This simple circuit will energize a relay at regular intervals. The length of the intervals is adjustable from seconds to days - and beyond.

COMMENTS SUGGESTIONS	Repeating Timer - Support Material	A SIMPLER REPEATING TIMER

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Circuit Description

Click Here For A Photograph Of The Prototype.

Circuit Diagram For A
Repeating Interval Timer

The Cmos 4060 IC has two built-in inverters - at pins 9, 10 and 11. 

They must be wired together to form an oscillator - the output of which is at Pin 9. 

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

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

The speed at which the oscillator ticks is controlled by R6, R7 & C4.
 
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Note that R6, R7 & C4 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 capacitor will begin to charge from Pin 10 - through R6 & R7. 

The speed at which it charges depends on the value of the capacitor - and the setting of R7. 

As the capacitor charges - the voltage at the junction of C4 & R7 will rise. 

When it reaches just over half the supply voltage - it takes Pin 11 high - through R8.

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Now everything reverses. 

Pin 11 is high - so Pin 10 is low - and Pin 9 is high. 

C4 begins to charge again - only this time it's in the opposite direction. 

As Pin 9 charges the capacitor - the voltage at the junction of C4 & R7 will fall. 

The speed at which it falls depends on the value of the capacitor - and the setting of R7. 

When it falls to just below half the supply voltage - it takes Pin 11 low - through R8. 

With Pin 11 low the cycle is complete - and the next cycle can begin.

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The oscillator runs because the polarity of the charge on C4 keeps reversing. 

As a result - the junction of C4 & R7 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 C4 keeps reversing. 

It's a sort of controlled feedback - where the frequency generated is determined by the values of R6, R7 & C4.

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The main purpose of R6 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 R7 was chosen to provide a maximum period in excess of 12-hours at Pin 3. 

This means that the timer can be set to re-trigger at intervals of anything from one minute up to at least 24-hours.

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Since the charge on C4 is constantly reversing in polarity - ideally the capacitor should be non-polarized. 

However - 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 R6 + R7 is 150k - very little current will flow through the resistors. 

So - even if C4 does leak in the reverse direction - the worst that can happen is that the oscillator won't run. 

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

Set R7 at about mid-point. 

If the green LED is turning on and off at about 3-second intervals - the oscillator is working properly.

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Note that R6, R7 & R8 are connected in series between Pins 10 and 11. 

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

The value of R8 needs to be very high. 

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 charges on C4. 

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The data sheet for the 4060 says that the value of R8 should be at least ten times the value of R6 + R7.

A 4M7 resistor was chosen because they're widely available. 

It's possible to use such high value resistors because Cmos inputs are operated by voltage - and not by current. 

Because no current flows through the R8 - there's no voltage drop across it. 

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The Cmos 4060 is a 14-bit binary counter. 

The output from the oscillator is connected internally to the counter.

And the counter counts the number of oscillations. 

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As it does so - the state of the count is reflected by the output pins. 

For example - after 8 oscillations - Bit 4 at Pin 7 will go high.

After 16 oscillations - Bit 5 at Pin 5 will go high.

After 32 oscillations - Bit 6 at Pin 4 will go high.

And so on - until after 8192 oscillations - Bit 14 at Pin 3 will go high. 

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

Bits number 1, 2, 3 & 11 do exist within the counter.

But they are not connected to output pins.

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To reset the counter to zero - Pin 12 must first be taken high - and then taken low.
 
At the exact moment that Pin 12 goes low - the counter resets.

That is - all of its outputs go low. 

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The purpose of C5 and R9 is to perform an automatic reset when power is applied. 
Initially - at power-up - C5 is discharged. 

In a discharged state it acts like a wire link connecting Pin 12 to the positive rail.

So Pin 12 is taken high. 

Almost immediately - C5 charges through R9.

So the resistor takes Pin 12 low. 

All of the outputs go low - and the count begins at Zero. 
 
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R7 controls the frequency of the oscillator. 

By altering its frequency - you can change the speed at which the count progresses.
 
So you can decide how long it will take for the count to reach any given bit - i.e. output pin. 

The output pin of your choice is connected to Pin 6 of the Cmos 4001.
 
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Gates 1 & 2 of the Cmos 4001 are connected to form a monostable. 

Basically, every time your chosen output pin takes Pin 6 of the Cmos 4001 high - the monostable will trigger.

And the relay will energize. 

It will remain energized until C1 charges through R3 - then it will drop out. 
 
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Provided you have a suitable relay - the circuit will work from a minimum of 5-volts up to a maximum of 15-volts. 

You need to make sure that the coil of your relay will operate at the voltage you are using.

Also - the relay coil shouldn't draw more than about 50 mA.

Otherwise the transistor might be overloaded.
 
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With a 12-volt supply - I specified a minimum coil resistance of about 270 ohms. 

This gives a maximum current of 12÷270 = 45 mA.

45mA is well within the limits of the BC547. 

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There's nothing special about the BC547. 

Any small transistor 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 transistor may be different from that of the BC547.

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When you're operating the timer you can use the reset button. 

It takes Pin 12 high and discharges C5. 

When you release the button - C5 will charge through R9 - and the resistor will take Pin 12 low. 

The moment Pin 12 goes low - all of the outputs go low - and the counter begins to count up from Zero. 

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However - when you are setting up the timer - you must reset the circuit by interrupting the power supply. 

The reset button only resets the counter - it does not reset the oscillator. 

In fact it stops the oscillator right at the point where it's about to deliver the first pulse. 

So the first cycle is shorter than normal. 

Consequently - the length of time - between the release of the reset button - and the arrival of the first pulse to be counted - is too short. 

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In everyday operation this amount of inaccuracy is insignificant. 

However - during setup the error is magnified by the factors in the setup table.
 
Interrupting the power for a few seconds not only resets the counter - it also resets the oscillator. 

This makes the time it takes for the yellow LED to light - much more predictable.

And makes its fine adjustment much easier.
 
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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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Coils of all sorts give off high reverse-voltage spikes that will destroy Cmos ICs. 

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

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

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

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Learn More About How A Cmos 4001 Monostable Works

Parts List

Suppliers

Worldwide RS Components

UK & Ireland Maplin

Construction Guide

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

The prototype of the timer was built using only the Stripboard Layout as a guide. So - if you reproduce the layout - you will have a working timer. 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 42 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 .

Repeating Timer - Pattern
For Cutting The Tracks

Repeating Timer - Pattern
For Cutting The Tracks

Actual Size

When you're satisfied that the pattern is right - cut the tracks. If you don't have the proper track-cutting tool - 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.

Next make and fit the Ten 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.

Recurring Timer - Make A

Then fit the 7 resistors - the 2 presets - the 5 capacitors - the 2 LEDs - the transistor - and the diode. The resistors are all shown lying flat on the board - but those connected between close or adjacent tracks are actually mounted standing upright. The same is true for the diode.

Repeat Timer - Make B

Fit the relay and the two IC sockets. Then examine the underside of the board 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 13 solder bridges to the underside of the board.

Finally - insert the 4060 and 4001 into the sockets. In each case, make sure that pin 1 is in the top left-hand corner - and check carefully 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.

You Are Now Ready To Test Your Finished Circuit Board.

General Information

Click Here For A Photograph Of The Prototype.

Repeating Timer Front Page	The Rest of Ron's Circuits	Write To Ron	More Free-to-Use Circuits	Circuit Exchange International

Repeating Timer - Support Material

Construction Guide

Actual Size

General Information