How To Build A Versatile Multi-Range Repeating Timer

Circuit Description

Breadboard Layout		Photo Of The Prototype		Circuit Simulation

Circuit Diagram For A
Cmos 4060 Repeating Timer

Ic2 is the relay timer. Its job is to energize the relay for a preset length of time.

Ic1 is the repeat timer. Its job is to restart the relay timer at regular intervals. 

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The Cmos 4060 contains a 14-bit binary counter.

The various bits of the counter are connected internally to the output pins.

You'll see from the diagram that bit-12 (Q12) is connected to pin 1. Bit-13 (Q13) is connected to pin 2 etc.

Although the 4060 contains a 14-bit counter - only 10 of those bits are available as outputs.

Q1, Q2, Q3 & Q11 do exist - but they are not connected to output pins.

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The Cmos 4060 also has two built-in inverters.

They are connected in series across pins  9, 10 & 11.

You can tell they're inverters because the symbols are drawn with a small circle at the output.

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As the name suggests - the output of an inverter is always the opposite of its input.

If the input pin is high - the output pin will be low.

If the input pin is low - the output pin will be high. 

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The four (green) components connected to pins 9, 10 & 11 turn the two inverters into a simple oscillator.

The purpose of the oscillator is to give the 14-bit counter - something to count.

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While the oscillator is running - pin 9 keeps switching from high to low and vice-versa. 

It's rather like the swing of a pendulum - or the ticking of a clock.

Pin 9 is connected internally to the binary counter - and the counter counts the ticks. 

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As the count progresses - its state is reflected by the output pins. 

For example - after eight oscillations - Q4 (pin 7) will go high.

After sixteen oscillations - Q5 (pin 5) will go high.

After thirty-two oscillations - Q6 will go high - and so on.

It takes 8192 oscillations for Q14 to go high. 

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Ic2 is the relay timer.

It controls the length of time the relay remains energized.

The "Range B" wire (R15) is connected to one of the ten output pins.

While that pin is low - it's connected internally to the negative line.

And base-current - flowing to ground through R15 - will keep Q2 switched on.

The transistor connects the negative side of the coil to ground - and this keeps the relay energized.

In other words - while the selected range pin is low - the relay will remain energized.

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R14 controls the frequency of the oscillator. That is - the speed at which the clock ticks. 

By adjusting R14 - you can speed it up - or slow it down.

In other words - you can control how long it takes for the count to reach the selected range pin.

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

Now - because R15 is no longer connected to ground - the flow of base-current ceases And Q2 switches off.

When the transistor switches off - it breaks the relay-coil's connection to ground. And the relay de-energizes.

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The high range pin also takes pin 11 high - through D2.

This stops the oscillator. So the count cannot proceed.

In other words - when the time has expired - and the relay has de-energizes - D2 disables the relay timer.

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It will remain disabled until Ic2 is reset.

That is - until pin 12 is taken briefly high - and then low again.

It can be reset manually at any time - by pressing the "Reset" button.

When the button is pressed - it takes pin 12 high - through D1.

And when it's released - R5 takes pin 12 low again. 

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The relay timer can also be reset (restarted) by Ic1.

Ic1 takes the reset pin high indirectly - by switching Q1 on briefly.

When the transistor switches on - it connects pin 12 to the positive line.

And when it switches off - R5 takes pin 12 low again. 

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The Ic1 circuit is practically identical to that of Ic2.

But whereas Ic2 uses Q2 to control the relay - Ic1 uses Q1 to restart the relay timer.

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R1 connects the base of Q1 to the positive line.

So the transistor is normally switched firmly off. 

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The "Range A" wire (C1) is connected to one of the ten output pins.

At power-up - that pin will be low. And while it's low - it's connected internally to the negative line.

So C1 will charge rapidly - through the emitter-base junction of Q1 - and R2.

While the capacitor is charging, this base-current switches the transistor on briefly.

And Q1 resets the relay-timer - by  connecting pin 12 of Ic2 to the positive line - momentarily.

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Because C1 switches Q1 on briefly - the relay-timer (Ic2) will restart every time C1 charges.

In other words - every time the selected (Ic1) range pin goes low - the relay will energize.

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Clearly - the capacitor has to be discharged - before it can be recharged.

In other words - it has to be charged and discharged - alternately.

It's discharged by the range pin when the range pin goes high.

So it's ready to restart the relay-timer - the next time the range pin goes low.

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      THE OSCILLATORS

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With the exception of the values of the potentiometers - the two oscillators are identical.

So the following description of the Ic2 oscillator - also applies to the Ic1 oscillator.

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Note that R13, R14 & C6 are connected in series between the outputs of the two inverters. 

If the cycle starts with pin 11 low - then pin 10 will be high - and pin 9 will be low.

So the high pin 10 will begin to charge C6 - through R13 and R14. 

The speed at which the capacitor charges - depends on the value of C6 - and the setting of R14. 

As the capacitor charges - the voltage at the junction of C6 & R14 will rise. 

And when the junction reaches just over half of the supply voltage - it takes pin 11 high - through R12.

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Now everything reverses. 

Because pin 11 is high - pin 10 is low - and pin 9 is high. 

C6 begins to charge again - only this time it's in the opposite direction.

As the high pin 9 charges the capacitor - the voltage at the junction of C6 & R14 will fall. 

Again - the speed at which it falls depends on the value of the capacitor - and the setting of R14. 

When the junction falls to just below half the supply voltage - R12 takes pin 11 low.

With pin 11  low - we're back where we started.

The cycle is complete - and the next cycle can begin.

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The oscillator runs because the polarity of the charge on C6 keeps reversing. 

As a result - the junction of C6 & R14 keeps taking pin 11 back and forth between high and low. 

And since pin 11 keeps changing back and forth between high and low - the polarity of the charge on C6 keeps reversing. 

It's a sort of controlled feedback - where the speed of the oscillator is determined by the values of R13, R14 & C6.

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The main purpose of R13 is to set an upper limit on the frequency. 

Its value was chosen to provide a minimum period of about 30 seconds at pin 7. 

The value of R14 was chosen to provide up to about 24-hours at pin 3. 

Thus - an overall range of about 30-seconds to 24-hours - is available.

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When they are biased in the reverse direction - polarized capacitors will conduct direct current.

Since the charge on C6 is constantly reversing in polarity - ideally the capacitor should be non-polarized. 

Modern electrolytic capacitors generally have a very low leakage current - even in the reverse direction. 

So you can usually get away with using a regular polarized electrolytic capacitor.
 
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Because the minimum value of R13 + R14 is 120k - very little current will flow through the resistors. 

So - even if C6 does leak in the reverse direction - the worst that can happen is that the oscillator won't run. 

It's easy to tell if the oscillator is working. 

Set R14 at about mid-point. 

If the green LED is turning on and off at about five-second intervals - the oscillator is running properly.

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By connecting two regular capacitors back-to-back - you can simulate a non-polarised capacitor.

Since one of the capacitors is always correctly biased - no DC current can flow through the pair.

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When two capacitors are connected in series - their combined capacitance is reduced.

The formula for capacitors connected in series - is similar to the formula for resistors connected in parallel.

Where just two capacitors are connected in series - the formula may be expressed as -

   (The Product / The Sum)
                     Or 
       (Ca x Cb) / (Ca + Cb)

So two 22uF capacitors - connected in series - have a combined capacity of -

          (22 x 22) / 44 
               = 11uF

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When - as here - both capacitors have the same value - there's a simpler calculation.

The combined capacitance is equal to half the value of one capacitor.

So two 22uF capacitors - connected in series - have a combined capacity of - 

          22 / 2 = 11uF.

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The Ic2 oscillator stops when D2 takes pin 11 high. 

The diode allows a high range pin to take pin 11 high.

But - it prevents a low range pin from holding pin 11 low. 

By clamping pin 11 high - D1 forces pin 10 to remain low - and pin 9 to remain high. 

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Since pin 11 can no longer change its state - the polarity of the charge on C3 cannot change. 

So the oscillator cannot run. 

In effect - the counter is frozen at the moment the range pin goes high.

It will remain frozen until Ic2 is reset. 

That is - until pin 12 is taken high - and then low again.

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Note that R12, R13 & R14 are connected in series between pins 10 and 11. 

They join the output of the first inverter to its own input. 

The value of R12 needs to be very high - at least 4M7. 

That way - any influence the output of the first inverter has on its own input will be very small. 

It will be swamped by the influence of the changing charge on C6. 

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Coils of all sorts give off high reverse-voltage spikes that will destroy Cmos ICs.

Always connect diodes across relay coils - and across any other device that might contain a coil. 

Note that the cathode of D3 - the side with the bar - is connected to the positive terminal of the coil. 

Any reverse-voltage spikes will be short-circuited at source - before they can do any harm.

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I've used a SPCO/SPDT relay in the drawing. 

But you can use a multi-pole relay - if it suits your application. 

I specified a minimum coil resistance of 270 ohms. 

A higher resistance value will improve battery life.

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With a 12-volt supply - the maximum current through the transistor will be 
(12v / 270 ohms) about 45mA. 

This is well within the limits of the BC557 - which has an IC(max) of 100mA.

There is nothing special about the transistors. 

Any small PNP transistors with a gain (hfe) greater than 100 and an IC(max) of at least 100mA should do.

But remember that the pin configuration of your transistors may be different from that of the BC557. 

Just because your transistors look the same as the BC557  - don't assume that the pins are arranged in the same order.

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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 turn-off the power - before inserting or removing the IC. 

Also - check that the IC is the right way round. Pin 1 should be in the top left-hand corner.

And check 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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The components used in the circuit should be widely available. However- with the exception of the two ICs - none of them are critical. If you can't find the specified parts - You're certain to find something that will do just as well.

Parts List

Component Suppliers	Worldwide RS Components	Uk & Ireland Maplin	How To Choose The Right Parts

Construction Notes

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

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 40 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

When you're satisfied that the pattern is right - cut the tracks. Make sure that the copper 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. If you don't have the proper track-cutting tool - a 6 to 8 mm drill-bit will do. Just use the drill-bit as a hand tool - there's no need for a drilling machine.

Next make and fit the Wire 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.

Then fit the thirteen resistors and the two potentiometers. The resistors are all shown lying flat on the board. However, those connected between close or adjacent tracks are mounted standing upright.

Next - fit the two transistors - the 3 diodes - the eight capacitors and the four LEDs. Pay particular attention to the orientation of the diodes and electrolytic capacitors. Because non-polarized electrolytic capacitors are not widely available - I used pairs of regular polarized capacitors - connected back-to-back.

Add the relay and the two IC sockets. Using the sockets reduces the risk of damage to the ICs - and makes them easier to replace should the need arise.

Next - use a magnifying glass to examine the underside of the board. 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 six (blue) solder bridges to the underside of the board. These are just small blobs of solder. I've used them to connect adjacent tracks. They are a simple and convenient alternative to wire links.

Finish off by inserting the two ICs into the sockets. Pin 1 of each 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 Timer

Repeating Timer No.9 - Support Material

Circuit Description

Parts List

Construction Notes

Actual Size