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

COMMENTS SUGGESTIONS	Toggle Switch No.3 - Support Material	MORE SWITCH CIRCUITS

Toggle Switches 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
Motor Reversing
Toggle-Switchs

This circuit uses a Cmos 4013.

I haven't drawn a realistic sketch of the IC.

In order to make the circuit diagram simpler and clearer - I've moved the individual pins to more convenient locations.

There's a more accurate image of the IC on the main schematic diagram.

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

The 4013 contains two distinct devices - known as Type D Flip Flops.

I've coloured one orange - and the other blue.

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

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

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

The flip flops are wired to form two identical toggle switches.

I'm going to describe how the orange switch works.

But the blue switch works in exactly the same way.

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

The outputs of the orange 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.

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

Pin 1 controls the relay.

For simplicity - I've only drawn the relay coils.

The actual relays can have as many sets of contacts as you like.

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

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

And it supplies base current to Q1 - through R3.

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 R2 - will 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 LED switches off - and the relay de-energizes.

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

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

So every time Sw1 is pushed - we want pin 1 to change from high to low - or vice-versa.

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

We've seen that the two output pins are always different from one another.

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

In effect - pushing Sw1 causes the high and low outputs to swap places.

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

This swap occurs the instant Sw1 & R4 take pin 3 high 

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

In standby mode - R1 holds pin 3 low.

It also keeps C1 discharged.

When the button is pushed - C1 charges rapidly through R4.

And the charge in C1 takes the Clock input high,

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

It's the act of taking pin 3 rapidly high - that 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.

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

It doesn't matter how long the push button is held closed.

Holding pin 3 high has no effect.

It's taking pin 3 high that causes the swap.

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

So before you can take pin 3 high a second time - you must first allow it to go low.

That is - you must first release the button - and allow C1 to discharge through R1.

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

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

That is - the outputs swap places.

We haven't considered WHY they swap places.

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

In our circuit - the output pins exchange states - every time Sw1 takes the Clock input high.

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

It's only doing so because we've connected pin 2 to pin 5 - via R8

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

Pin 5 is the Data input pin.

When Sw1 takes the Clock input 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.

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

Of course - if the state of the Data pin hasn't changed since the last time the button was pushed - the state of pin 1 won't change either.

For example - if you connect pin 5 to ground - pin 1 will remain low - no matter how many times you push Sw1.

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

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

We know that the state of pin 2 is always different from the state of pin 1.

So - if we connect pin 2 to pin 5 - 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 you push the button - 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 pin 5.

And since pin 5 is - once more - different from pin 1

It's "all change" again - the next time Sw1 is pushed.

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

         THE ROLE OF C1

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

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 - unreliable.

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

Thanks to C1 - there's only one very fast clock pulse.

It lasts for a few microseconds - the time it takes for R4 to charge the capacitor.

We can't produce a second clock pulse - until after C1 has been discharged by R1.

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 R8 & C6

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

R8 & C6 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 C6 discharges into a low pin 2.

And if pin 5 is low - it will stay low until C6 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 R4 and R8 - 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. 

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

    THE ROLE OF C5 & R7

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

The circuit powers-up with C5 in a discharged state.

In that state - it acts like a wire link - joining pin 4 to the positive line.

In other words - C5 takes pin 4 high.

Almost immediately - R7 charges C5 - and takes pin 4 low again.

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

R7 charges C5 by removing positive charge from the capacitor's negative plate.

It doesn't take very long.

But it's long enough to perform an automatic reset. 

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

The brief high - at pin 4 - resets the orange flip flop.

A similar pulse - at pin 10 - resets the blue flip flop.

As a result - we always start off with pins 1 & 13 low

In other words - we always start off with both relays de-energized.

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

You can reset the orange flip flop at any time - by taking pin 4 high.

Of course - if pin 1 is already low - the reset will have no apparent effect.

But - if pin 1 is high and Q1 is switched on - taking pin 4 high will switch the transistor off - and de-energize the relay.

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

Taking pin 10 high will have a similar effect on the blue flip flop.

If pin 13 is already low - the reset will have no apparent effect.

But - if pin 13 is high and Q2 is switched on - taking pin 10 high will switch the transistor off - and de-energize the relay.

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

    THE ROLE OF D3 & D4

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

D3 & D4 allow each of the push buttons to reset the opposing flip flop.

Sw1 can take pin 10 high - and Sw2 can take pin 4 high.

So - if the blue relay is energized - the orange button will de-energize it.

And - if the orange relay is energized - the blue button will de-energize it.

As a result - the two relays can never be energized at the same time.

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

    THE ROLE OF D2 & D5

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

We've seen that pin 1 always copies the state of the data pin.

If pin 5 is high - the clock pulse will cause pin 1 to go high - or to remain high.

And if pin 5 is low - the clock pulse will cause pin 1 to go low - or to remain low.

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

While the orange relay is energized - Q1 holds pin 9 low - through D2.

That is - the blue data input pin is clamped low.

When Sw2 is pushed - pin 13 copies the state of pin 9.

And because pin 9 is clamped low - pin 13 will remain low.

In other words - while the orange relay is energized - the blue toggle switch is disabled.

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

D5 has a similar effect.

While the blue relay is energized - the orange toggle switch is disabled.

As a result - you can only operate one relay at a time.

You must switch off the orange relay - before you can energize the blue relay.

And you must switch off the blue relay - before you can energize the orange relay.

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

If you include all four diodes - a different switching pattern occurs.

Suppose that the blue relay is energized - and Q2 is taking pin 5 low - through D5.

Sw1 can't energize the orange relay because pin 1 is copying a low.

But Sw1 can take pin 10 high - through D3 - and so de-energize the blue relay.

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

As a result - the first push on the orange button will turn Q2 off.

This allows pin 2 to take pin 5 high.

Then a second push on the orange button will turn Q1 on - and energize the orange relay.
 
            ============

Sw2 behaves in a similar way.

If the orange relay is energized - the first push on the blue button will de-energize it.

Then the second push will energize the blue relay.

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

This behaviour is described by the flow chart.

Basically - you can't switch directly from one relay to the other.

There is always an intermediate stage - when both relays are de-energized.

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

Of course - you can play mix and match.

One switch can take precedence over the other.

In other words - you can create a master/slave relationship.

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

If you include D3 - but leave out D4 - the orange button can de-energize the blue relay.

But the blue button cannot de-energize the orange relay.

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

Similarly - if you include D5 and leave out D2 - the blue switch can disable the orange switch.

But the orange switch cannot disable the blue switch.

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

        WHY USE DIODES?

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

The four diodes create one way paths. 

They allow the clock inputs to take the reset pins high.

But they prevent the reset pulse from taking the clock input high.

Similarly - they allow the transistors to take the data pins low.

But - while the transistors are switched off - they prevent the relay coils from taking the data pins high.

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

             THE RELAYS

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

I used SPCO/SPDT relays in the prototype - but you can use multi-pole relays if they suit 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 coils of your relays will operate at the voltage you are using.

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

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 = 45mA.

45mA is well within the limits of the BC547. 

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

There's nothing special about the BC547. 

Any small NPN 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 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 cathodes of D1 & D6 - the side with the bar - are connected to the positive terminal of the coil. 

The small arrowhead indicates the direction in which the diodes 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.
 
            ============

Changing The Behaviour

If you leave out D2, D3, D4 & D5 you'll have two independent toggle switches. Successive pushes on the orange button will energize and de-energize only the orange relay. And the blue button will behave in a similar manner.

If you keep D3 and D4 then - as well as energizing its own relay - each button will de-energize the opposing relay. This means that you can switch back and forth from one relay to the other. It also means that the two relays can never be energized at the same time. As soon as you energize one - the other drops out.

If you keep D2 and D5 - the blue switch can disable the orange button - and the orange switch can disable the blue button. This means that you have to de-energize the blue relay - before you can energize the orange relay - and vice versa. In other words - you can't switch directly from one relay to the other. There has to be an intermediate stage - during which both relays are de-energized.

You can also mix and match these effects. If you leave out D2 & D3 - you can create a Master/Slave relationship. The blue switch can disable the orange push button - but the orange switch cannot disable the blue push button. In other words - Sw2 can both de-energize the orange relay and energize the blue relay - at any time. But Sw1 can only energize the orange relay - if the blue relay is already de-energized.

Of course - if you can use diodes to interconnect two toggle switches - you can use diodes to interconnect any number of toggle switches. So - once you have a clear understanding of the behaviour you require - all you need do is design a suitable diode network.

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 29 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 .

Toggle Switch No.3 - Make A

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.

Next fit the Thirteen Wire Links. 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.

Toggle Switch No.3 - Make B

Then fit the twelve resistors. For clarity - all the resistors are drawn lying flat on the board. However - R1 is actually mounted standing upright. See the Photograph Of The Prototype.

Toggle Switch No.3 - Make C

Next - fit the six diodes - the two transistors - and the seven capacitors. Pay particular attention to the orientation of the diodes and transistors. See the Photograph Of The Prototype.

Toggle Switch No.3 - Make D

Finally - fit the two relays - the IC socket - and the LEDs. Pay particular attention to the orientation of the LEDs.

Toggle Switch No.3 - Make E

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 6 solder bridges. The solder bridges are just small blobs of solder connecting the two tracks together. They're like very short wire links.

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