Conventionally, mechanical devices such as, a micro-switch or a limit-switch, have been used to detect the presence of a detecting object or changes of the situation thereof through contact with the object. However, the improvement in the energy-saving views has required devices capable of performing functions not only to detect objects containing less voluminous objects at a certain distance without mechanical contact, but also to respond quickly for detection of the objects. As a result, electrical non-contact type switches were introduced.
Among the non-contact type switches, optical switching devices have advantages of long distance detection and a rapid response time. Such optical switching devices are those of transmission-type, reflection-type and radiation-type as shown in FIGS. 1(A), 1(B) and 1(C) and so on.
Referring to FIG. 2, it depicts a schematic block diagram of a conventional optical switching device for a light modulation type (E3S type) employing an infrared light-emitting diode (hereinafter referred to as "LED") which was developed in Japan and realized by a custom integrated circuit. FIG. 3(A) through FIG. 3(G) illustrate a waveform timing diagram of each part of FIG. 2.
A pulse train generated by an oscillator 1 as shown in FIG. 3(A) is amplified by a current amplifier 2. The amplified pulse train from the amplifier 2 is supplied to an infrared LED 3 which turns on or off depending on the frequency thereof to emit an optical pulse train to a detection object 4. A reflection optical pulse train from the object 4 as shown in FIG. 3(B) is converted into an electrical signal via a photo-transistor 5 and delivered to a light receiving circuit 6. The light receiving circuit 6 includes a variable resistor RR for adjusting the light receiving sensitivity according to the light detecting distance and provides an electrical detection pulse train as shown in FIG. 3(C) corresponding to the reflection optical pulse. The detection pulse train from the light receiving circuit 6 is amplified by an alternating current amplifier 7 and thereafter is supplied to a gate circuit 8. The gate circuit 8 synchronizes the amplified detection pulse train from the alternating current amplifier 7 with the pulse train from the oscillator 1. That is, the gate circuit 8, whenever the pulse from the oscillator 1 is high, makes the amplified detection pulse train from the amplifier 7 pass through and then provides a gated pulse train as shown in FIG. 3(D). A detector 9 receives and converts the gated pulse train into a direct current detection signal as shown in FIG. 3(E). A switching circuit 10 compares the detection signal from the detector 9 with a predetermined potential level so as to eliminate noise signals and then provides a switching ON/OFF control signal as shown in FIG. 3(F). Therefore, the switching circuit 10 provides a logic high level at the time of the presence of the detecting object, while the switching circuit 10 provides a logic low level at the time of the absence of the object.
Therefore, since the conventional optical switching device employs a gate circuit for gating the detection pulse train with the pulse train from the oscillator arranged in a light transmitting portion and a detector for detecting the level of the gated pulse train, incorrectness of positions of the detection pulse train and the pulse train from the oscillator makes the pulse width of the gated pulse train from the gate circuit narrow and as a result, DC detection level of the detector becomes too low to sense the detected object. Also, incomplete synchronization in the gate circuit of the detection pulse train due to the delay with the pulse train from the oscillator can make the detection of a remote object impossible.