What does it do?
| The Schmitt inverter subsystem, also known as a Schmitt NOT gate, provides an output signal which is opposite to the input signal. The Schmitt inverter is ideal for converting analogue signals into digital signals. |
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How does it operate?
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Schmitt Inverter Circuit
Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit. |
When the input signal goes above an upper threshold voltage, the output signal goes low and remains low until the input signal falls below the lower threshold, when the output signal goes high.
This action provides a cleaner output signal by removing electrical noise and makes the Schmitt inverter ideal for converting analogue signals into digital signals. |
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When used with a digital input signal, the Schmitt inverter acts like a normal inverter subsystem.
The Schmitt inverter circuit uses a Schmitt trigger. Schmitt triggers are logic gates that have been designed to include 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 applications
- Removing the effects of noise due to fluorescent lighting on a light sensor
- Debouncing mechanical switches.
- Producing a digital signal from an analogue sensor, e.g. a thermistor or light sensor.
Making
Pins of 40106B Schmitt inverter |
How part of the PCB might look |
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.
Build and test the unit that will provide the input signal before building the Inverter.
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);
- the voltage on pin 14 is high (the supply voltage);
- the voltage on pin 1 (the blue PCB track) goes high and low in response to the unit that provides the input signal.
Insert the IC the right way round.
Testing
Make sure that the signal going out (on the green PCB track) changes from high to low.
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
- An ordinary inverter provides simple inverting action
- A comparator can be used for converting an analogue signal to digital
- PICs – more flexible but more expensive.
Web links
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