| QTC comes in the form of sheet, ‘pills’ and cable. QTC sheet can be used to make very low cost touch switches that can be placed on the case of products. QTC ‘pills’ can be used to produce low cost force/pressure sensors whose resistance varies with the applied force. |
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How does it operate?
Quantum tunnelling composite is a recently developed ‘smart’ material that exhibits extraordinary electrical properties. In its normal state it is an insulator, but when it is compressed it conducts. The material owes its unusual properties (and its name) to a strange phenomenon of quantum mechanics which mean that electrons are able to ‘tunnel’ through some materials i.e. conduct, if their physical structure is slightly changed (by pressure).
QTC sheet
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The main application of QTC sheet in schools is in making low cost touch switches.
QTC sheet consists of three layers:
- a thin layer of (light grey) QTC
- a conductor, and
- a (white) plastic insulator
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To make up a simple switch on the case of an electronic product, self adhesive copper track can be trapped under screw heads and the screws connected to the electronic circuit.
The copper track can then be stuck onto the product surface so that the two sides of the switch come to the required switch position. A small piece of QTC sheet (cut from the larger sheet in which it is supplied) with the white plastic side on top is used for the switch, held in position by sellotape. |
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Only stick the tape over one side of the QTC sheet, not on the area where there is a gap between the copper tracks; sticking it over the entire sheet can create pressure on the switch and make it close all the time.
Graphics can be added on top of the switch, but should not be stuck directly onto the QTC sheet because again this can ‘press’ the switch. A thin sheet of foam can be placed over the QTC switch and the graphics stuck onto the foam. This prevents the graphics from pressing the switch. |
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QTC sheet switch circuit |
The QTC sheet switch is included in the circuit in the usual way. A pull-down resistor (R1) of about 3k3 is suitable. The value of this resistor is not critical. If a larger value is used the switch becomes more sensitive (operates with lower force). The value of 3k3 gives a switch that only requires moderate force and is not triggered by light pressure e.g. from the weight of the foam sheet over the switch.
The resistance of the QTC sheet switch when moderate pressure is applied is around 1 – 2k. |
Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit. The ‘QTC sheet’ in the Livewire circuit is represented by a variable resistor. |
QTC ‘pills’
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Unlike QTC sheet (which switches quite quickly between a high and low resistance), QTC pills are pressure sensitive variable resistors.
They can be used as input sensors in electronic systems and the signal from them can go to analogue processing units (such as PICs with analogue to digital converters or amplifiers) to produce a system that responds to weight or force. |
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One way to arrange for contact between the pill and the electronic circuit is to use a pair of self adhesive copper tracks, with the QTC ‘pill’ sandwiched between them (it is not necessary to remove the glue).
QTC pills are more expensive than QTC sheet and so QTC sheet is a better choice if all that is needed is a simple touch switch. |
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To represent a QTC pill in a circuit simulation on a computer, use a variable resistor. |
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Possible applications
QTC sheet can be used to produce:
- low cost touch switches
- switches that can be placed on the surface of products and decorated with graphics
QTC pills can be used to produce:
- input sensors that respond to force or weight
- devices that can control quite high currents, allowing motor speed control using force
Making
QTC sheet switches can be incorporated into the product design for the case, creating adaptable membrane switches. It is best to protect them with a thin foam cover to which graphics can be applied.
When using QTC sheet to make touch switches, it is important to make sure that:
- the QTC sheet is placed with the grey side (the QTC material) next to the copper track and the white side (the insulating plastic layer) away from the copper track
- the stick-on copper track is in good electrical contact with the electronic circuit. Just sticking it onto a terminal or copper track on the circuit does not give reliable electrical contact. It is important to trap the stick-on copper track under a screw head or a hexagonal metal spacer.
Testing
QTC sheet touch switches should be tested by checking the signal going out with a multimeter on the voltage setting. The signal should be low if the switch is not pressed and high when it is pressed.
QTC pills should be tested (before connecting the power supply) using a multimeter on the resistance setting. When no force is applied the resistance should be megohms. Applying a force of about two kg weight should reduce the resistance to less than 100W.
Fault finding
If there is a fault with QTC sheet, check that:
- The sheet is the correct way up
- Nothing (such as the sellotape or graphics) is pressing on the QTC in the gap between the copper tracks
- The copper tracks are not touching each other
If there is a fault with a QTC pill, check that:
- There is good electrical contact between the electronic circuit and the QTC pill
- There is no short circuit across the QTC pill
Alternatives
- Push switches can be used in place of QTC sheet touch switches. They are more expensive but may be easier and clearer to use in some situations.
- Load cells can be used in place of QTC pills, but they are much more expensive.
Web links
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