Abstract: A magnetic field alternation detecting apparatus includes a magnetic member sensor enclosed in a case and having a core (i.e., a yoke) to which a magnet is attached and around which a coil is wound. The magnetic member sensor obtains as an electromotive voltage a change in magnetic flux linking the coil on the basis of a magnetic field distribution disturbed by an approaching magnetic member present near the case from a pair of output terminals of the coil. An analog-to-digital converter is connected to the output terminals of the coil using at least one control terminal of a transistor such as a bipolar transistor or an enhancement operation field effect transistor. A main current terminal of the transistor is connected to a pair of output terminals.

Abstract: A constant-capacitance capacitor is series-connected with a variable-capacitance capacitor whose capacitance varies with the pressure. The other end of the variable-capacitance capacitor is connected to the ground potential terminal of a D.C. power source. The terminal voltage of the variable-capacitance capacitor is compared with a threshold voltage provided by the D.C. power source by means of a comparator. An output voltage of the comparator is stored into a memory in synchronism with a clock pulse signal. An output voltage of the memory is integrated by an integrator. An output voltage of the integrator is supplied to a buffer amplifier for lowering the signal source impedance with respect to the output voltage. An output voltage obtained from the buffer amplifier is fed back to the other end of the constant-capacitance capacitor and at the same time supplied to an output terminal.