Here is a pressure-sensitive alarm that won't break the bank. The alarm makes use of a pressure sensor that was constructed at home. This pressure sensor functions as a variable capacitor and is made up of two copper-clad boards with a piece of sponge sandwiched in between them. Take two conventional copper-clad boards measuring 6.5 by 6.5 centimeters each and use them as the basis for the pressure sensor (variable capacitor C1). On the substance made of copper, apply varnish or a green mask. To insulate the space between the copper-clad boards, place a layer of soft sponge approximately 2.5 centimeters thick and secure it with insulating tape, as illustrated in figure 1. Ordinary insulated flexible wires should be soldered to the copper plates on both of the cladded boards. These wires are what are required to link the pressure sensor to the alarm circuit's input device. The pressure sensitive alarm's circuit is depicted in Figure 2, below. When there is no pressure applied to the sensor plates, the capacitance of the home-made sensor drops to

less than 10 pF. This causes the IC 555, which is connected as an astable multivibrator, to become inoperable. When there is pressure applied to the sensor, the gap between the copper-clad boards gets closer together. This causes the capacitance of the sensor to increase, and it might reach up to 50 pF, depending on how close together the copper-clad boards were before. Therefore, the sensor functions within an IC 555- based astable multivibrator as a variable capacitor. The IC 555 has a frequency of oscillation that is approximately 350 kHz, which is inaudible. This output signal is sent to two 4017 decade counters in the system. The decade counter IC2 changes the signal from 350 kHz to 35 kHz, which is also inaudible to the human ear. The output of IC2, which is operating at 35 kHz, is sent to the decade counter IC3, which alters the frequency such that it may be heard as 3.5 kHz. These frequencies can be altered by adjusting either the preset controls VR1 and VR2 or the pressure applied to the sponge sandwiched between the copper-clad boards. The audio frequency signals coming from IC3 at 3.5 kHz are amplified by the transistors T1 and T2 before they are sent to the loudspeaker. This results in the loudspeaker emitting a high-pitched audible tone. Circuit Application You could put the speaker in a convenient spot within your home, and the home-made pressure sensor could go beneath the doormat at the front of your property. The alarm will activate whenever somebody steps on the mat. It is possible to power the circuit with a supply of 6-12 V DC. Constructions and quality assurance Assemble the circuit on any PCB that can be used for general purposes, and then enclose it in an appropriate cabinet. Establish a connection between the sensor and the speakers using the flexible wires. You can play around with different sizes of pressure sensors to achieve a variety of tones and levels of volume.