MAT SWITCH
This simple circuit produces a warning beep when somebody crosses a protected area in your
home or office. The switch, hidden below the floor mat, triggers the alarm when the person walks over it. The circuit uses a conductive foam as the switch. It can be two small pieces of conductive pads usually used to pack sensitive ICs as antistatic cover. Alternatively, you can make the switch by coating conducting carbon ink on two small pieces of a copper-clad board.
Working
When the circuit is in standby mode, transistor T1 does not conduct, since its base is floating. When the person walks, the switch is pressed and current flows through R1 and the switch to
provide positive bias to transistor T1. Transistor T1 conducts and its collector voltage drops, which acts as a negative trigger input for the monostable
Circuit Diagram
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