Schmitt trigger
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In electronics, a Schmitt (or Schmidt) trigger is a special kind of comparator circuit.
A Schmitt trigger has one input voltage and one output voltage. The output can be either high or low. When the input is below a certain threshold, the output is high; when the input is above a certain (higher) threshold, the output is low; and when the input is between the two thresholds, the output retains its value. The trigger is so named because the output retains its value until the input changes sufficiently to trigger a change. This delayed action is called hysteresis, and implies that the Schmitt trigger has some memory.
The benefit of a Schmitt trigger over a similar system with a single input threshold is that the Schmitt trigger is more stable. With only one input threshold, a noisy input signal near that threshold could rapidly switch back and forth, causing the output to switch back and forth from low to high. With the Schmitt trigger, a noisy input signal near one threshold could cause only one switch in output value, after which it would have to move to the other threshold in order to cause another switch.
The Schmitt trigger was invented by US scientist Otto H. Schmitt; today, they are typically built using operational amplifiers, and the reference voltage levels can be adjusted by controlling the resistances of R1 and R2:
Image:Opampschmitt xcircuit.png
An op-amp comparator simply gives out the highest voltage it can, when the positive input is at a higher voltage than the negative, and then switches to the lowest output voltage it can, when the positive input drops below the negative.
For instance, if the trigger is currently in the on state, the output will be at the positive power supply rail (+VS). V+ is then a voltage divider between Vin and +VS. The comparator is comparing V+ to ground. VinR2 must be equal to −VSR1 for V+ to equal zero, so Vin must drop below −(R1/R2)VS to get the output to switch. At this point, the output becomes −VS, and the threshold becomes +(R1/R2)VS to switch back on.
Image:Hysteresis sharp curve.svg
So this circuit creates a switching band centered around zero, with trigger levels ±(R1/R2)VS. The input voltage must rise above the top of the band, and then below the bottom of the band, for the output to switch on and then back off. If R1 is zero or R2 is infinity (an open circuit), the band collapses to zero width, and it behaves as a standard comparator. The output characteristic is shown in the picture on the right. The value of the threshold T is given by (R1/R2)VS and the maximum value of the output M is the power supply rail.
The speed at which the op-amp switches between power rails is the slew rate. The output can also be configured to drive digital logic.
The symbol for Schmitt triggers in electronic diagrams is a triangle with a hysteresis symbol:
Image:Schmitttrigger symbol.png
A possible structure of a more realistic configuration is the following:
Image:Opampschmitt realistic xcircuit.png
The output characteristic has exactly the same shape of the previous basic configuration; moreover the threshold values are the same as well. On the other hand, in the previous case the output voltage was depending on the power supply, while now it is defined by the Zener diodes: this way the output can be modified and it is much more stable. The resistor R3 is there to limit the current through the diodes, while R4 has to limit the current due to the offset voltage on the input of the amplifier.
Schmitt trigger with two transistors
Schimtt trigger is also frequently composed from two transistors. The chain RK1 R1 R2 works as a voltage divider, setting the base voltage for the transistor T2 base. This divider, however, is affected by the transistor T1, providing lower voltage if T1 is open. Hence the threshold voltage for switching between the states depends on the current state.
Image:Schmitt with transistors.png
Under low input voltage, T1 is closed and T2 is opened. The base volatage of the transistor T2 is determined by the mentioned divider. Due negative feedback, the voltage on the shared emitter resistor RE is close to the voltage. The T1 can only open if the input voltage (base voltage) raises a little above the voltage on the RE (emitter voltage). When T1 opens, T2 closes, because its the voltage divider now provides lower base voltage and the emitter voltage does not drop too fast as the open T1 delivers some current accoss RE.
Under higher input voltage, T1 is open and T2 is closed. When the input voltage reduces, T1 starts to close, increasing the base voltage for T2. T2 opens at some time, increasing the current across RE, further reducing the T1 base-emitter potential and closing this transistor.
In the "state 1" the output voltage is close to V+, but in the "state 0" it is still high enough and may not be sufficiently low to be a "logical zero voltage" for the digita circuits. To work arount this, the additional amplifiers are added after the output cascade or the trigger is feeded by the two voltage sources (V+ and V-), the ground level being between them
Use as an oscillator
Schmitt triggers are sometimes used to implement a simple type of relaxation oscillator, or multivibrator. This is achieved by connecting a single resistor-capacitor network to the device — the capacitor connects between the input and ground and the resistor connects between the output and the input. The output will be a continuous square wave whose frequency depends on the values of R and C, and the threshold points of the Schmitt trigger. Since multiple Schmitt trigger circuits can be provided by a single integrated circuit (e.g. the 4000 series CMOS device type 40106 contains 6 of them), a spare section of the IC can be quickly pressed into service as a simple and reliable oscillator with only two external components. It should be noted that frequency stability is not that great however.
External links
es:Disparador Schmitt pl:Przerzutnik Schmitta lt:Šmito trigeris