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Pressure Pad – Step On It!

by:Thomas Scarborough,

Cape Town, South Africa

EPE june 04

 

 

 

TRADITIONAL pressure mats (or pads) may be useful in a variety of applications, particularly for security. A smaller sized pressure pad may be especially useful in soft toys, to trigger a sound or an action. However, pressure mats (or pads) of any kind tend to be pricy, even without the electronics. They also tend to be tailored to specific applications, and this may limit their uses. While one could resort to a simple pressure switch, such switches often pose problems with mounting.

 

Fig.1. Versatile Pressure Pad circuit diagram.

 

The design in Fig.1 uses a sandwich made of conductive foam as the pressure pad. Conductive foam sheets are widely and inexpensively available, being used in particular for the safe storage of static sensitive devices. A small piece, say 2cm × 2cm, can be sandwiched between two similar sized pieces of copper-clad board, with the copper making contact with the conductive foam on each side. If the conductive foam is a little brittle to begin with, it may be softened up by squeezing it. The “sandwich” may be held together with sticky-back tape. This is then wired to the simple circuit as shown in Fig.1, which is a common j.f.e.t. op.amp wired as a non-inverting comparator. Supply voltage may vary between wide margins (3·5V to 18V). Although a 12V relay is shown here (RLA), the output (pin 6) may be used to switch any logic circuit of corresponding supply voltage. When minimal pressure is applied to the “sandwich”, the resistance of the foam may represent around half a mega ohm. When pressure is applied, resistance drops dramatically. It will easily drop to 10k, and with heavy pressure may drop below 1k. Depending on the application, VR1 adjusts the circuit to respond to the appropriate amount of pressure. Different sizes of pressure pad may be built, and in this case one only needs to ensure that potentiometer VR1 is suitably chosen and adjusted to the resistance of the pressure pad, with the voltage at pin 3 rising above that of pin 2 (half supply voltage) when pressure is applied. More innovative uses are possible, e.g. switching off the light when getting into bed, or monitoring the amount of time someone spends in a seat.