BASIC 
ELECTRONICS COURSE 
Page 29 INDEX

There is one more GATE we need to cover. It is the . . .
THE SCHMITT TRIGGER INVERTER
The SCHMITT TRIGGER INVERTER is an INVERTER with a SCHMITT TRIGGER input.
 
It is basically a GATE and can be used as an INVERTER. But the Schmitt Trigger input gives it a lot more features.
The Schmitt Trigger Inverter is shown on the left and the Schmitt symbol must be included to show it is not an ordinary inverter. 

This is a very useful BUILDING BLOCK and can be used in many different ways.  
It can be used as an Inverter, an oscillator, a Buffer as well as other "Building Blocks." It all depends on the surrounding components. 
When you add components to create a circuit that performs a "function," the result is called a BLOCK, or BUILDING BLOCK or STAGE. 
There are 6 Schmitt Inverters in the chips we will be discussing and this allows 6 separate "stages" or "blocks" to be produced.  That's why complete projects can be designed around a single chip. 
When wired as an oscillator, only two external components are needed. If you require the oscillator to be turned on and off, a low-frequency oscillator can gate the oscillator (as shown previously). If you require the oscillator to drive an output device, two Schmitt Inverters can be wired in parallel to deliver the drive-current. If you require a load but WITHOUT inversion, two Schmitt gates can be placed in series. (This is called "double inversion" and is equivalent to a BUFFER.)
The chip we are referring to is the HEX SCHMITT TRIGGER with the basic identification of 74C14. 
This chip is also known as 40106 or 74F14  or 74HC14. They are all pin-for-pin compatible devices. The advantage of the 74HC14 is it will operate on a voltage as low as 3v. 

The range of circuits you can design with this chip is endless but before you can start designing, there are two features we need to cover. 
1. The Hysteresis of a Schmitt Trigger
2. The Time Delay Circuit.


1. THE HYSTERESIS OF A SCHMITT TRIGGER
The Hysteresis of a Schmitt Trigger is the gap between the low point where the gate changes state and the high point. This gap is typically 33% of rail voltage and because this gap is so wide, it is very difficult for noise to enter the gate and cause false triggering. This makes the gate ideal for noisy situations but it will not amplify low-level signals and therefore cannot be used where small signals are required to be amplified. This is the only drawback of a Schmitt Trigger. 
The hysteresis is the "dead gap" between 33% and 66% of rail voltage and these two levels are discussed fully below.

2. THE TIME DELAY CIRCUIT
 The time delay circuit is also know as a "TIMING CIRCUIT," "DELAY CIRCUIT," or "R-C CIRCUIT". These names all refer to a CAPACITOR and RESISTOR in series. It does not matter if the capacitor is placed above or below the resistor as the time delay will be the same. The only difference will be the value of the voltage at the beginning and end of the timing cycle. 
If the capacitor is above the resistor, as shown in the first diagram below, the voltage will RISE from zero to rail voltage. 
If the capacitor is below the resistor, as shown in the second diagram, the voltage will fall from rail voltage to  zero. 
The join of the two components is the point where the voltage is detected and is called the "Detection Point." 
The Detection Point is monitored by a Detection Circuit. This can be any of the gates we have described or a transistor or a multimeter. 
The detection circuit  must not load the timing circuit. In other words the detection circuit must have a very high input impedance and as we have already described, CMOS gates have a very high input impedance. That's why they are ideal for detecting the voltage on a DELAY CIRCUIT. 

When voltage is applied to a TIMING CIRCUIT, the capacitor begins to charge. If we monitor the voltage across the capacitor, we can determine when it is at a particular voltage level. It will take a PERIOD OF TIME to reach this level and this is the TIME DELAY we require. 
In the animation below we see the capacitor charging via a resistor, with a meter showing the approx voltage across the capacitor. We have already mentioned the capacitor does not charge at a constant rate, but this characteristic does not concern us at the moment. 
The point to remember is the TIME it takes for the capacitor to charge. 

In the animation below, the meter is monitoring the voltage across the RESISTOR. As you can see, the voltage across it is falling as the capacitor charges. 
As the voltage across the capacitor INCREASES, the voltage across the resistor DECREASES because the total voltage across the combination is 10v, and the voltage across each must add up to 10v.   

If we connect the DELAY CIRCUIT to the output of a gate (any gate can be used - but we will choose an INVERTER and since we are discussing the Schmitt Inverter, we will use it)  we can CHARGE the capacitor when the output of the Schmitt Inverter is HIGH and DISCHARGE the capacitor when the output is LOW. The animation below shows how the voltage across the capacitor rises and falls during the cycle. The actual shape of the graph does not concern us. We are only interested in VOLTAGE LEVELS and the TIME TAKEN for the voltage to rise and fall.  
We will discuss the voltage levels in a moment. For the moment you need to know the levels detected by a SCHMITT INVERTER are: 33% and 66% of rail voltage. These two levels are shown on the graph. 


Here comes the clever part. Instead of the voltmeter monitoring the voltage across the capacitor, the input of the Schmitt Inverter can be connected to the capacitor. 
If the voltage across the capacitor is less than 66% of rail voltage, the output of the gate is HIGH and the capacitor begins to charge. When the voltage reaches 67%, the output goes LOW and the capacitor begins to discharge. When the voltage across it reaches 32% of rail voltage, the Schmitt Inverter changes state and the output goes HIGH. In this way we need only one gate to create an oscillator.
There are two very important things to observe in the animation below.
1. The output is a square wave. In other words the output goes from one state to the other VERY QUICKLY  and this produces the characteristic waveshape. 
2. The voltage across the capacitor is EXACTLY 32% to 67% of rail voltage. 
The animation below shows the gate in operation. 
You will notice that the diagram does not show the chip connected to the positive and negative rail. It is ASSUMED the chip is connected to the supply voltage and that's how the output produces the HIGH. 


A point to remember:
1. The output of the chip is always the reverse (opposite) of the input. The chip INVERTS the level (HIGH or LOW) on the input and makes the output the OPPOSITE. If the Input is HIGH, the output will be LOW etc. 

HOW THE SCHMITT OSCILLATOR WORKS
Suppose the input is LOW. The output will be HIGH. The voltage across the resistor will cause current to flow through it and charge the capacitor. When the voltage on the capacitor reaches 67% of rail voltage, the gate will change state. 
The energy in the capacitor will "bleed" through the resistor and the voltage across the capacitor will gradually fall. When it reaches 32% of rail voltage, the gate will change state.  

By selecting the correct values for R and C, the Schmitt Oscillator can flash an LED at a low flash rate.  The 470R resistor in series with the LED has nothing to do with the flash rate. It must be included so that the output of the gate goes HIGH. If it is omitted, the output will not rise above 1.7v. This is the characteristic voltage across a LED and has been discussed in the first pages of this course. If the resistor is reduced in resistance, it will load the output and the output will not rise to rail voltage. If this occurs, the input will not see 67% of rail voltage and the gate will not change state. 
The output of a 74C14 Schmitt gate will deliver about 15 - 20mA. If the load requires more than about 20mA, a buffer (driver) transistor will be needed. The output will deliver more than 20mA but the output will not be full rail voltage. A small drop will not affect the performance of the oscillator, but if the load current is increased, a point will come when the output will not rise to 67% of rail voltage and the input will not allow the gate to change state. 

  

If more than 25mA is required by a load, a buffer transistor will be needed between the gate and the load. This will allow a globe or relay to be driven by the oscillator. The buffer resistor is needed for two reasons. 1. It allows the output of the oscillator to rise to rail voltage, and 2. It limits the current into the base of the transistor. 
Go to page 30  page 31 for interactive Schmitt Trigger Oscillator circuits.

The diagram below shows how the Schmitt gate can be used to create other features such as a BUFFER, high-current driver and a high-voltage driver. 

  Sometimes you will see a NAND Schmitt Trigger oscillator used in a circuit. When both inputs of a NAND gate are connected together as show in the diagram on the left, the gate becomes an inverter and you can use the inverter we have described above. The advantage of the inverter is 6 gates in a chip. The NAND Schmitt Trigger has 4 gates in a chip. Talking Electronics has designed many projects around the Hex Schmitt Trigger and we encourage you to design around this chip too.  



The pinout of the Quad gate and Hex gate Schmitt Trigger:

Circuits that use the Schmitt Trigger NAND gate can be converted to use the Schmitt Trigger INVERTER by making a few simple changes. We have already covered the theory for this and can now put our knowledge into practice. The diagram below shows a low-frequency NAND gating a NAND oscillator. 
Go to the Truth Tables on page 26 and study the NAND gate. When one input is HIGH, the other input controls the output. This is what we want for the circuit below. 
When the gating line is HIGH, the waveform from the high-frequency oscillator will appear on the output.
When the gating line is LOW, the output of the high-frequency oscillator remains HIGH. In other words the oscillator is JAMMED (jammed LOW). When we talk about Jamming, it is the input we jam and the output may be HIGH or LOW, depending on the type of gate. The gating line can also be called the control line. 

If the circuit is re-designed using two Schmitt INVERTERS, the oscillator can be jammed HIGH or LOW depending on the placement of the gating diode. This is one of the advantages of using INVERTERS and diode gating. The two diagrams below show how this is done. 


Question 131: What are two other names for a TIMING CIRCUIT?
Ans: An R-C circuit, a TIME DELAY circuit. 

Question 132: Name the two components in a Timing Circuit.
Ans: A Capacitor and a resistor.

Question 133: Are the two components in a Timing Circuit connected in parallel or series?
Ans: Series. 

Question 134: The join of the two components in a timing circuit is monitored. What are we monitoring?
Ans: The voltage at the join. 

Question 135: Is a Schmitt Trigger an INVERTER?
Ans: No a Schmitt Trigger refers to the characteristic of an input where the gate changes at a specific voltage level. 

Question 136: What is the "Hysteresis" of a Schmitt Trigger?
Ans: The gap between the high and low points where the gate changes state. 

Question 137: When both inputs of a NAND gate are connected together, it is converted into a ______ gate.
Ans: An INVERTER.

Question 138: What is the voltage levels on a capacitor in a Schmitt Oscillator?
Ans: Between 33% and 66% of rail voltage. 

Question 139: When the input of a Schmitt Trigger Inverter rises above 67% of rail voltage, the output changes to _______ (HIGH/LOW).
Ans: LOW

Question 140: When the input of a Schmitt Trigger is LOW, the output is _______ (HIGH/LOW)
Ans: HIGH
Question 141: What happens to the output of a Schmitt Trigger if the input rises from 0v to 50% of rail voltage then back to 0v again?
Ans: The output remains HIGH.

Question 142: Why do we say the Schmitt Trigger has high noise rejection?
Ans: Because noise has to have a fairly high amplitude to cause the gate to change state. If the gate is LOW, the noise will have to be 67% of rail voltage to change the state of the gate. 

Question 143: Why does the writer of this course prefer the Hex Schmitt Trigger rather than the Quad Schmitt Trigger?
Ans: Two extra gates in a chip.

Question 144: If the output of a Schmitt gate is connected directly to the base of a transistor, will the output go HIGH?
Ans: No. The output will not rise above 0.7v 

Question 145: If the output of a Schmitt gate is connected directly to a LED, will the output go HIGH?
Ans: No. The output will not rise above 1.7v

Question 146: If two inverters are connected in series, name the equivalent gate produced.
Ans: A BUFFER

Question 147: Give a reason why two inverters are connected in parallel:
Ans: To provide a higher driving current

Question 148: Can both the input and output of an Inverter be HIGH at the same time?
Ans: No. 

Question 149: For the Interactive problem on page 30,  what is the value of the timing resistor?
Ans: 100k

Question 150: If an inverter is Jammed HIGH,  is the input HIGH or LOW?
Ans: HIGH

You will notice we are concentrating on features that are absolutely essential to designing circuits. The mathematics has been left out to prevent anyone "dropping-out." You can very easily go to a technical reference for the mathematics. 
We are showing you how to design circuits via EXPERIMENTATION. Even if you design something via mathematics, you must physically create the circuit to make sure it works as expected. 
A lot of the more-complex blocks such as shifters, adders, counters,  etc  will not be covered as these operations can be performed with an 8-pin microcontroller.

Using a microcontroller is not only cheaper and easier to get a circuit operational, it is the way circuits are heading. With a microcontroller you have total control over the operation of the circuit and since everything is held in the form of a secure program, you can be rewarded for your  "intellectual efforts."  
But just before we launch into microcontroller design, we will provide a little more on the Schmitt Trigger. 

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