This simple toggle switch circuit will energize and de-energize a relay - at the push of a button.

COMMENTS SUGGESTIONS	Toggle Switch No.1 - Support Material	MORE SWITCH CIRCUITS

Toggle Switch No.1 Front Page	The Rest of Ron's Circuits	Write To Ron	More Free-to-Use Circuits	Circuit Exchange International

Circuit Description

Click Here For A Photograph Of The Prototype.

Circuit Diagram For
A Cmos 4013 Based
Toggle-Switch

This circuit uses a Cmos 4013.

It contains two distinct devices - known as Type D Flip Flops.

One of the flip flops is connected to the pins on the left of the IC - and it's labelled in red.

The other is connected to the pins on the right of the IC - and it's labelled in blue.

Both flip flops share the same connections to the power supply - at pins 14 & 7.

But apart from that - they function entirely independently of one another.

                   ============

We're only using one flip flop - the red one on the left.

The blue one - on the right of the IC - is a spare. 

It's good practice to connect the input pins of the unused flip flop - to ground.

                   ============

Generally speaking - Cmos ICs use very little current.

The only time a significant current flows - is while the output pins are in the process of changing state.

The outputs of the unused flip flop are at pins 12 & 13.

If you connect pins 8, 9, 10 & 11 to ground - it will prevent pins 12 & 13 from changing state randomly.

So no energy is wasted.
 
                   ============

The outputs of the red flip flop are at pins 1 & 2.

They are both labelled Q.

But one Q has a bar above it - and the other doesn't.

This indicates that the outputs are - "complementary".

                   ============

Complementary simply means that the two outputs will always have different states. 

In other words - when pin 1 is high - pin 2 will be low.

And when pin 1 is low - pin 2 will be high.

                   ============

We could use either output to control the relay.

I chose to use the output at pin 1.

                   ============

When pin 1 goes high - it's connected internally to the positive line.

And it supplies base current to the transistor - through R4.

The resistor limits the base current to under 5mA.

This is more than enough to switch the transistor on.

When the transistor switches on - it connects the negative side of the relay coil to ground.

And this causes the relay to energize.

                   ============

The transistor does a second job.

It connects the negative side of the LED to ground.

The LED is acting as an indicator.

If the LED is lighting - you know that the relay is energized.

The 4mA or so - provided by R3 - should make it bright enough for this purpose.

                   ============

When pin 1 goes low again - it's connected internally to the negative line.

So it can no longer supply base current to the transistor.

This causes the transistor to switch off.

When Q1 switches off - the relay de-energizes - and the LED is extinguished.

                   ============

Since this is a toggle switch - we want the push button to energize and de-energize the relay - alternately.

So every time the button is pushed - we want pin 1 to change from high to low - or vice-versa.

                   ============

For present purposes - we don't need to use the Set and Reset inputs.

So pins 6 & 4 are connected to ground.

This disables the two unused inputs - and prevents them from having any unwanted influence on the output pins.

                   ============

We are only using four of the IC's pins.

The Clock and Data inputs at pins 3 & 5 respectively.

And the two outputs - at pins 1 & 2.

                   ============

The output pins are always different from one another.

If pin 1 is high - pin 2 will be low - and vice-versa.

Pushing the button causes the high and low outputs to swap places.

The high pin will go low - and the low pin will go high.

This swap takes place the instant the Clock input goes high.

                   ============

Consider what happens when the button is pushed.

Thanks to the 1 Meg resistor - C1 always starts off in a discharged state.

In this state - it acts like a wire link.

So - when the button is pushed - C1 connects pin 3 directly to ground.

In other words - C1 takes the Clock input low.

                   ============

In a matter of microseconds - the 47 ohm resistor charges C1 - and takes the Clock input high again.

This act of taking the Clock input rapidly high - causes the outputs to swap places.

If pin 1 is high - it will go low.

And if pin 1 is low - it will go high.

Of course - the same applies to pin 2.

                   ============

So far we've only been looking at WHAT happens when the button is pressed.

That is - the outputs swap places.

But we haven't considered WHY they swap places.

                   ============

In our circuit - the state of pin 1 changes every time the Clock input is taken high by R1.

But the IC itself doesn't have to behave like this.

It's only doing so because we've connected pin 2 to the Data input - via R5.

                   ============

When the Clock input goes high - pin 1 copies the state of the Data input pin.

If the Data input pin is high - pin 1 will go high.

And - if the Data input pin is low - pin 1 will go low.

                   ============

However - if the state of the Data input pin hasn't changed since the last time the button was pushed - it should be clear that the state of pin 1 won't change either.

For example - if you take the Data input permanently low - by connecting pin 5 to ground - pin 1 will remain low - no matter how many times you push the button.

                   ============

The way to create the toggle action - is to connect pin 2 to pin 5.

The state of pin 2 is always different from the state of pin 1.

So - when we connect pin 2 to pin 5 - we know that the state of pin 5 will always be different from the state pin 1.

And - because the state of pin 5 is always different from the state of pin 1 - when the clock pulse arrives - pin 1 will always change state.

                   ============

Of course - when pin 1 changes state - so does pin 2.

Consequently - Pin 2 will change the state of the Data input.

And since the Data input is - once more - different from pin 1

It's "all change" again - when the next clock pulse arrives.

                   ============

           THE ROLE OF C1 & R2

                   ============

We could use the push switch to connect pin 3 directly to ground.

And when the switch is released - R1 would take the Clock input high.

But Cmos ICs are high-speed devices.

They are capable of switching state millions of times every second.

And - contact bounce would almost certainly deliver several clock impulses in rapid succession.

This would make the outcome of pushing the button - uncertain.

                   ============

Thanks to the capacitor - there's only one very fast clock pulse.

It lasts for a few microseconds - the time it takes for R1 to charge C1.

We can't produce the next clock pulse until after C1 has discharged again - through R2.

And this can't happen until after the button has been released.

This results in predictable and reliable operation - every time the button is pushed.

                   ============

           THE ROLE OF R5 & C2

                   ============

R5 & C2 make the data on the Data input pin - a bit more persistent.

Instead of changing quickly - when pin 2 changes - the state of the Data input pin will change slowly.

If pin 5 is high - it will stay high until C2 discharges into a low pin 2.

And if pin 5 is low - it will stay low until C2 charges from a high pin 2. 

These two delays mean that the state of the Data input pin will always survive for a brief period - after the output pins have swapped states.

                   ============

The period is very short.

It doesn't take long for a 2k7 resistor to charge or discharge a 100nF capacitor.

But - if you compare the values of R1 and R5 - you'll see that the pin 5 delay is about 50 times longer than the clock pulse.

So - pin 1 will have completed its change of state - long before the data it's reading - is changed by pin 2. 

And if - for any reason - our brief clock pulse were to split into a number of briefer pulses - it will have no effect.

While the state of pin 5 remains unchanged - the extra clock pulses will not change the state of pin 1. 

                   ============

I used an SPCO/SPDT relay in the prototype - but you can use a multi-pole relay if it suits your application.

The circuit will work from a minimum of 5-volts up to a maximum of 15-volts. 

You just 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.
 
                   ============

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. 

                   ============

There's nothing special about the BC547. 

Any small NPN 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.

                   ============

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 D1 - the side with the bar - is connected to the positive terminal of the coil. 

The small arrowhead indicates the direction in which the diode will conduct.

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

                   ============

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.
 
                   ============

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 toggle switch. 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 15 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. 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.

Toggle Switch No.1 - Make A

Next fit the Two Wire Links - the five resistors - and the three capacitors. 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.

Then fit the diode - the transistor - the relay - the IC socket - and the LED. Pay particular attention to the orientation of the diode. See the Photograph Of The Prototype. Note that it's facing upwards.

Toggle Switch No.1 - Make B

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 8 solder bridges.

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 14 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.

General Information

Click Here For A Photograph Of The Prototype.

Toggle Switch No.1 Schematic	The Rest of Ron's Circuits	Write To Ron	More Free-to-Use Circuits	Circuit Exchange International

Toggle Switch No.1 - Support Material

Construction Guide

Actual Size

General Information