substrate type PNP 
transistor TR10. The substrate type PNP transistor TR10 has a collector in 
common with a substrate and is connected to the ground. Furthermore, the 
transistor TR10 constitutes an emitter follower circuit. The emitter of 
transistor TR10 is connected to a power supply Vcc through a resistor R10 
and to an emitter of a NPN type transistor TR20 through resistor R20. The 
base of transistor TR20 has a constant voltage Vo, e.g. 2.8 volts supplied 
from voltage source 30, connected thereto. The collector of transistor 
TR20 is connected to the base of a PNP type transistor TR30. The emitter 
and collector of transistor TR30 are connected to the power supply Vcc and 
an output terminal Vout respectively. Transistors TR20 TR30 constitute a 
comparison circuit 40, wherein the constant voltage Vo serves as a 
threshold level. The structure of an integrated circuit using a substrate 
type PNP transistor TR10 will be described hereinafter. 
FIG. 17 shows transistor TR10 and its emitter resistor R10. A substrate of 
the integrated circuit comprises a P layer 100. N layer 110 is formed over 
the P layer 100. In P layer 100 there are provided P+ domains 12 13 
protruding towards the top. Mutual electrodes are provided thereupon and 
are connected to the ground and output terminal Vout respectively. 
In N layer 110, N+ domain 14 and P+ domain 15 are closely provided as a 
base and emitter respectively, constituting the substrate type PNP 
transistor TR10. Furthermore, in N domain 11, P+ domain 16 is formed as 
resistor R10. Resistor R10 is connected to P+ domain 15 by means of a 
metal body. P+ domain 16, constituting resistor R10, is connected to the 
power supply Vcc through N+ layer 17. 
Operation of the sensor depicted in FIGS. 16 17 will now be described. FIG. 
18 (a) (b) shows output signals of comparison circuit 40 and emitter 
voltage of transistor TR10 wherein a metal ball is either in close 
proximity to or away from the oscillation coil L. The metal ball is away 
from the coil L at time T1, wherein the oscillating circuit 20 maintains 
oscillation at a constant frequency. The emitter of transistor TR1 
provides an amplitude wave shown in FIG. 18 (a). In this state, transistor 
TR10 turns on and off repeatedly. Accordingly, a signal dropping below the 
threshold voltage value Vo is supplied to transistor TR20. Thus, at time 
T1, transistor TR20 also repeatedly turns on and off. As a result, as 
shown in FIG. 18 (b), transistor TR30 turns on, voltage output Vout is 
high, and the absence of metal is detected. On the other hand, when a 
metal ball passes closely to the coil L at time T2, the oscillation 
circuit 20 ceases oscillation, as shown in FIG. 18 (a). At this time T2 
the emitter of transistor TR20 surpasses the threshold voltage value Vo, 
transistors TR20 TR30 turn off, and the voltage output Vout drops, 
indicating the proximity of metal. 
As shown in FIG. 19, an intense radio wave, e.g. 430 mh (40 watts), is 
directed towards the coil L in the oscillation circuit 20 at time T3. At 
this time, PNP transistor TR10 turns on and the emitter voltage falls to 
almost zero volts. This happens because the substrate serves as a kind of 
antenna, losing a function as a transistor. As shown in FIG. 19 (b), 
transistors TR20 TR30 also turn on during time T3. At time T3, the output 
of transistor TR20 goes down and the output of transistor TR30 goes up, 
indicating the absence of metal. Therefore, when a radio wave is directed 
to the coil L, an output showing no metal ball is produced irrespective of 
a ball's presence. When radio wave emission ceases, the apparatus returns 
to normal operation. 
FIG. 20 shows a signal wave wherein metal is in close proximity to the coil 
L and a radio wave is alternatingly being directed at the apparatus. When 
the radio wave is not emitted towards the coil L, transistors TR10 TR20 
TR30 are all turned off and metal ball presence is detected. On the other 
hand, when a radio wave is emitted, the emitter voltage of transistor TR10 
drops to around zero volts and transistors TR10 TR20 TR30 all turn on, 
producing a signal corresponding to metal ball absence. In this 
description, the radio wave is emitted at 430 mh (40 watts). However, the 
embodiment is not limited by this figure as the effect can be obtained for 
signals ranging from 10 to 1000 mh. 
The above descriptions and accompanying drawings are merely illustrative of 
the applications of the principles of the present invention and are not 
limiting. Many other embodiments falling under the spirit and scope of 
this invention may be devised by those skilled in the art. Accordingly, 
this invention is only limited by the scope of the appended claims.