Schmitt Trigger

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Symbol and Truth Table

The Schmitt trigger is a special circuit which acts like a switch that changes state at two different thresholds. These are called the upper and lower threshold or the positive and negative going threshold. The difference in these two threshold levels is called the hysteresis voltage. The Schmitt trigger does not react to any input voltage level in the range between the two thresholds, which for a 74C/HC14 corresponds precisely to the "forbidden zone".

A Schmitt trigger can also be likened to two comparators controlling an RS flip-flop at the output. In fact, the schematic of the 74C14 shows it to be designed that way. The upper threshold comparator sets the output latch and the lower threshold comparator resets the output latch.

These two thresholds (balance points) makes the 74HC14 Schmitt trigger different from an ordinary 74HC240 inverter with a single threshold at 1/2 Vcc.

Each of the six inverters in a 74C14 Schmitt trigger uses 12 MOSFETs, so by comparison the discrete version of the Schmitt trigger using 3 transistors and 11 other components is about as complex.

Normally the 74HC14 threshold parameters are a fixed ratio of Vcc. This keeps the device functionally simple and as a result the 74HC14 Schmitt trigger is one of the most popular devices for interfacing real world signals to digital electronics. It just doesn't get any simpler compared to the other versions we will discuss.

However it is a little known fact that it is possible to alter the thresholds of a 74HC14 by using negative feedback from output to input. For example, by adding a 5.1M resistor from output to input and a 1M input resistor this will provide about 15% negative feedback. That is subtracted from the internal 30% positive feedback and with Vcc=5V, it effectively changes the thresholds at the input of the 1M resistor to approximately 2.1V and 2.9V. Very useful if the signal of interest has smaller transistions than the normal 74HC14 hysteresis voltage.

Beside the 74HC14, there are a number of ways to implement a Schmitt trigger in CMOS logic. The simplest is to use a non-inverting buffer like the 74HC245 and connect a 3M resistor from output to input to provide positive feedback which is summed with the input signal through the 1M series resistor. These values will give the same thresholds as a 74HC14 but keep in mind the input resistance is 4M to GND or Vcc depending on the current output state.

The same circuit can be achived with two inverters like the 74HC240 version shown. The two inversions of the signal generate the required positive feedback. Both the true and inverted output signals are available. One variation on the last circuit is to add some negative feedback with a 4.7M resistor from the inverted output to the input. This cancels out part of the positive feedback and reduces the hysteresis voltage while permitting a larger input resistor.

Ideal amplifiers called opamps (a.k.a., operational amplifiers) can be used to make a simple (compared with the discrete version) Schmitt trigger but don't let it lure you away from the object of the exercise showing just what goes on "under the hood" of a transistor Schmitt trigger:

I have included the inverting and non-inverting examples of opamp Schmitt trigger which have adjustable threshold and hysteresis voltage. Note that in both cases the potentiometers settings interact so that the threshold and hysteresis must be adjusted by trial and error.

The above diagram reproduces the basic Schmitt trigger circuit of Richard Piotter. It consist of two inverters (NPN and PNP) which give a double inversion to the input signal. The output of the second stage is fed back and summed with the input signal and a resistor to ground. Think of those resistors as forming a voltage divider which determines the input voltage required to cross the 0.6 V threshold of the NPN base emitter junction to turn the transistor on of off. With the values given the positive going threshold is 1.95 V and the negative going threshold is 1.34 V assuming a Vcc of 5V. The output signal at the PNP collector is non-inverting with respect to the input signal.

This diagram shows how the basic circuit is modified to give the symmetrical 1/3 - 2/3 Vcc thresholds equal to the 74C/HC14 Schmitt trigger. This is done by setting NPN emitter voltage to 1/2 Vcc - 0.6 V which makes the on / off switching threshold at the input NPN base exactly 1/2 Vcc. The 1M input and 3M feedback resistors form a voltage divider that sets the values of the positive going input threshold to 2/3 Vcc and the negative going input threshold is 1/3 Vcc.

The positive feedback signal at one end of the 3M feedback resistor is alternately Vcc or Gnd depending on the state of the non-inverted output signal at the collector of the PNP transistor. The other end of the 3M feedback resistor is at the base of the input NPN which is at 1/2 Vcc during switching. The voltage at the input of the 1M resistor is therefore 1/3 * 1/2 Vcc = 1/6 Vcc above and below 1/2 Vcc which at Vcc = 5V is 3.33 V and 1.66 V respectively. This ignores the effect of the <0.1 uA that sinks into the base of the NPN at switching.

The inverting NPN output stage provides isolation, and input protection diodes were added to simulate the 74C/HC14 inverting Schmitt trigger so that this circuit can now be used in the same kind of applications as that device but at Vcc up to 24 V or higher depending on the transistors. The NPN output drive to positive is limited by the 4.7K resistor but it could be replaced with a smaller resistor or even a pager motor. The resistor in the PNP collector was chosen for low power but can be replaced with 4.7K to increase base drive for the NPN output transistor.

The input diodes are not needed for protection since the 1M resistor takes care of that but may be needed to clamp an input coupling capacitor (i.e. Nv) to the negative and positive rails. This is called DC restoration since it removes the residual charge from the capacitor so it has no memory of any previous switching operation which might otherwise affect the timing of a subsequent switching operation.

While this circuit is not an economical substitute for the 74HC14, it gives a good insight into the design of trigger circuits in general.