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FUNCTION BLOCK
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DESCRIPTION
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THIS USES LOGIC DEVICES
Logic 1 = High = +V
Logic 0 = Low = 0V
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- This monostable is built around 2 LOGIC gates.
- It produces an output pulse, either high or low by using
NOR or NAND gates.
- The time of the pulse is set by 2 components.
- Useful for producing brief pulses for latches and clocks, from 1us to 1 second, for longer durations use a
555 monostable.
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CIRCUIT DETAILS
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Input
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NOR GATE MONOSTABLE - Circuit Diagram
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- Trigger
This is activated by a low to high input signal. A high to low transistion has no effect.
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Output
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When triggered, the output will pulse high for the pre-determined time. The output can only sink/source about 2-3mA, just enough for an LED (but this will reduce the output voltage).
It should really only be used to trigger other logic devices - but you can add a driver for higher current.
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NAND GATE MONOSTABLE - Circuit Diagram
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- Trigger
This is activated by a high to low input signal. The opposite to the NOR monostable.
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Output
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When triggered, the output will pulse low for the pre-determined time.
Again the output is only 2-3mA and should be connected to another logic device.
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CALCULATING THE TIME...
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The duration of the pulse is calculated as follows:
Time (secs) = 0.7 x C1 x R1
It can be tricky to choose values to start with, so try the following:
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1
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Choose one of the following values for the capacitor, depending on the time you require:
Less than 1 ms. = 1nF,
Up to 100 ms. = 47nF,
Up to 1 sec. = 470nF
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2
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Now rearrange the formula to work out the required resistor value R1
R1 = Time / (0.7 x C1)
To ensure that the caluclations are correct, use either Mohms & uF or Ohms & Farads as units
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Try using a preset  or a potentiometer  for R1. Then you can adjust the time manually. |
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PACKAGE DETAILS
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Both the NOR and NAND gates come in a 14pin DIL package. Any gate can be used for any purpose. CMOS types have been chosen since their supply voltage can range from 3 to 15v.
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4001B
2ip NOR GATE
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4011B
2ip NAND GATE
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When using ICs, it is always best to use an IC socket so the IC can be removed easily if needed.
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Unused gates should have their inputs connected to 0v
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www.edutek.ltd.uk - Working Electronics For Students & Teachers
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