What does it do?
| Provides a switch that produces a sharp change from low to high, suitable for use with a counter. |
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How does it operate?
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Whenever a mechanical switch (such as a push switch, slide switch, microswitch or reed switch) is pressed, the switch contacts will bounce, producing several very quick on and off signals. Each of these signals would be counted by a counter subsystem.
To overcome this, a debouncing circuit is used to produce a clean output signal from the switch. |
NB: the graph is purely illustrative and shows stylised bounce patterns |
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Possible debounced switch circuit
Click on the circuit diagram to download a Livewire file of the debounced switch that you can investigate and add to your own circuit. |
The debounced switch circuit uses a push-to-make (PTM) switch:
A capacitor along with a pair of Schmitt triggers are used to clean the signal.
Schmitt triggers are logic gates that have both an upper and a lower threshold level.
This ability of the Schmitt trigger to switch on and off at different voltage levels is known as hysteresis. |
Possible Actions
Making
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Pins of 40106B Schmitt inverter |
How part of the PCB might look
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The PCB shows the basic circuit. Several gates in the IC are not used in this simple design; they can be applied in other subsystems. Any unused input pins should be connected to 0V or Vs, to prevent damage by static electricity.
Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that the voltage on pin 7 is low (0V) and the voltage on pin 14 is high (the supply voltage).
Connect the negative lead of the capacitor to 0V. Insert the IC the right way round.
Testing
Make sure that the signal going out (on the green PCB track) changes from low to high when the switch is pressed.
Fault finding
If there is a fault, check that:
- The voltage on pin 7 is low (0V)
- The voltage on pin 14 is high (the supply voltage)
If there is a fault, check the tracks and solder joints.
Alternatives
- Push Switch – cheaper, but the switch bounces, which can give a problem with counters.
Web links
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