JPH034138B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Technical field to which the invention relates The present invention relates to a capacitive proximity switch that detects a nearby object by a change in capacitance between a detection electrode and the ground.

(b) Prior art and its disadvantages A capacitive proximity switch generally includes an oscillation circuit whose oscillation frequency changes as the capacitance of a capacitor changes. FIG. 1 is a block diagram of a typical conventional capacitive proximity switch.

In the figure, an oscillation circuit 1 oscillates at a frequency based on the magnitude of capacitance between a detection electrode 2 and the ground. The oscillation output is rectified by a detection circuit 3, further integrated by an integration circuit 4, and converted into a signal with a voltage level proportional to the magnitude of the oscillation frequency. The signal is subjected to voltage discrimination by a Schmitt circuit 5, and if the output level after integration is above a certain level, an output circuit 6 is driven. A constant voltage circuit 7 supplies a constant voltage to each circuit in order to operate the proximity switch stably.

By the way, a proximity switch is usually required to have good temperature characteristics, and also needs to be as small as possible. However, in the conventional capacitive proximity switch with the above configuration, each circuit is entirely composed of discrete components, so the number of elements is extremely large, making it difficult to downsize. The drawback is that the threshold level in the Schmitt circuit 5 tends to be unstable, making it difficult to compensate for temperature.

(c) Object of the Invention An object of the invention is to provide a capacitive proximity switch that has a small number of components and is extremely stable against temperature.

(d) Structure of the Invention The present invention was made based on the fact that integrated circuits (hereinafter referred to as ICs) for high-frequency oscillation type proximity switches having a current-biased input transistor on the input side are in practical use. It is something.

In summary, the present invention includes: an oscillation circuit whose oscillation frequency changes depending on the capacitance between a detection electrode and the ground; a capacitor, a resistor, and a resistor connected to the output terminal of the oscillation circuit;
An LCR series circuit of a coil, and an IC for the above-mentioned high frequency oscillation type proximity switch having a current-biased input transistor, so as to apply the terminal voltage of the coil of the above-mentioned LCR series circuit to the base of the input transistor of the IC. This is what I did.

(e) Description of Embodiment FIG. 2 is a circuit diagram of a capacitive proximity switch which is an embodiment of the present invention.

In the figure, 10 is a detection electrode, 11 is a shielding electrode, and a detection signal from the detection electrode 10 is input to an oscillation circuit A including a transistor TR1 and a transistor TR2. Capacitor C1 of oscillation circuit A
is a capacitor for negative feedback, and capacitor C2 is a capacitor for positive feedback. In addition, the variable resistor R7 is a resistor for adjusting the detection distance, and the transistor TR
Adjust the amount of positive feedback to 1. The output of oscillation circuit A,
In other words, the emitter output of transistor TR2 is
LCR of capacitor C3, resistor R8, and coil L1
Input to series circuit B. Capacitor C3 cuts off the DC component, and resistor R8 divides the AC component to appropriately set the voltage across coil L1. IC1
is an integrated circuit for the above-mentioned high frequency oscillation type proximity switch, and the input terminal 5 is connected to the coil L1 and the resistor R.
8 contacts are connected.

FIG. 3 is an internal configuration diagram of the IC1.

In the figure, terminal 1 is for connecting a capacitor that lowers the alternating current impedance of the constant voltage output. In Figure 2, capacitor C5
corresponds to this. Transistors Q1 and Q2 constitute a current mirror circuit, and provide a current of the same magnitude as the current determined by resistor R1 to transistor Q3 as a bias current. The voltage VB connected between the terminal 5 and the base of the transistor Q3 is a bias voltage that also serves as temperature compensation for the voltage between the base and emitter of the transistors Q3 and Q4. this voltage
VB is given by, for example, the forward drop voltage of a diode. Terminal 5 is a signal input terminal, and this IC
When the circuit is used in a general high frequency oscillation type proximity switch, an LC resonant circuit is connected between the terminal 5 and the ground terminal 6. A series circuit of emitter resistors R2, R3, and R4 is connected to the emitter of the transistor Q3, and a base of the transistor Q4 is connected to a contact point between the resistors R2 and R3. transistor Q
The emitter terminal 3 of No. 4 is used to connect a resistor R9 for adjusting the operating distance between the emitter terminal 3 and the ground terminal 6. Transistors Q5 and Q6 connected to the collector of transistor Q4 constitute a current mirror circuit, and the collector terminal of transistor Q6 is connected to terminal 4.

With the above configuration, the transistor Q5 detects the collector current of the transistor Q4 determined by the adjustment resistor R9, and flows out a current having the same magnitude as the detected current to the terminal 4. In this case, terminal 5 and terminal 6
If an LC resonant circuit is connected between them, and terminals 4 and 5 are connected, the transistors Q1 to
Q6 performs LC oscillation by current feedback.
The oscillation output is taken out from the emitter of transistor Q3, and waveform-shaped by comparator circuit 12.
Further, the integrator circuit 13 converts it into a voltage level that increases in proportion to the magnitude of the oscillation frequency, and the comparator circuit 14 performs voltage discrimination on the voltage level signal and outputs it to the output circuit 15. Terminal 2 is for connecting an integrating capacitor of the integrating circuit 13. The output circuit 15 has complementary output terminals 8 and 9, and the terminal 8 is always "low" during oscillation.
Terminal 9 is always "high" during oscillation. A constant voltage circuit 16 supplies a constant voltage to each circuit, and a power supply reset circuit 17 turns off the complementary configuration output circuit 15 for a certain period of time after the power is turned on, so that the terminals 8 and 9 are in a non-voltage state. Terminal 7 is for connecting a capacitor for a timer that determines the power reset time of this power supply reset circuit. Transistor Q7 is driven by comparator circuit 14, and resistor R is driven according to the output of comparator circuit 14.
2. Change the partial pressure ratio of R3 and R4. That is, the bias of the transistor Q4 is changed according to the result of voltage discrimination, and the magnitude of the feedback current in the oscillation circuit is adjusted to perform a hysteresis operation.

The high frequency oscillation type proximity switch IC 1 having the above circuit configuration is connected as shown in FIG.
That is, the contact of coil L1 and resistor R8 is connected to terminal 5, terminal 4 is connected to ground, integrating capacitor C4 is connected between terminal 2 and ground, and operating distance is set between terminal 3 and ground. Connect the adjustment resistor R9. Further, a capacitor C5 is connected between the terminal 1 and the ground to reduce the impedance of the constant voltage circuit 16, and a capacitor C6 for setting the power supply reset period is connected between the terminal 7 and the terminal 6 (ground terminal). Note that the output signal of terminal 8 drives LED1 and transistor TR
3 and guided to output terminal b. Diode D1 is used to prevent reverse connection, and is also a Zener diode.
ZD1 and ZD2 are connected to protect the element.

Next, the operation of the capacitive proximity switch having the above structure will be explained.

When power voltage is supplied between power terminals a and c,
Oscillation circuit A oscillates. The oscillation frequency at that time is determined by the capacitance between the detection electrode 10 and the ground. The oscillation output is supplied to the LCR series circuit B from the emitter of the transistor TR2. The DC component of the oscillation signal is cut off by the capacitor C3, and only the AC component is divided by the resistor R8 and the inductance of the coil L1 and guided to the terminal 5 of the IC1.
The oscillation voltage input to terminal 5 is applied to transistor Q3.
and is supplied to the comparator circuit 12 by the emitter follower. The signal whose waveform has been shaped by the comparator circuit 12 is sent to the integrator circuit 13.
The signal is converted to an integral level by the comparator circuit 14, subjected to voltage discrimination, and then guided to the output circuit 15. If the integral level is above a certain level, the comparator circuit 14 drives the output circuit 15 to make the terminal 8 "high". Otherwise, terminal 8 remains "low". That is, if the oscillation frequency in the oscillation circuit A is above a certain level, the terminal 8 becomes "high". When terminal 8 is "high", LED1 emits light and transistor TR3 is driven to set output terminal b to "low". When the object approaches the detection electrode 1 and the capacitance increases, the output terminal b becomes "low", and when the object moves away from the detection electrode 1, the output terminal b becomes "high". A control circuit (not shown) connected to terminal b of the proximity switch can detect when the level of terminal b becomes "low" and know that an object has approached the detection electrode 1.

By the way, in this proximity switch, the AC component of the oscillation output is divided by the resistor R8 and the inductance of the coil L1, and inputted to the terminal 5 of the IC1. The current impedance can be made small, and the alternating current impedance can be made high. That is, by causing the bias current of transistor Q3 to flow to the terminal 5 side, unnecessary bias voltage can be prevented from being generated. If a series circuit of resistors is simply used to divide the voltage of the oscillation output (that is, a resistor is connected in place of the coil L1), an unnecessary bias voltage will be applied to the terminal 5 by the bias current of the transistor Q3. This causes transistor Q3 to be locked in the on state. In order to avoid this situation, terminal 5
If the resistance connected to the oscillation circuit A is made smaller, the output impedance of the oscillation circuit A will be lowered, resulting in the inconvenience that it will no longer oscillate. Therefore, with a current biased input transistor like this
When connecting the IC to an oscillation circuit, by connecting the coil L1 to the terminal 5, the above-mentioned inconvenience can be completely solved.

As described above, by connecting the LCR series circuit between the high frequency oscillation type IC circuit and the oscillation circuit A, it is possible to configure a capacitance type proximity switch that operates appropriately. As is clear from Figure 2, the number of parts in this proximity switch is significantly reduced, and since IC1 is used in the waveform processing section, stability against temperature increases and temperature compensation can be easily performed. It is. Therefore, the threshold level does not fluctuate up and down, and stable operation can be performed over a long period of time.
Furthermore, since conventional waveform processing units use discrete components, they require relatively large-capacity capacitors, but by using ICs in this way, the capacitors can be made smaller.

(f) Effects of the Invention According to the present invention, it is possible to construct a capacitive proximity switch with an extremely small external size compared to the conventional one, and the operation stability is also extremely improved, so the range of application of the switch can be expanded. This has the advantage that it can be made wider and reliability can be greatly improved.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a typical conventional capacitive proximity switch. FIG. 2 is a circuit diagram of a capacitive proximity switch according to an embodiment of the present invention. FIG. 3 is an internal configuration diagram of IC1 used in the above embodiment. 10...Detection electrode, 12...Comparator (for waveform shaping), 13...Integrator circuit, 14...Comparator (for voltage discrimination), Q3...(current biased) input transistor, A...Oscillation circuit ,
B...LCR series circuit.

Claims (1)

[Claims] 1. An oscillation circuit whose oscillation frequency changes depending on the capacitance between a detection electrode and the ground, and an LCR series circuit of a capacitor, a resistor, and a coil connected to the output terminal of this oscillation circuit. , an integrated circuit including an input transistor whose base is applied with the terminal voltage of the coil and current biased, and a waveform processing circuit that produces a predetermined output according to the frequency of the output signal of the input transistor. Capacitive proximity switch. 2. The waveform processing circuit includes a waveform shaping circuit that shapes the output signal of the input transistor, an integrating circuit that integrates the output of the waveform shaping circuit, and a voltage discrimination circuit that discriminates the voltage of the output of the integrating circuit. A capacitive proximity switch according to claim 1.