Proximity switch remote control circuit

The proximity switch is a transistor contactless position switch. Its function is to send an action signal when an object approaches a certain distance
to control a relay or logic element. In addition to stroke control and limit protection, its uses can also be used to detect the presence of metal
bodies, high- speed counting, speed measurement, positioning, changing movement direction, detecting part size, liquid level control, and as a non-contact button, etc. It has the characteristics of reliable operation and
long life, and has been widely used in machine tools in recent years.
    When a metal object enters the magnetic field of a high-frequency oscillation coil that oscillates stably at a constant frequency, eddy current losses are generated inside the object, and
there is hysteresis loss for ferromagnetic metal objects, which increases the resistance of the oscillation circuit and increases the energy loss. So that the oscillation weakens until it terminates. Therefore,
by connecting an amplifier circuit and an output circuit behind the oscillation circuit, the presence or absence of metal objects can be detected, and the corresponding control signal can be given to control
the relay K. to achieve control purposes.
    Now we take the LXJ series, which is currently widely used, as an example to analyze the circuit. The circuit is as shown in the figure.
    In the figure, L is the inductance of the magnetic oscillation coil. It forms a capacitive three-point oscillation circuit with capacitors C3 and C4, and uses the capacitor voltage division feedback
signal.
    Under normal circumstances, that is, there is no metal plate close to the magnetic head, the transistor VT1 is in an oscillating state, and the
oscillation , causing point a to present a negative potential and cause the transistor VT2 to cut off, point b The high potential causes VT3 to conduct,
the relay K is closed, and the indicator light is off.
    When the metal plate is close to the magnetic head (oscillation coil), the transistor VT1 stops oscillating. There is no additional negative oscillation signal on the VTZ base,
so VT2 turns from cut-off to conduction, gate '3 is cut off, relay K is released, and the indicator light is on.