Abstract: In a switching circuit formed as an integrated circuit, a series circuit comprising a diode (D.sub.1) and a resistor (R.sub.1) is connected between a base and an emitter of an npn transistor (TR.sub.1) requiring a high speed switching operation. Therefore, a high speed operation is made possible. Furthermore, in a circuit constructed such that the above npn transistor is driven by a pnp transistor (TR.sub.3, TR.sub.4), a leakage current produced in the above pnp transistor at high temperature is allowed to flow in the above series circuit. Accordingly, a malfunction of the above npn transistor is prevented. Consequently, an integrated circuit operable even under high temperatures is achieved.

Abstract: The detection system comprises a sensor and a controller which are connected by two wires. The sensor can take more than two states as opposed to conventional detection systems. According to this arrangement, for instance, a small leakage current exists even when the sensor is in normal off state, and, therefore, electric current in the range of I1 to I2 flows even in normal off state. On the other hand, electric current of I3 to I4 flows in normal on state. When there is a wire breakage, no electric current flows. Conversely, when there is a shortcircuiting, electric current exceeding I4 flows. Electric current in the range of I3 to I4 corresponds to an unstable state which is neither normal on state nor normal off state.

Abstract: An oscillation circuit of a proximity switch includes a parallel LC resonance circuit (L, C), an oscillation transistor (Tr6) controlled by an output voltage from the resonance circuit for switching a constant current and a feedback circuit (Tr2, Tr3) for feeding back a current flowing through the oscillation transistor to the resonance circuit. In the circuit, there is further disposed a feedback current increase circuit including a second transistor (Tr7) controlled by an output voltage from the resonance circuit and a resistor (R) connected in series to the second transistor. The resonance circuit is supplied via the feedback circuit with a feedback current obtained by adding a current flowing through the feedback current increase circuit and a current passing through the oscillation transistor, thereby developing a large change in the amplitude of the oscillation.