What does it do?
| The positive latch produces an output signal that goes high and remains high when the input signal has been high. It is useful for turning something on until a second signal switches it off. |
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How does it operate?
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Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit. |
The positive latch subsystem is a resetable memory block. The output signal goes high as soon as the input signal goes high. Pressing the push switch resets the output signal back to low.
An alternative form of latch is provided by the negative latch subsystem. The positive latch is easier to understand and work with.
The positive latch circuit uses a pair of cross-coupled NOR gates to provide a simple form of memory.
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Rather than using a push switch, the reset signal can come from another sensor or processing block.
The positive latch is an example of positive feedback – the output signal from each NOR gate is fed back to an input pin of the other NOR gate. It is this feedback that produces the latching action. |
Possible applications
- Keeping an output device turned on until it is switched off by a separate switch.
Making
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Pins of 4001 NAND gate IC
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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 latch.
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.
Add the switch and resistor and test that the signal on pin 6 goes low when the switch is pressed. Insert the IC the right way round.
Testing
Make sure that the signal going out (on the green PCB track) goes high and stays high after a high signal on the input pin.
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
- The negative latch – may be useful if there are ‘spare’ NAND gates but it is more difficult to understand
- PICs – more flexible but more expensive.
- The thyristor acts as a combined latch and driver
Web links
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