How to build a repeating / non-repeating sequential timer - using a Cmos 4017.

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Circuit Description

Click Here For A Photograph Of The Prototype.

Circuit Diagram For
A Cmos 4017 Based
Sequential Timer

This circuit uses a Cmos 4017.

The 4017 is a decade counter.

The count starts at zero.

And it advances by one - each time pin 14 is taken high. 

When the count reaches nine - it goes back to zero - and starts all over again.

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As the count progresses - each of the output pins goes high in turn.

The first is pin 3 - it represents zero.

The second is pin 2 - it represents one.

Pin 4 represents two - and so on.

The number represented by each output pin is shown in red.

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Although the 4017 will count up to nine - we are only using it to count up to four.

Consequently - some of the output pins are unused.

Unused Cmos output pins should always be left unconnected.

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The circuit uses the first five outputs. These are - pins 3, 2, 4, 7 & 10

The first four outputs pins are each connected to a 2k7 resistor.

With a 12v supply - the resistor limits the current available to 12v ÷ 2k7 = just under 5mA.

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This is more than enough to operate a transistor switch.

And the transistor can be used to sound a buzzer - or energize a relay.

If you increase the current to about 10mA - it will drive an optical isolator.

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To increase the current - you should reduce the value of the resistor.

As a rough guide - it's safe to draw up to about 1mA per supply volt.

This means that the value of the resistor - should never be below 1k.

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Every step in the sequence is the same.

Each output charges C5 through a timing resistor.

And - when the voltage on C5 takes pin 14 high - the sequence moves on to the next output.

For clarity - I've only highlighted the first step in the sequence.

Once you understand the first step - you'll understand the remaining three steps as well.

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Consider what happens when we turn on the power.

The count begins at zero.

In other words - pin 3 will be high.

And the rest of the output pins will be low.

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Pin 3 does three different jobs.

It provides the necessary output current - through R1.

It discharges C5 rapidly - through R13 & Q1.

And it recharges C5 slowly - through D5 & R5.

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It's true that - at power up - C5 doesn't need discharging.

But it does have to be discharged at the start of every subsequent event in the sequence.

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For the count to advance to one - pin 14 must be taken high.

A Cmos input pin is high when it's just over half the value of the supply voltage.

As pin 3 charges C5 - the voltage on pin 14 rises.

When it reaches roughly 6v5 - the counter will advance to one.

And the second event in the sequence will commence.

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That is - pin 2 will supply the necessary output current.

It will also discharge C5 rapidly - through R13 & Q1.

And it will begin the process of taking pin 14 high again - by recharging C5 - through D6 & R6.

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It's the act of taking pin 14 high - that causes the timer to advance to the next step in the sequence.

But pin 14 can't be taken high - if it's already high.

That's why - at the beginning of each step in the sequence - C5 must be discharged.

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The job of discharging C5 is carried out by Q1.

When the transistor switches on - it discharges the capacitor rapidly - through R13.

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We can't simply use pin 3 to supply base current to the transistor.

If we did - Q1 would remain permanently switched on.

As a result - the transistor would hold pin 14 permanently low.

Consequently - C5 would never charge - and the sequence would never progress.

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When pin 3 goes high - we only want it to supply base current to the transistor for a short period.

We want Q1 to switch on - discharge C5 - and then switch off again. 

Thus leaving R5 free to recharge the capacitor.

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We can achieve this by connecting a capacitor between the output pin and the base of Q1.

When pin 3 goes high - C1 is in a discharged state.

It will take about 10 seconds or so to charge - through R11 & R12.

But while it's charging - it will supply base current to Q1.

And the transistor will switch on.

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Once C1 is fully charged - the base current will cease.

And the transistor will switch off.

Then pin 3 can get on with the job of charging C5 - through D5 & R5.

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When the sequence moves on - and pin 3 is no longer keeping C1 charged - the capacitor will discharge through R9 & D9.

This only takes a few seconds.

Once it has discharged - it's ready to conduct again.

So it will switch Q1 on briefly - the next time it's required to do so.

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The length of time the transistor remains switched on - depends mainly on the values of C1 and R11.

It's currently about ten to twelve seconds.

If you need a shorter discharge time - reduce the value of R11.

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R12 connects the base of Q1 to ground.

It keeps the transistor switched off until it's needed.

R13 limits the maximum surge current through the transistor - as C5 discharges.

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There's no theoretical limit on the value of C5.

If the value of C5 is increased to 4700uF - or  10000uF - the individual events in the sequence can last many hours.

The considerable amount of charge stored in such a large capacitor could destroy the BC547 - if the maximum discharge current were not limited.

R13 prevents the maximum current from exceeding about 30mA.

This is well within the limits of the BC547.

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If you examine the main schematic - you'll see that the circuitry surrounding C1 has been duplicated.

The duplicate circuit is coloured green - and it uses C2 to supply the brief base current to Q1.

Both the red and the green circuits are identical - and work in an identical manner.

They just take turns at switching Q1 on.

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The reason for the two circuits is simple.

The capacitors need time to discharge - before they can supply the necessary base current to switch Q1 on.

If there were only one capacitor - it would never discharge.

The changeover from one output to the next is very fast.

C1 would be kept charged by successive output pins.

And - while it remains charged - it can't switch Q1 on.

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But - by connecting the two capacitors to alternate outputs - the next capacitor in the sequence is always discharged.

And when its turn comes - it will switch the transistor on.

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This explains why a repeating sequence has to have an even number of events.

With an odd number of events - the last event would charge C1.

So pin 3 would be presented with a charged capacitor when the sequence attempts to repeat.

If necessary - you can always create an even number of events - by splitting one of the events in two.

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At the end of the fourth event in the sequence - the count will move on to four - and pin 10 will go high

When pin 10 goes high - it takes pin 15 high - through D13.

This resets the counter to zero.

Causing pin 10 to go low - and the zero output to go high again.

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Now we're back where we started.

Pin 3 is high - and the first event in the sequence has begun.

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We're in a sort of loop.

We start off at zero - with the first event in the sequence.

This is followed by events two, three and four.

Whereupon D13 resets the counter to zero - and the sequence starts all over again.

If you don't want the sequence to repeat - simply leave out D13.

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The circuit uses a lot of diodes.

But they're all doing the same type of job.

They are all creating one-way paths.

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D1 & D3 allow pins 3 & 4 to supply current to C1.

But they prevent pins 3 & 4 from shorting together.

The same is true of D2 & D4 - in relation to C2 and output pins 2 & 7.

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D5, D6, D7 & D8 also provide one-way paths.

For example - D5 allows a high pin 3 to charge C5.

But it prevents a low pin 3 from discharging C5.

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D9 allows R9 to discharge C1 - even though the negative plate of C1 is not connected to ground.

The same is true for D11, R10 & C2.

D10 & D12 allow both C1 & C2 to supply base current to Q1 - without their being connected together.

And D13 allows a high pin 10 to take pin 15 high - but it prevents a low pin 10 from holding pin 15 low.


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      THE ROLE OF C4 & R14

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To be certain that the count will start at zero - C4 & R14 perform a reset when power is first applied. 

To reset the counter to zero - pin 15 must first be taken high - and then taken low again.

The first task - that of taking pin 15 high - is performed by C4.

And the subsequent task - that of taking pin 15 low again - is performed by R14.

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At power-up - C4 is in a discharged state.

In this condition - it acts like a wire link joining pin 15 to the positive line.

In other words - C4 takes pin 15 high.

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R14 charges C4 by removing positive charge from its negative plate.

As R14 charges C4 - the voltage on pin 15 falls.

In other words - R14 takes pin 15 low.

This act of taking pin 15 high - and then low again - resets the counter to zero.

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C4 & R14 also reset the counter when it attempts to advance to four.

When pin 10 goes high - it's connected internally to the positive line.

And D13 will discharge C4 by applying a positive charge to its negative plate.

This causes pin 15 to go high.

When pin 15 goes high - all the output pins go low.

This includes pin 10. 

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The instant pin 10 takes pin 15 high - pin 10 itself will go low.

Since D13 is no longer keeping C4 discharged - the capacitor will charge again - through R14.

In other words - pin 10 discharges C4 - and R14 recharges C4.

In so doing - they takes pin 15 briefly high - and then low again.

The act of taking pin 15 high - and then low again - is what causes the 4017 to reset.

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        THE CLOCK INPUT

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The clock input works in a very similar manner to the reset.

To advance the count by one - you must take pin 14 high.

This is what happens when one of the timing resistors charges C5.

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Not all Cmos ICs will allow you to take the clock input high - slowly.

Many require a very fast rising voltage on the clock input pin.

But the 4017 clock input includes a Schmidt-Trigger device.

It's a sort of buffer between the ICs internal trigger - and pin 14. 

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It doesn't matter how slowly we charge C5.

Sooner or later the Schmidt-Trigger will activate.

And when it does - it will take the ICs internal trigger - sharply high.

What this means for us - in practical terms - is that there's no theoretical maximum limit on the length of any event in our sequence.

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There are two more pins we haven't mentioned.

One is an input - called Inhibit.

And the other is an output - called Carry.

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Inhibit can be used to switch the counter on and off.

To switch the counter off - you need to take pin 13 high.

While pin 13 is high - the counter will ignore the clock pulses.

In other words - it doesn't matter how many times you take pin 14 high - the count won't advance.

We don't need this feature.

So I've disabled it - by connecting pin 13 to ground.

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The Carry output is at pin 12.

It allows you to connect several 4017s together.

A single 4017 will count up to ten. That is (0 to 9)

Two 4017s can count up to one hundred (0 to 99)

Three will count up to a thousand - and so on.

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The carry pin of each 4017 is connected to the clock input of its successor.

The carry pin of the first 4017 is connected to the clock input of the second.

The carry pin of the second is connected to the clock input of the third - and so on.

This feature is not used in our circuit.

And - because pin 12 is an output pin - we must leave it unconnected.
 
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Although static electricity can destroy Cmos ICs - they are extremely reliable in operation. 

Most modern versions will survive a fair degree of abuse.

But it's worth taking a couple of basic precautions. 

Avoid touching the pins - and don't overheat them with the soldering iron. 

A socket will reduce the chances of damaging the IC - and will make it easier to replace if necessary.

If you're using a socket - always switch-off the power before inserting or removing the IC.

Also - check that the IC is the right way round - and that all of the pins are correctly inserted into the socket. 

Sometimes - instead of entering the socket - a pin will curl up underneath the IC.
 
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Increasing The Number Of Events

To increase the number of events beyond four - connect the additional outputs as follows:

Each additional output pin requires its own 2k7 current limiting resistor.

Each additional output pin is connected to C5 - through a diode and a timing resistor.

Each additional output pin is connected through a diode to either C1 or C2.

The output pins are connected to the two capacitors alternately.

Output four (pin 10) goes to C1. Output five (pin 1) goes to C2. Output six (pin 5) goes to C1 again - and so on.

If the sequence is to repeat - it must have an even number of events (n).

And output (n+1) should be connected to pin 15 - through D13.

There's one exception to this.

If the sequence has ten events - it will repeat anyway. So there's no need for D13.

If you have an odd number of events - and still want the sequence to repeat - split one of the events in two.

If you don't want the sequence to repeat - it must have nine events or fewer.

And you should leave out D13 altogether.

Parts List

Suppliers

Worldwide RS Components

UK & Ireland Maplin

Construction Guide

Click here if you're new to constructing stripboard projects.

The prototype of this circuit was built using only the Stripboard Layout as a guide. So - if you reproduce the layout - you will have a working circuit. Details of how to Test Your Finished Circuit Board are also provided.

The terminals are a good set of reference points. To fit them - you may need to enlarge the holes slightly. Then turn the board over and use a felt-tip pen to mark the 19 places where the tracks are to be cut. Before you cut the tracks - use the "actual size" drawing to Check That The Pattern is Correctly Marked .

Actual Size Of Pattern

When you're satisfied that the pattern is right - cut the tracks. If you don't have the proper track-cutting tool - then a 6 to 8mm drill-bit will do. Just use the drill-bit as a hand tool - there's no need for a drilling machine. Make sure that the copper strip is cut all the way through. Sometimes a small strand of copper remains at the side of the cut and this will cause malfunction. Use a magnifying glass - and backlight the board. It only takes the smallest strand of copper to cause a problem.

Sequential Timer - Make A

Next fit the Four Wire Links and the fourteen resistors. For the links - I used bare copper wire on the component side of the board. Telephone cable is suitable - the single stranded variety used indoors to wire telephone sockets. Stretching the core slightly will straighten it - and also allow the insulation to slip off.

If you haven't finalized the values of your timing resistors - you could fit veropins in place of R5, R6, R7 & R8. Then you can experiment with different value resistors - until you achieve the time periods you require.

At the bottom of the prototype - I left room to add variable resistors. They would let you set the time periods more accurately. You should still use R5, R6, R7 & R8 to take you close to your target times - then make fine adjustments with the pots.

Sequential Timer - Make B

Next - fit the thirteen diodes. Pay particular attention to their orientation. See the Photograph Of The Prototype. Note that D5, D6, D7 & D8 are facing downwards.

Sequential Timer - Make C

Next - fit the five capacitors - the transistor - and the IC socket. Pay particular attention to the orientation of the electrolytic capacitors. Note that the positive terminal of C5 faces downwards.

Then turn the board over and examine the underside carefully - to make sure that there are no unwanted solder bridges or other connections between the tracks. If you backlight the board during the examination - it makes potential problem areas easier to spot. When you're satisfied that everything is in order - add the 2 solder bridges.

Sequential Timer - Make D

Finish off by inserting the Cmos IC into the socket. Pin 1 of the IC should be in the top left-hand corner. Check that all 16 pins have entered the socket. Sometimes - instead of entering the socket - a pin will curl up under the IC.

You Are Now Ready To Test Your Finished Circuit Board.

Devising Complex Sequences	The Rest of Ron's Circuits	Write To Ron	More Free-to-Use Circuits	Circuit Exchange International

Sequential Timer - Support Material

Increasing The Number Of Events

Construction Guide

Actual Size Of Pattern