This car alarm was obtained in a Signetics documentation from the 1980s. The circuit uses the 555 and has a pulsed output to activate a relay that controls the horn. The sensors are parallel connected.
In this circuit of a 1991 publication, we can use the 2N2646 unijunction transistor and the sensor...
Measuring capacitances and resistance this simple instrument was found in Radio Electronics...
Found in a 90’s 303 Circuits, this circuit uses components that are still common and has a common...