Electronic Thermostat Circuit - with adjustable hysteresis

Circuit Description

Click Here For A Photograph Of The Prototype.

Circuit Diagram For A
Temperature Controlled Relay

You can use a Cmos 4001 or a Cmos 4011 in this circuit.

These ICs have four - two-input - gates.

In each case - the two inputs are joined together.

So the gates are being used as simple inverters.

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

This means that when the two inputs are high - the output will be low.

And when the two inputs are low - the output will be high.
 
                 ============

The output from the circuit is at pin 10.

When pin 10 goes low - it's connected internally to the negative line.

So it connects the base of Q1 to ground - through R6.

This allows current to flow through the transistor's emitter-base junction.

And R6 limits the size of that current - to a maximum of about 5mA.

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

When current flows through the emitter-base junction of Q1 - it switches the transistor on.

The transistor connects the negative side of the relay coil to ground  - and the relay energizes.

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

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

So it connects the base of Q1 to the positive line - through R6.

This cuts off the transistor's base current.

Without its base current - the transistor switches off.

This disconnects the negative side of the relay coil from ground  - and the relay de-energizes.

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

In other words - every time pin 10 goes low - the relay will energize.

And every time pin 10 goes high - the relay will de-energize.

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

If you use an NPN transistor - instead of the PNP - the opposite is true.

Every time pin 10 goes high - the relay will energize.

And every time pin 10 goes low - the relay will de-energize.

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

All the gates are being used as simple inverters.

So - if pins 12 & 13 are low - pin 11 will be high.

Pin 11 takes pins 8 & 9 high - so pin 10 will be low.

In other words - while pins 12 & 13 remain low - the relay will remain energized.

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

The opposite is also true.

If pins 12 & 13 are high - pin 11 will be low.

Pin 11 takes pins 8 & 9 low - so pin 10 will be high.

In other words - while pins 12 & 13 remain high - the relay will remain de-energized.

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

While the temperature is low - we want the relay to be energized.

So - while the temperature is low - we want pins 12 & 13 to be low.

And while the temperature is high - we want the relay to be de-energized.

So - while the temperature is high - we want pins 12 & 13 to be high.

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

The state of pins 12 & 13 is controlled by gates 1 & 2.

Gate 1 is responsible for taking pins 12 & 13 high.

And Gate 2 is responsible for taking pins 12 & 13 low.

In other words - Gate 1 decides when the relay will de-energize.

And Gate 2 decides when the relay will energize.

Having independent control over these two events - lets you choose the width of the hysteresis.

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

 THE ROLE OF THE THERMISTOR

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

A cmos input pin is high - when it's over half the supply voltage.

And a cmos input pin is low - when it's below half the supply voltage.

I've used a 12-volt supply in the diagram.

This means that - roughly speaking - anything above 6-volts is a high.

And anything below 6-volts is a low.

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

R1, R2, R3 and the Thermistor form a potential divider.

And the potential divider controls the voltages on the inputs of Gates 1 & 2. 

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

A potential divider is no more than a number of resistors connected in series.

In our case it's four resistors.

A portion of the 12v supply voltage appears across each resistor.

And the size of that portion depends on the value of the resistor concerned.

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

For example - if we connect four 10k resistors in series across our supply - there will be a drop of 3v across each one.

In other words - the 12-volt supply potential will be divided into four equal portions - of 3-volts each.

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

But the resistors don't have to be the same value.

When resistors with different values are connected in series - the voltage across each resistor - is in proportion to its value.

For example - if you connect a 10k resistor and a 15k resistor in series across the supply - two-fifths of the 12v will appear across the 10k resistor - and the remaining three-fifths will be across the 15k resistor. 

In other words - there'll be 4v8 across the 10k resistor - and 7v2 across the 15k resistor.

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

Back to Gates 1 & 2 - and the Thermistor.

R1, R2, R3 and the Thermistor form a potential divider.

And the 12v supply is divided up between them.

The state of the Gate 1 inputs is controlled by the voltage at the junction of R2 & R3.

The state of the Gate 2 inputs is controlled by the voltage at the junction of R3 and the Thermistor.

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

Note that the voltage on pins 5 & 6 is simply the voltage across the thermistor.

But the voltage on pins 1 & 2 is the voltage across the thermistor PLUS the voltage across R3.

It should be clear that the voltage on pins 1 & 2 will always be higher than the voltage on pins 5 & 6.

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

           RISING TEMPERATURE

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

Lets start with the temperature low.

This means that pins 12 & 13 are low.

So pin 10 is low - and the relay is energized.

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

Consider what happens when the temperature begins to rise.

As it rises - the value of the thermistor falls.

So the voltage drop across the thermistor falls.

And the voltage on the inputs of Gates 1 & 2 - falls also.

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

Since the voltage on the inputs of Gate 1 is always higher than the voltage on the inputs of Gate 2 - the inputs of Gate 2 will be the first to go low.

This will cause pin 4 to go high.

But when pin 4 goes high - it has no immediate effect.

The presence of D1 means that pin 4 cannot take pins 12 & 13 high.

The only change is that pins 12 & 13 are no longer being held low by pin 4.

But - they will continue to be held low by pin 10 - through R5.

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

As the temperature rises further - the voltage at the junction of R2 & R3 will continue to fall.

Eventually - it will take pins 1 & 2 low.

This will cause pin 3 will go high.

And pin 3 will take pins 12 & 13 high - through D2.

This causes pin 11 to go low.

Pin 11 takes pins 8 & 9 low.

So pin 10 will go high - and the relay will de-energize.
 
                ============

         FALLING TEMPERATURE

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

Now - lets start again.

This time - with the temperature high.

Pins 12 & 13 are high.

Pin 11 is low.

Pin 11 takes pins 8 & 9 low.

So pin 10 is high - and the relay is de-energized.

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

Consider what happens when the temperature begins to fall.

As it falls - the value of the thermistor rises.

So the voltage drop across the thermistor rises.

And the voltage on the inputs of Gates 1 & 2 rises also.

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

Since the voltage on the inputs of Gate 1 is always higher than the voltage on the inputs of Gate 2 - the inputs of Gate 1 will be the first to go high.

This will cause pin 3 to go low.

But when pin 3 goes low - it has no immediate effect.

The presence of D2 means that pin 3 cannot take pins 12 & 13 low.

The only change is that pins 12 & 13 are no longer being held high by pin 3.

But - they will continue to be held high by pin 10 - through R5.

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

As the temperature falls further - the voltage at the junction of R3 & the Thermistor will continue to rise.

Eventually - it will take pins 5 & 6 high.

This will cause pin 4 will go low.

And pin 4 will take pins 12 & 13 low - through D1.

This causes pin 11 to go high.

Pin 11 takes pins 8 & 9 high.

So pin 10 will go low - and the relay will energize.

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

To sum up - the inputs of Gate 1 are always the first to go high - and the last to go low.

And the inputs of Gate 2 are always the first to go low - and the last to go high.

Gate 2 decides the temperature at which the relay will energize.

And Gate 1 decides the temperature at which the relay will de-energize.

The difference between these two temperatures - the hysteresis - depends on the voltage drop across R3.

And the voltage drop across R3 - the amount of hysteresis - depends on the value of R3.

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

               THE ROLE OF R2

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

R2 is a padding resistor.

Its value is chosen to place you at roughly the required temperature.

Then R1 is used to provide the fine adjustment.

A lower value R2 - will give access to higher temperatures.

And a higher value R2 - will give access to lower temperatures.

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

        THE ROLES OF C1 & C2

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

Cmos inputs react very fast.

The state of a cmos gate can be changed by high frequency interference.

This might be electro-magnetic waves picked up by the wiring - or the power lines.

To high frequencies - C1 & C2 are like a short circuit.

They conduct any stray signals to ground - before they can do any harm.

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

        THE ROLES OF D1 & D2

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

D1 allows Gate 2 to take pins 12 & 13 low - but it prevents Gate 2 from taking pins 12 & 13 high.

Thus - Gate 2 is able to switch the transistor on - and energize the relay.

But it cannot switch the transistor off - so it cannot de-energize the relay.

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

Similarly D2 allows Gate 1 to take pins 12 & 13 high - but it prevents Gate 1 from taking pins 12 & 13 low.

Thus - Gate 1 is able to switch the transistor off - and de-energize the relay.

But it cannot switch the transistor on - so it cannot energize the relay.

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

R4 is not really necessary.

Pin 3 should never be high - while pin 4 is low.

So a high Gates 1 output - should never short to a low Gate 2 output - through D1 & D2.

But - because it might just happen - I included R4 as a precaution.

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

        THE ROLES OF C3 & C4

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

Changes in temperature can happen slowly.

So the inputs of Gates 1 & 2 may spend a significant time hovering around the 6-volt mark.

In this area - the status of the input is uncertain.

It's neither high nor low.

In other words - the circuit can't tell if the temperature is above or below the preset level.

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

Although Cmos gates are digital devices -during this period of uncertainty - their outputs can behave in an analogue manner.

Instead of switching sharply from high to low - and vice-versa - the pin 10 voltage rises and falls slowly.

This may mean that your relay will energize and de-energize slowly.

And the uncertainty about its status can cause the relay coil to tend to oscillate.

In other words - the relay may rattle or buzz for several seconds.

C3 & C4 help prevent the oscillation.

You may find that you can leave out C4 - and rely solely on C3.

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

R5 creates a Schmidt-Trigger effect - to further sharpen the switching point.

It provides a little positive feedback.

Once pins 12 & 13 start to go low - pin 10 will start to go low.

And as pin 10 starts to go low - R5 helps pins 12 & 13 to go low.

A similar effect happens in the opposite direction - when pins 12 & 13 start to go high.

The result is that the relay changes state cleanly - and without the noise.

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

I used a single pole 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  BC557. 

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

There's nothing special about the BC557. 

Any small PNP 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 BC557.

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

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 D3 - 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.
 
                 ============

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 .

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 Seven Wire Links - the preset - and the five fixed 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.

Next - fit the three diodes and the fours capacitors. Pay particular attention to the orientation of both the diodes and the electrolytic capacitors. See the Photograph Of The Prototype. Then - fit the relay and the IC socket.

Thermostat With Hysteresis - Support Material

Construction Guide

Actual Size

Fit The Transistor

BC557 PNP

BC547 NPN