Doctronics

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Discovering Digital Electronics


 4 : MONOSTABLES

A monostable produces a single pulse when triggered. In this Chapter, you can find out about a variety of logic gate monostables. All of them produce a single pulse when triggered. Most often the period of the output pulse is determined by an RC, resistor/capacitor, combination.

Monostables differ in how they are triggered. Some monostables have a RESET input which allows the output pulse to be terminated early. In a retriggerable monostable, a second trigger pulse received before the end of the first ouput pulse initiates a new timed period. If the monostable is non-retriggerable, a second trigger pulse received during a timed period is ignored. All of this and more will become clear.

 


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4.1 Why would you want one?

You can see a monostable in action in a garden floodlight/security light:

security light
Garden floodlight/security light

These devices have a PIR, or passive infra-red, detector which responds to the heat emitted by an intruder, or by the neightbour's cat when it comes into the range of the detector. In addition, the device contains an LDR, or light-dependent resistor.

You can find the PIR and the LDR on the circuit boards inside:

security light PCBs
Garden floodlight/security light PCBs

The PIR is the round device located in the centre of the left hand PCB. The glass window allows heat to reach the detector inside. The LDR is level with the PIR at the left hand edge of the same PCB.

Don't take your own security light to pieces. These devices are mains operated and you can't guarantee their safety when reassembled.

The circuit is triggered and the floodlight is switched on when:

  • it is dark
  • an intruder/cat is within range of the PIR

The floodlight stays on for a definite period even when the neighbour's cat has scuttled past into the bushes. This happens because the floodlight control circuit includes a monostable. The block diagram for the circuit looks like this:

Garden floodlight/security light block diagram

Typically, you can change the ON period of the monostable by adjusting the setting of a varible resistor. A second variable resistor allows you to set the threshold level for the light/dark detector. You can see these resistors above and below the LDR on the circuit board, and also from the underside of the control unit:

floodlight controls
Garden floodlight/security light controls

The LDR is visible behind the round plastic window between the two variable resistor adjustment knobs.

You can probably think of other electronic devices which show a monostable action. Monostables are important subsystems in many digital circuits, producing control signals which help to make sure that the right things happen at the right time.

 
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4.2 Edge detectors


 

An edge detector, sometimes called a half-monostable, is used to generate a short pulse on the rising or falling edge of a longer period pulse waveform.

4.2.1 Rising edge detector with positive output pulse

The diagram shows a practical circuit for a rising edge detector:

Rising edge detector with positive output pulse

Schmitt trigger gates, such as the 40106, should be used for this type of circuit to avoid glitches as the capacitor discharges.

Click 'play' in the animation to see how the input and output signals compare:

Rising edge detector waveforms, positive output pulse

The RC network generates a spike corresponding to the rising edge of the input waveform. This results in a short pulse at the output. The spike corresponding to the falling edge of the input signal is suppressed by protection diodes in the input circuit of the Schmitt trigger NOT gate.

The width of the output pulse should be equivalent to a two thirds charge/discharge time, that is:

two thirds charge time

This depends on the thresholds of the Schmitt trigger device. The thresholds vary with power supply voltage so that this is an approximate value.

 

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Here is the circuit in prototype form:

down open in a new window 40106 pins Rising edge detector with positive output pulse

One of the spare 40106 40106 from Rapid Online gates is used to make a Schmitt trigger astable. With 1 MΩ and 1 μF, the frequency will be around 1 Hz. Use the oscilloscope to monitor the various points in the circuit to confirm that you understand how the circuit works.

4.2.2 Rising edge detector with negative output pulse

Sometimes it is convenient to have a negative pulse corresponding to the rising edge of the input signal. In this case, just one of the 40106 gates is needed:

Rising edge detector with negative output pulse

Click 'play' in the animation to see the waveforms for this circuit:

Rising edge detector waveforms, negative output pulse

The prototype board layout for this circuit is identical to the version for the positive output pulse. Monitor the output at pin 2 of the 40106, instead of at pin 4.

4.2.3 Falling edge detector with positive output pulse

To build a falling edge detector, the resistor in the RC network is tied to the positive power supply, instead of to 0 V:

Falling edge detector with positive output pulse

Click 'play' in the animation to see the waveforms for this circuit:

Falling edge detector, positive output pulse

Modify your prototype board layout as follows:

down up open in a new window 40106 pins

Monitor the output at pin 2 of the 40106, as indicated.

4.2.4 Falling edge detector with negative output pulse

For a negative-going output pulse, two Schmitt trigger NOT gates are needed:

Falling edge detector with negative output pulse

Click 'play' in the animation to see how this affects the waveforms for this circuit:

Waveforms for falling edge detector, negative output pulse

To see the circuit in action on your prototype board, move the oscilloscope probe to pin 4 of the 40106.

Edge detector, or half-monostable, circuits are useful in producing control signals which you can use within a larger digital circuit. The most important limitation is that the period of the input pulse must be longer than that of the output pulse. Usually, you want a short output pulse. Choose R and C values accordingly.

 
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4.3 Debouncing switches

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The metal contacts of push button and other switches do not switch over cleanly. Often, the contacts make a number of rapid intermittent connections before settling into their new position. These intermittent connections result in 'bounce' signals. Electronic counting systems respond individually to these signals and count several pulses even although you think you pressed the button only once.

The solution is to use a 'debouncing' circuit. The diagram shows one possibility:

Debounced switch

When the switch is closed, the 220 nF capacitor charges up instantly. There isn't enough time for the capacitor to discharge between intermittent connections. The output pulse switches HIGH cleanly. When the switch is released, the capacitor discharges with the time constant set by 100 kΩ/220 nF. The time taken to reach the lower threshold of the Schmitt trigger NOT gate should be:

two thirds charge time

That is, switching LOW is delayed by 1.1x100 kΩx0.2μF=22 ms, beyond the time when bounce connections are likely to occur.

It is easy to modify your previous prototype board layout to test this circuit:

down up open in a new window 40106 pins Debounced switch

Monitor the output using the oscilloscope and press the button to see output pulses. What would you do if you wanted a negative-going output pulse instead of a positive-going pulse?

 
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4.4 Power on RESET


 

When power is connected to a circuit containing electronic counters, or similar sequential logic devices, the initial states of these devices cannot be predicted.

The bistables (more about bistables in Chapter 5) which form the counting circuit can flip either way when power is first connected, so that the counter does not automatically start from zero.

To force counters and similar devices to start from the correct initial state, you need to provide a pulse which RESETs the counter when power is first connected:

Power on RESET circuit

The 1 MΩ/1 μF provides a delay. Initially, the capacitor is discharged and Vout snaps HIGH.

When power is applied, the capacitor starts to charge up and should reach the first threshold of the Schmitt trigger NOT gate after 1.1RC, that is, after 1.1 s. This value is approximate because the thresholds of the gate may not be at exactly one-third and two-thirds of the power supply voltage. When the threshold is reached, Vout snaps LOW.

Vout remains LOW until the power supply is disconnected.

Modify your prototype board layout from earlier in this Chapter to test the circuit:

down up open in a new window 40106 pins Power on RESET circuit

Disconnect and then reconnect the power supply and follow the output from pin 2 of the 40106 on the oscilloscope. You should be able to tell that the output goes HIGH immediately and remains HIGH for around 1 s after the power is connected.

You can use this signal to force counters and other sequential electronic logic devices into the desired initial state. What would you do if you wanted a negative-going RESET signal instead of a positive-going signal?

 
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4.5 NAND gate monostables

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You can build a monostable using NAND gates following this circuit:

NAND gate monostable

The trigger input is held HIGH and pulsed LOW by pressing the trigger switch. The period of the monostable pulse is determined by R and C and, assuming the switching threshold of the 4011 gates is half the power supply voltage, the period of the pulse will be:

monostable period

The monostable output pulse is positive-going at Q and negative-going at Q.

To understand how the monostable works, think first about the truth table for a NAND gate:

input B input A output
0 0 1
0 1 1
1 0 1
1 1 0
NAND gate truth table

When the circuit is in its resting state, capacitor C is discharged via resistor R so that the input to the second NAND gate, pins 5 and 6, is held LOW. The output of this gate, Q, is HIGH.

The 100 kΩ pull up resistor in the input voltage divider holds pin 2 of the 4011 HIGH unless the push switch is operated. In the resting state, both inputs of the first NAND gate are held HIGH and its output is LOW.

Operating the push switch briefly causes the output of the first NAND gate to snap HIGH while the output of the second NAND gate snaps LOW. The ouptut of the second NAND gate remains LOW until the capacitor discharges to the threshold voltage at the input of the gate.

Click 'play' in the animation to see the waveforms for this circuit:

Waveforms for NAND gate monostable

The trigger pulse is usually shorter than the output pulse of the monostable. If it is longer the output of the monostable remains in its active state until the trigger pulse terminates.

Build this test circuit on prototype board:

down up open in a new window 4011 available from Rapid Online 4011 pins NAND gate monostable

Operate the push button to trigger the monostable. How could you double the period of the monostable pulse?

The circuit below shows how a RESET input can be added to the NAND gate monostable:

NAND gate monostable with RESET input

Modify your prototype board layout as follows:

down up open in a new window 4011 pins NAND gate monostable with RESET input

Some of the existing links have been rearranged and the blue coloured links are new. The timing capacitor has been increased to 10 μF.

Operate the trigger switch as before and confirm that the monostable period has been increased to around 7 s. Operate the trigger switch again and then press the RESET switch. The monostable output pulse terminates immediately.

 
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4.6 NOR gate monostables

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With appropriate changes to the circuit, NOR gates can be used to make a monostable:

NOR gate monostable

In the trigger input voltage divider, the 100 kΩ is a pull down resistor. R and C are the timing components and, as with the NAND gate monostable, the period of the output pulse is given by:

monostable period

Before clicking 'play' in the animation, think about the waveforms you would expect in this circuit:

Waveforms for NOR gate monostable

To understand how the monostable works, think about the truth table for a NOR gate:

input B input A output
0 0 1
0 1 0
1 0 0
1 1 0
NOR gate truth table

When the circuit is in its resting state, both inputs to the first NOR gate are LOW and its output is HIGH. The trigger pulse is positive-going. What effect will this have on the output of the first gate?

Pins 5 and 6 are held HIGH by resistor R and drop LOW when the output of the first gate changes. Capacitor C starts to charge and when the gate threshold is reached, the output pulse is terminated.

Build the circuit on prototype board as follows:

down up open in a new window 4001 pins NOR gate monostable

The prototype board layout is different from the NAND gate version. Note the polarity of the 4.7 μF capacitor. One end of the 1 MΩ resistor is tied to +9 V.

Operate the miniature tactile switch to see the monostable in action.

The circuit is easily modified to include a RESET input:

NOR gate monstable with RESET input

Modify the prototype board version of the circuit as follows:

down up open in a new window 4001 pins NOR gate monstable with RESET input

It is instructive to build some of these circuits on prototype board. Remember that you can open any of these drawings in its own window. Right-clicking gives you the option to print out the layout so that you can follow a paper version if the electronics lab is a long way from the computer room.

 
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4.7 Retriggerable monostable

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A retriggerable monostable circuit allows you to initiate a new monstable pulse before any ongoing pulse has been completed. One version uses a 555 timer 555 timer available from Rapid Online. This is not a logic gate circuit but can help you to understand what is meant by retriggering:

Retriggerable monostable

As you can see, a PNP transistor, BC557B, has been included in the trigger circuit. To understand how the circuit works click 'play' in the animation:

Waveforms for retriggerable monostable

The first trigger pulse results in a normal ouput pulse with a period determined by t=1.1RC. Note that the timing capacitor is fully discharged during the trigger pulse: the timing period starts when the trigger input goes HIGH.

The second trigger pulse is followed by a third and then a fourth before the normal output period has been completed. The additional trigger pulses discharge the timing capacitor, giving an extended output pulse.

To see the circuit in action, build it on prototype board, as follows:

down up open in a new window 555 pins Retriggerable monostable circuit

Operate the push switch. The normal output of the monostable should be around 5 s. When the LED switches OFF, press the push switch again and then press is once more after 2-3 s.

Provided you keep pressing the switch at intervals of less than 5 s, the output LED remains ON.

If the input voltage divider is replaced by a source of pulses, this circuit can be used as a 'missing pulse' or 'low rate' detector. Any decrease in the frequency of the input pulses below the design level, will allow the monostable to complete its cycle, driving the output LOW.

More information on the 555 timer is provided at: beastie zone www.doctronics.co.uk/555.htm.

 
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4.8 4538B dual precision monostable

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The 4538B 4538B available from Rapid Online is a special purpose IC containing two monostables which can be used independently. The pin connections for the 4538B are as follows:

4538B pin conections

Pin connections for monostable A are on the left hand side of the IC, with pins for monostable B on the right. The next diagram shows these connections in more detail:

4538B internal arrangement

Each monostable has two trigger inputs. The monostable can be triggered/retriggered by a rising edge at provided is held HIGH. Alternatively, the monostable can be triggered/retriggered by a falling edge at provided is held LOW.

The two outputs have opposite logic states. In its resting or quiescent state, output is LOW: goes HIGH during the monostable pulse. Output is normally HIGH and goes LOW during the monostable pulse.

is an overriding active LOW direct reset input. In other words, the output of the monostable is reset, forcing output LOW, whenever is LOW, regardless of the logic states of the trigger inputs.

This behaviour can be summarised in truth table form:

inputs outputs
L H
H H
X X L L H
4538B function table

where:

H = HIGH state (the more positive voltage)
L = LOW state (the more negative voltage)
X = state is immaterial
= positive-going transition (rising edge)
= negative-going transition (falling edge)
= positive output pulse
= negative output pulse

The timing components are connected to and as follows:

Connecting timing components

The period of the output pulse is given by:

4538B monostable period

To test one of monostables inside the 4538B, you need to add suitable trigger and indicator circuits. With CMOS logic, power supply decoupling is essential:

4538B test circuit

Build the circuit on prototype board, as follows:

up open in a new window 4538 pins R4538B test circuit

Check the pin connections of the 4538 by clicking 4538 pins. From the function table, which mode of operation is selected?

Operate the push switch to trigger the monostable. Is the monostable retriggered if you press the button again before the timing period is complete? Experiment to see if you can change the mode of operation so that the monostable is triggered by a falling edge. You will need to rearrange the resistor/switch voltgae divider as well as the connections to the trigger inputs, pins 4 and 5.

The 4538 is a versatile device and is a good choice if you need short pulses linked to particular events in a complex logic circuit.

Other monostable circuits are described in the beastie zone pages for the beastie zone 555 timer and the beastie zone 4060 counter/divider.

 
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4.9 What have you learned?


DOCTRONICS recommends:

  • A monostable produces a single pulse when triggered.
  • Edge detectors, or half monostables, generate a pulse corresponding to the rising or falling edge of a longer period pulse waveform.
  • Edge detectors are built using Schmitt trigger logic gates and can be designed to give positive-going or negative-going output pulses. The period of these pulses approximates to the two thirds charge/discharge time set by the timimg components:
    two thirds charge time
  • Monostables can be built with NAND gates and NOR gates. The monostable period is determined by:
    monostable period
  • RESET inputs can be added to these circuits, allowing the monostable output pulse to be terminated early.
  • A retriggerable monostable circuit allows you to initiate a new monostable pulse before any ongoing pulse has been completed. One easy to understand version uses a 555 timer.
  • The 4538B dual precision monostable is a versatile device containing two retriggerable monostables which can be used independently. Trigger inputs can be programmed for rising edge or falling edge triggering.
  • Monostables can be made with other devices, including the 555 timer and 4060 counter/divider.
 
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