| The retriggerable monostable subsystem produces a delay after the input signal goes high. |
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How does it operate?
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Retriggerable monostable circuit
Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit. Note that Livewire does not include the 4098B monostable IC, so the circuit diagram uses a 16 pin IC outline as a substitute. The circuit operation therefore cannot be simulated.
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The timing period begins as soon as the input signal changes from low to high.
If the input signal changes from low to high again during the timing period, the timing starts again.
The retriggerable monostable is slightly different from the delay subsystem where the timing period begins when the input signal goes low.
The retriggerable monostable circuit uses the ability of an electrolytic capacitor to store charge.
The 4098B IC actually contains a pair of monostables as shown in the pin diagram below. So the IC can be used to produce a pair of independent monostables. For full details see the IC data sheet. |
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The length of the delay can be calculated:
Delay time (t) = 0.5 ´ R1 ´ C1 where R1 is the resistance in megohms and C1 is the capacitance in mF.
So, if R1 = 1M and C1 = 10mF, the delay is 0.5 ´ 1 ´ 10 = 5s.
If a longer delay time is needed then a larger capacitor can be used. |
Possible applications
- Making a buzzer sound for a fixed length of time after a switch has been pressed.
Making
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Pins of 4098B IC
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How part of the PCB might look
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Build and test the unit that will provide the input signal before building the retriggerable monostable unit.
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 pins 3, 4 and 8 is low (0V);
- the voltage on pins 5, 11, 13 and 16 is high (the supply voltage);
- the voltage on pin 12 (the green 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 blue PCB track – from pin 10) changes from high to low.
Fault finding
If there is a fault, check that:
- The voltage on pins 3, 4 and 8 is low (0V)
- The voltage on pins 13 and 16 is high (the supply voltage)
If there is a fault, check the tracks and solder joints.
Alternatives
- The delay unit – is triggered as long as the input signal is high. Timing is not as accurate. There are no ‘spare’ monostables on the IC.
- 555 monostable – the voltage levels are not reliably compatible with CMOS ICs
- PICs – more flexible and accurate but more expensive.
Web links
Return to list of datasheets
