BASIC 
ELECTRONICS COURSE 
Page 32 INDEX

THE TRANSISTOR SCHMITT TRIGGER
The Schmitt Trigger can be made with two transistors and for some applications, this has a definite advantage over the Schmitt Trigger in the form of a digital gate (in a chip). 
 We have already covered the characteristics of a Schmitt Trigger. Its output changes very rapidly from a LOW state to a HIGH state and back again as the input voltage rises and falls SLOWLY. 
A typical application is turning a fan or heater ON and OFF at particular temperature levels. The circuit operates a relay and, depending on the current-rating or the relay, it can operate a low or high current device. There is a certain amount of "setting up" required to get the circuit to turn ON and OFF at the required temperatures, but once these are worked out, the circuit is very simple and reliable. 
Firstly we will look at how the circuit works. The animation below shows a potentiometer moving across a small percentage of its range and turning the relay on and off. The angular movement of the potentiometer will depend on a number of things:
1. The resistance of the top resistor, 
2. The resistance of the potentiometer, 
3. The value of the emitter resistor, and
4. The current required by the relay. 
The value of these resistors will have to be worked out by experimentation, so we will look at the basic operation:

 The animation above shows the first transistor turns ON when the wiper is moved towards the positive rail and this turns the second transistor OFF.  The wiper has to be moved down slightly for the first transistor to turn OFF, and this turns the second transistor ON. The potentiometer and resistor form a voltage-divider network and the voltage at the join is the "turn-on voltage" for the transistor. 
The same "turn-ON" effect can be created by adjusting the top resistance in the voltage divider network. This is shown in Fig: 2. below. This is only a theoretical circuit as the potentiometer should have a small-value of resistance between the pot and positive rail to prevent the pot being taken to the positive rail causing a very high current to flow into the base. This resistor is called a "stop resistor" and is not shown on the diagram. The pot will really only have to be moved slightly from its lower position but everyone loves to turn a pot fully clockwise to see what happens!

The gap between the ON and OFF states of the circuit is called the Hysteresis Gap and this can be widened or narrowed by changing the value of the emitter resistor. But before we go into this value and all the other component values, we need to explain the secret behind the "snap action" of the relay. It closes very quickly when the potentiometer is turned very slowly in one direction.
Normally a single transistor driving a relay will gradually energise a relay or turn a LED on slowly as the potentiometer is turned. 
But the action of the two transistors creates a SWITCHING EFFECT where the output changes very rapidly from one state to the other. 
This is very important if you are driving a relay as it prevents the relay "chattering" and burning out the contacts. The secret in the snap action lies in the feedback line between the two emitters. This is shown in bold in the diagram below:


Here is how the feedback line works:

The animation above shows how the first transistor gets "jacked-up" by 0.1v by the action of the second transistor. This is due to the voltage drop developed across the emitter resistor being fed back to the first transistor. But it still doesn't explain the SNAP ACTION. 
Let us take it in slow motion. 
When the voltage on the base of the first transistor is low, the second transistor is turned ON via the resistor on its base.  This resistor is also the collector resistor for the first transistor but at the moment it is acting as the "turn on" resistor for the second transistor. 
The relay is energised and current flows through the coil. This current also flows through the emitter resistor and say it puts a 0.1v drop across the resistor. This voltage is passed to the first transistor but at the moment it does not affect the first transistor. It merely means the first transistor has to rise to 0.8v       (0.7v - the normal turn on-voltage for a transistor PLUS 0.1v = 0.8v.)
Nothing magic happens until the base voltage on the first transistor rises to about 0.75v.
At this point the transistor begins to turn ON and the voltage between its collector-emitter terminals reduces. At 0.751 volt on the base, the collector voltage reduces further and as the base voltage rises, the collector-emitter voltage reduces EVEN MORE. During this time the collector-emitter voltage will drop to 1v, then 0.85v then 0.75v and at about this level it begins to rob the second transistor of its TURN-ON voltage. 
This causes the second transistor to begin to TURN OFF slightly and the current through the relay drops. 
This causes less current to flow in the emitter resistor and the voltage across this resistor falls. This drop in voltage is passed to the first transistor and the voltage on the emitter falls slightly.   
This has exactly the same effect as raising the voltage on the base of the first transistor BUT we are not increasing the base voltage. This means the circuit is turning on the first transistor for us and a REGENERATIVE action takes place in the circuit where the first transistor gets turned on MORE AND MORE until the two transistors have SWITCHED states VERY QUICKLY. This is the SNAP ACTION we discussed above. 
The circuit now has the first transistor fully turned ON and the second transistor fully turned OFF. The voltage on the base of the first transistor is about 0.7v because the 0.1v on the emitter has disappeared and the transistor "pulls" the base down to 0.7v as this is the maximum voltage that can exist between  base and emitter. 
To snap the circuit into the other state, the voltage on the base of the first transistor is reduced slightly. 
At about 0.65v, the transistor begins to turn off. The voltage between its collector-emitter terminals increases and this turns on the second transistor. Current flows through the coil of the relay and this current causes a voltage drop to appear across the emitter resistor. This voltage is passed to the emitter of the first transistor and you can see the base-emitter voltage is reduced slightly without us having to do any work. This causes the first transistor to turn OFF slightly and the regenerative action of the circuit takes over.

"SEE" THE CIRCUIT OPERATING:
The animation below shows exactly how to "SEE" the circuit in operation. When a transistor is drawn as a round circuit it indicates a high voltage across the collector-emitter terminals. When it is squashed,  it indicates the voltage across it is very low (less than 0.5v) and any other transistor connected between collector-emitter will be robbed of "turn-on" voltage. This is how one transistor is able to turn off another transistor.

OK
Now we know how the circuit works we can interface it to external devices. A thermal sensor can be added to the circuit as shown in the diagram below. The thermal sensor in our prototype had a cold resistance (@ room temperature) of 10k and and we were told this type of sensor changed 0.5% per degree C. It is an NTC type and this means it has a Negative Temperature Co-efficient. In other words the resistance DECREASES by 0.5% for each degree rise in temperature. At 100°C the resistance will be about 6k (see below for the actual resistance values for the probe we used in our prototype  - that's why you cannot rely on what you are told).

By referring to the circuit above, you can see the voltage on the base of the first transistor is set by the potentiometer and the circuit will change states when the temperature rises. You must set the LOW threshold by turning the potentiometer towards the 0v rail until the relay is energised. 
Increasing the probe temperature will increase the voltage on the base and turn ON the first transistor. This will cause the second transistor to turn OFF and the relay will release.

COMPONENT VALUES
Now we come to putting component values on the circuit diagram. The diagram below shows the values for our prototype. 

AGAIN
Going over the operation of the circuit once again  . . . the voltage on the base is increased via an external device such as a potentiometer or thermal sensor, and at a high temperature the circuit takes over and snaps the relay OFF. 
When the temperature is reduced, a point is reached when the circuit takes over and the relay snaps ON. 
The gap between the HIGH and LOW is called the Hysteresis gap and this can be made wider or narrower by adjusting the value of the emitter resistor. 
If it is made too narrow, the feedback voltage will not be large enough to snap the circuit into its opposite state and the relay will chatter.
The reason for this is very technical. For instance, in our design, the emitter resistor is 3R9. If it is reduced to 2R2, the relay will chatter. 
When experimenting, you should take values of resistors and capacitors to their extreme so that you know how close the value is to an unreliable design. If you don't do this, you may be creating a circuit that is extremely close to non-operating. 
3R3 is the lowest value you should use and the highest value in our prototype was 10R. 
These values depend heavily on the current taken by the relay (25mA in our case) and different relays will require different emitter resistors. 

 SETTING UP
The first variable to set is the gap resistor. This is the feedback resistor on the emitters. 
Its main purpose is to adjust the gap between the turn-ON and turn-OFF points. It will also move the actual turn-ON and turn-OFF points so the gap is the first setting. 
Once this is set, the actual LOW VALUE is determined by adjusting the trimmer resistors. These are the only two settings you can adjust as the high setting is determined by the gap-value. 

 OUR CIRCUIT
The circuit we used is shown in the diagram below. The relay pulled in at 26°C and released at 34°C. The 330R "set resistor" puts a small "set voltage" on the base so that the 1k pot has a wider adjustment to make it easier to set the low value. 
When setting up the circuit, the probe is placed in the low temperature medium and the pot is turned towards 0v rail until the relay pulls in. The probe is then placed in the high temperature medium and the relay will release. This circuit gives an 8 degree range (8 degree gap) and this gap can be widened by increasing the value of the emitter resistor. The characteristics for the probe used in the circuit are as follows:
9,500 ohms at 26°C and  6,900 ohms at 34°C  The probe we used obviously had a much higher resistance change per degree than the figures we quoted above. It changes nearly 4% /°C 

If you need to detect higher temperatures, say 40°C to 55°C, the 330R "set resistor" will have to be lowered to 220R so the 1k pot will still have a range. The emitter resistor will also have to be changed to 3R9 or 4R7. This is shown below:

This concludes our description of this very handy circuit. It proves basic electronics is still needed in this microprocessor/digital world. Now for a few questions . . .

QUESTIONS    Cover the answer and scroll down to reveal

Question :  Name a feature of a Schmitt Trigger Circuit
Answer:   The "snap-action" from one state to the other. 

Question : What is the "secret" behind the Snap Action?
Answer: The feedback line between the emitter of the second transistor and the emitter of the first transistor. 

Question : Can both transistors be ON at the same time?
Answer: No. The first transistor turns the second OFF and the second transistor turns the first ON and the situation is reversed for the other state.

Question:  What is the name given to the gap between the low and high "change-points" where the
circuit changes from one state to the other.
Answer:   The Hysteresis gap.

Question: If the emitter resistor is increased in resistance, what happens to the Hysteresis gap?
Answer:   It widens

Question: If the voltage on the base is increased, does the relay become energised or released?
Answer:   It is de-energised.

Question:  The probe in our prototype has a NTC. Does the resistance of the probe increase as the temperatures rises?
Answer:   No. It decreases.

Question: What is the purpose of a "stop resistor"?
Answer:   To prevent a high current flowing if a potentiometer is turned fully in one direction. 


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