■ What is it for?
Since the word "pressure" can have various meanings, let us specify at once that we do not propose to make a barometer here. Our montage of the day makes it possible to detect the increase of a pressure such as that exerted, for example, on a carpet by a thief who enters your home. It can also be used in various skill games that are very popular at school fairs and festivals.
To better understand what it is, know that it is a circuit that lights up an LED, triggers a buzzer and provides logical information for any other assembly (an alarm center for example) when the pressure exerted on its sensor increases.
The difficulty of the operation lies not in the scheme, which remains very simple, but in the sensor. The one we used today is free but proves, nevertheless, very effective.
■ The schema
This "free sensor" is none other than conductive foam in which all statically sensitive integrated circuits should be provided (we wrote "should" because some resellers insist on supplying them with stitches in polystyrene , Which is the worst of the electrostatic materials!). This foam is in fact ordinary soft foam, made conductive by a car-one particle charging, which gives it its black color.
To detect variations in pressure, it is therefore sufficient to choose such a foam plate of adequate size and shape; And put a thread on each side. Any decrease in strength between the two wires will indicate a more or less significant crushing of the foam.
Hence our scheme is very simple to understand. The sensor constitutes the low part of a resistance voltage divider placed at the input of a NAND trigger gate of Schmitt. As long as the foam is not crushed, the voltage at the inlet of this door is high, and its output is therefore low, blocking the door and the transistor which follow. If the foam is crushed, its resistance decreases, the tension also, and the output of the door then passes to the high level. This saturates Ti which lights a LED and delivers a low level for a possible alarm center. In addition, this opens the door connected to the buzzer, which then passes the low-frequency signal generated by the third door mounted, in turn, into an oscillator. When the pressure is relaxed, and provided that the foam is sufficiently flexible, the assembly returns to its initial state.
No difficulty in supplying components or construction is to be expected with such a simple assembly, which works at the last welding performed. Only the choice of foam according to your applications requires a little attention. Several qualities are indeed present on the market: hard foams, hardly compressible, which can then be used for relatively strong pressures (anti-marigold carpet for example) and highly ventilated foams, very flexible, to reserve for much lower pressures.
This foam is sold in large sizes but, for this application, you already have what you need in your drawers, or your dealer will be happy to help you out. The cost price to the square centimeter of such a product is indeed derisory.
The contacts on both sides will be made with simple strands pressed in force for the rigid foam. For soft foam, 4.5 V battery type blade contacts will go very well.
The potentiometer Pi obviously serves to adjust the sensitivity of the mounting depending on the foam used and the pressure that must trigger the mounting.
■ Nomenclature of components
ICi: 4093 CMOS T1: BC 547,548, 549 LED1: LED
Resistors 1/4 W5%
Ri: 5.6 KR - R2: 33 KR - R3: 12KR - R4: 1 KR
C1, C2: 0.1 uF mylar
BZ: piezo buzzer Sensors: conductive foam
P1: adjustable potentiometer for IC, horizontal, 100 KR