The Schmitt Trigger is a type of comparator with two different threshold voltage levels. Whenever the input voltage goes over the High Threshold Level, the output of the comparator is switched HIGH (if is a standard ST) or LOW (if is an inverting ST). The output will remain in this state, as long as the input voltage is above the second threshold level, the Low Threshold Level. When the input voltage goes below this level, the output of the Schmitt Trigger will switch.
The HIGH and LOW output voltages are actually the POSITIVE and NEGATIVE power supply voltages of the comparator. The comparator needs to have positive and negative power supply (like + and -) to operate as a Schmitt Trigger normally. The following drawing shows how a Schmitt Trigger would react to an AC voltage input:
The orange line is the AC input. The horizontal RED line indicates the High Threshold Level, while the BLUE horizontal line indicates the Low Threshold Level. The green line is the output of the Schmitt Trigger. When the input voltage level goes above the High Threshold Level, then the output of the ST goes High. When the input voltage level goes below the Low Threshold Level, then the output of the ST goes Low. This is the basic operation of a Schmitt Trigger.
Schmitt Trigger circuits
The simplest Schmitt Trigger using OP-Amp
The most simple Schmitt Trigger circuit is implemented with a comparator with a positive feedback. Look at the following circuit:
When the non-inverting input (+) is higher than the inverting input (-), the comparator output switches to the POSITIVE voltage supply. On the contrary, the non-inverting input (+) is lower than the inverting input (-), the output switches to the NEGATIVE voltage supply.
The inverting input (-) is grounded, so someone would expect that the turn-on and off point would be the ground (0). The function of the ST comes from the feedback resistor RFB. When for example the output of the comparator is to the POSITIVE voltage supply, then the non-inverting input has through the RFB this voltage! The same happens when the output is to the NEGATIVE power supply.
The voltage needed to switch the output of the comparator must be above or below zero (ground), according to the POSITIVE and NEGATIVE power supply and according to the resistors RI and RFB. More specific, the formula to calculate the threshold voltage is:
| VTHRESHOLD = VSUPPLY x | RI |
| RFB + RI |
So, if the output is to the POSITIVE voltage, the required negative voltage that must be applied to Vin is:
VINPUT <= - VTHRESHOLD
If the output is to the NEGATIVE voltage, the required positive voltage that must be applied to Vin is:
VINPUT >= VTHRESHOLD
The above circuit is non-inverting Schmitt Trigger. It can be easily converted into an inverting Schmitt Trigger as follows:
The same formula is used to calculate the threshold level. But the output of this circuit is inverted in comparison to the previous one. When the input voltage is above the High Threshold Level, the output goes LOW.
These are the most basic Schmitt Trigger. The threshold voltage is an equal offset from the center of the POSITIVE and NEGATIVE power supply (usually this is 0). Thus, we can call it "symmetrical Schmitt Trigger". I have include a Symmetrical Schmitt Trigger calculator in the Dr.Calculus page. Do not forget to visit and try it!
Non-symmetrical Schmitt Trigger using OP-Amp
This circuit can have non-symmetrical to zero threshold levels
This circuit is widely used in a variety of applications. It provides the functionality of the basic Schmitt Trigger circuit described above, but it has the ability to set the High and Low Threshold Levels to any voltage desired. For example, you can set the circuit to turn on at +2.5 Volts and turn off at +1.8 volts. Take a look at the circuit:
This circuit is rather weird compared to the previous one. First of all, the feedback resistor RFB is not connected to the input of the circuit. The inverting input (-) is also not connected directly to the ground. There is a resistor network composed of the RFB, the R1 and the R2. The values of those resistors will finally determine the High and Low Threshold Levels.
To calculate the High and Low threshold levels of this circuit, you need at first to calculate the total resistance of the three resistors connected in parallel:
| RTOT = | 1 |
| 1/R1 + 1/R2 + 1/RFB |
Following, you calculate two values, the A and B:
| A = | VREF x RTOT |
| R1 |
| B = | VSUP x RTOT |
| RFB |
Now it's easy to calculate the threshold levels:
VTHRESHOLD_HIGH = A + B
VTHRESHOLD_LOW = A - B
I have include a Non-Symmetrical Schmitt Trigger calculator in the Dr.Calculus page. Do not forget to visit and try it!
Non-symmetrical Schmitt Trigger using OP-Amp with single power supply
A variation of the previous circuit is the single power supply op-amp Schmitt trigger circuit. This circuit does not require positive and negative voltage for the op-amp to operate. The negative voltage of the op-amp is connected directly to the ground (0V) of the circuit. This is the schematic drawing:
The calculation of the two threshold points is different. To calculate the High Threshold Level, you solve the following formula:
RTOT = (R1 x RFB) / (R1 + RFB)
VTHRESHOLD_HIGH = V * R2 / (R2 + RTOT)
For the Low Threshold Level, you solve the following formula:
RTOT = (R2 x RFB) / (R2 + RFB)
VTHRESHOLD_HIGH = V x RTOT / (R1 + RTOT)
I have include a Non-Symmetrical Schmitt Trigger calculator using Op-Amp with single power supply in the Dr.Calculus page. Do not forget to visit and try it!
Transistor Schmitt Trigger Implementation
Since a transistor implementation of a Schmitt Trigger is rather important due to the single voltage supply that requires to operate, i present you a basic Schmitt Trigger circuit with two NPN transistors:
The operation of this circuit is simple. Suppose that the input voltage is zero. Q1 will not conduct. The resistors R1+R2 and R4 will perform a voltage divider. The output from this voltage divider will determine the state of the transistor Q2.
As the input voltage is increased, a very small current will start flowing through Q1. This will have a result on the base voltage of Q2 that it will gradually be decreased, and also the emitter voltage of Q2 will be decreased. But the emitters of Q1 and Q2 are connected together. Therefore, the voltage difference VBE of Q1 will be increased. There will be a point that the current flowing through Q1 will be very high, and the Q2 will be sent to cut-off. When the Q2 is to cut-off area, no current flows through it and thus, the output voltage is the power supply voltage.
Now let's assume that the input voltage is decreasing. The base current of Q1 is decreased, and so does the current IC. This will increase the base voltage VB on Q2. There will be a point that input voltage will be very low, and the base voltage of Q2 will become slightly higher than the emitter voltage. This will cause a small base current to flow through Q2, and thus an emitter current that will flow through R3. This will cause the emitter voltage to be increased. Because the voltage difference between the base and the emitter of Q1 will become smaller, less current will flow through Q1 and the base voltage of Q2 will be further increased. This loop will cause Q2 to start conducting, and Q1 to be sent to cut-off almost simultaneously. There is only a very narrow voltage area where this shift is done.
To calculate the High Threshold Level of this circuit, you can use the following formula:
VTHRESHOLD_HIGH = VSUP x R4 / (R1 + R2 + R4) - 0.62
Where 0.62 is the typical VBE of a silicon transistor.
To calculate the Low Threshold Level, you can use the following formula:
VTHRESHOLD_LOW = VSUP x R4 / (R1 + R2 + R4 + R1 x R4 / R3) + 0.61
I have include a Transistor Non-Symmetrical Schmitt Trigger calculator in the Dr.Calculus page. Do not forget to visit and try it!
Use of the Schmitt Trigger
As usual, there are not specific applications for a Schmitt Trigger. It can be used for example as a window comparator. Or it could be used with a thermistor for heating control, or with an LDR for light control circuits. But there are specific applications where a Schmitt Trigger is just perfect for the job. This is the squaring of a signal. Many times, the input signal from a source is noisy. If it is directly coupled to an IC input, is is most likely that it will receive false pulses due to the noise. Look at the following drawing with a noisy signal:
The noisy input signal is supposed to be just two pulses. But there is a specific voltage level, that each IC will read the input as HIGH or LOW. This level is marked with the red line. Due to the noise of the input signal, you can see that this level is crossed more than once during one positive pulse. The IC input will read false pulses and produce incorrectly results.
The following drawing indicates the same noisy input that is first filtered through a Schmitt Trigger circuit:
The difference can be clearly seen. Due to the High and Low threshold levels of the Schmitt Trigger, the two pulses can be squared again with a very good precision. The false pulses that the IC would read due to the instant voltage drop of the signal are smoothed. This is why the Schmitt Trigger is so widely known and used for digital signal squaring and filtering.
