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23 December 2009
Author: Giorgos Lazaridis
The Schmitt Trigger

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:



Now it's easy to calculate the threshold levels:



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)

For the Low Threshold Level, you solve the following formula:

RTOT = (R2 x RFB) / (R2 + RFB)

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.

Relative pages
  • Dr.Calculus: Non-symmetrical Schmitt Trigger calculator using Op-Amp
  • Dr.Calculus: Symmetrical Schmitt Trigger calculator using Op-Amp
  • Dr.Calculus: Transistor Schmitt Trigger calculator
  • The transistor theory of operation
  • Op-Amp IC Pinouts

  • Comments


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  • At 23 November 2015, 21:50:54 user Sumit wrote:   [reply @ Sumit]
    • helped a lot :)

  • At 8 October 2013, 23:06:46 user farokh wrote:   [reply @ farokh]
    • wao,, very simple and useful information. I was trying to understand the feedback resistor since last two days. This page solved my problem.
      thanks for posting this.

  • At 2 July 2013, 5:32:55 user kashif wrote:   [reply @ kashif]
    • thankx for simple and very informative details about schmitt trigger

  • At 10 May 2013, 5:17:37 user Kapil wrote:   [reply @ Kapil]
    • Thanks

  • At 6 November 2012, 12:00:51 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @kiranvarma read these pages that you posted this message (2 pages).

  • At 6 November 2012, 9:16:32 user kiranvarma wrote:   [reply @ kiranvarma]
    • Really neat and clean explanation. It is easy to understand, thank you Giorgos for this presentation. Hi Giorgos, i need help about schimit trigger.

      Some circuits in the internet designed with three resistors values. What is the use of three resistors and how do we calculate them, if i provide the threshold values. Any expressions for resistor calculations. I am waiting for your reply.

  • At 24 July 2012, 16:21:12 user binod hamal wrote:   [reply @ binod hamal]
    • thankss very much,it helped me so much

  • At 23 April 2012, 19:55:47 user paris chasmaan wrote:   [reply @ paris chasmaan]
    • itx very helpful and intelligently explained.....

  • At 30 March 2012, 8:54:59 user anmar wrote:   [reply @ anmar]
    • thanks ssssssssssssss very thanksss

  • At 27 March 2012, 0:59:41 user dharani wrote:   [reply @ dharani]
    • nice!
      easy to understand

  • At 17 March 2012, 4:26:50 user ramesh wrote:   [reply @ ramesh]
    • It is nice article with good representation

  • At 8 February 2012, 19:01:17 user Christian wrote:   [reply @ Christian]
    • Thanks for this clear explanation of the Schmitt Trigger.

  • At 7 February 2012, 15:24:35 user aamina wrote:   [reply @ aamina]
    • i am now very clear about this circuit.thanks.

  • At 3 January 2012, 13:12:39 user abhimanyu singh wrote:   [reply @ abhimanyu singh]
    • thangs for this complement

  • At 30 December 2011, 13:47:07 user Behnaz wrote:   [reply @ Behnaz]
    • that was good and useful thanks a lot

  • At 2 December 2011, 7:11:53 user omer wrote:   [reply @ omer]
    • thank you very much to this usful information

  • At 25 November 2011, 7:02:20 user sunitha wrote:   [reply @ sunitha]
    • thanks for the good explanation...

  • At 8 November 2011, 8:28:29 user Kammenos wrote:   [reply @ Kammenos]
    • @Brian S make R1 greater than RFB

  • At 8 November 2011, 0:57:18 user Brian S wrote:   [reply @ Brian S]
    • For the forward Schmitt Trigger - How would you get below ground to have the Trigger reverse or turn off.

  • At 3 November 2011, 6:07:41 user Maggie wrote:   [reply @ Maggie]
    • Very useful! really furthered my understanding!

  • At 27 October 2011, 17:37:27 user Raz wrote:   [reply @ Raz]
    • Very nice and balanced descriiption. :)

  • At 4 October 2011, 12:10:45 user Jeswanth wrote:   [reply @ Jeswanth]
    • Very nice description...I liked it ....and also easily understandable..
      Thank u...

  • At 1 October 2011, 15:19:42 user pallavi wrote:   [reply @ pallavi]
    • i want lab notes on ic 741

  • At 26 September 2011, 16:28:47 user Ashish wrote:   [reply @ Ashish]
    • ver good

  • At 5 September 2011, 17:55:05 user jeves wrote:   [reply @ jeves]
    • Nice job, especially in the font of the equations, that copy to text adequately.

  • At 5 August 2011, 23:44:28 user Gaurav wrote:   [reply @ Gaurav]
    • i am find for exact meaning of thershold which i find here thanx for ur such a keen representation

  • At 13 July 2011, 22:13:30 user Kerim wrote:   [reply @ Kerim]
    • Thank you for the well presented article.
      I was interested to find out the formulas of Vhi and Vlo when using npn transitors (I became lazy lately to derive again a formula I forgot).
      I noticed that Rout was dropped in the VTHRESHOLD_HIGH formula. As you know, lowering the value of Rout lets the voltage on R3 be higher hence Vhi increases as well (and vice versa). What do you think?

      Thank you.

      Sorry, your formula is right if Q2 is not saturated. So perhaps, but I am not sure, one may add another VTHRESHOLD_HIGH formula in case Q2 is saturated though it is no a desirable state for high speed signals.

  • At 17 June 2011, 12:44:26 user admirer wrote:   [reply @ admirer]
    • it has been truly of great help!!! thanks a lot :)

  • At 7 April 2011, 7:38:53 user Bruno BR wrote:   [reply @ Bruno BR]
    • PERFEITO 5 *****
      me ajudou bastante ! :D

      ( Perfect 5 *****

  • At 30 March 2011, 15:36:59 user Kammenos wrote:   [reply @ Kammenos]
    • @slv better make a window comparator. much easier to setup.

  • At 30 March 2011, 15:21:41 user slv wrote:   [reply @ slv]
    • The circuit to control car accumulator voltage. The circuit on at 12.2V and off at 14.8V. There is the transistor TS circuit, but hysteresis too narrow on it, I need wider.

  • At 30 March 2011, 15:10:03 user Kammenos wrote:   [reply @ Kammenos]
    • @slv i think that you need a window comparator and not a schmitt trigger. Are you sure you want an ST? What is the application?

  • At 30 March 2011, 15:06:37 user slv wrote:   [reply @ slv]
    • I mean the trigger ON at 12.2V and OFF at 14.8V. This is the inverting trigger right?

  • At 30 March 2011, 15:01:23 user slv wrote:   [reply @ slv]
    • Non-symmetrical Schmitt Trigger using OP-Amp with single power supply.
      I have Low Threshold Level 12.2V and High Threshold Level 14.8V. How to calculate resistors in this case? Thank you.

  • At 16 March 2011, 17:21:55 user Sreejith wrote:   [reply @ Sreejith]
    • Thanks Sir........
      With your help I submitted my assignment neatly


  • At 2 February 2011, 5:37:25 user vijay wrote:   [reply @ vijay]
    • its very easy to understand...thanx

  • At 1 February 2011, 2:28:25 user vijayan wrote:   [reply @ vijayan]
    • it is help full for me,thank you

  • At 26 January 2011, 19:12:57 user dpk malviya wrote:   [reply @ dpk malviya]
    • very gud..

  • At 25 January 2011, 10:33:10 user amateur wrote:   [reply @ amateur]
    • Very good tutorial! Many thanks!

  • At 8 December 2010, 19:33:11 user Kammenos wrote:   [reply @ Kammenos]
    • sc_chan what do you mean?

  • At 7 December 2010, 1:25:32 user sc_chan wrote:   [reply @ sc_chan]
    • The formula of VTHRESHOLD_HIGH & VTHRESHOLD_LOW may be exchange.

  • At 6 December 2010, 13:31:43 user SAA wrote:   [reply @ SAA]
    • Clear explanation.. Thank you so much.. :)

  • At 23 November 2010, 8:51:12 user varun jain wrote:   [reply @ varun jain]
    • this metter is very useful in my project.

  • At 22 October 2010, 19:37:58 user Kammenos wrote:   [reply @ Kammenos]
    • If you make a single-power supply circuit, then requires only positive and ground.

  • At 22 October 2010, 19:28:17 user Mark wrote:   [reply @ Mark]
    • Does the schmitt trigger require a Pos and Neg voltage or can I use it with a Pos and ground?

  • At 21 October 2010, 11:01:32 user technoholic wrote:   [reply @ technoholic]
    • very good explanation. helped me a lot!!

  • At 20 October 2010, 18:38:33 user mrudu wrote:   [reply @ mrudu]
    • nice!!!

  • At 9 August 2010, 8:26:07 user vinzz wrote:   [reply @ vinzz]
    • perfect!

  • At 8 August 2010, 13:16:19 user lipu wrote:   [reply @ lipu]
    • thanks for the graphical representation.it cleared the doubts

  • At 5 July 2010, 5:38:08 user RONY CHAKRABORTY wrote:   [reply @ RONY CHAKRABORTY]
    • great article. I have cleared my miss conception. Many many thanks for this.
      thank you PcbHeaven

  • At 18 May 2010, 13:57:51 user andreea wrote:   [reply @ andreea]
    • how can i simulate a Schmitt Trigger in Pspice?

  • At 21 April 2010, 17:33:04 user pratik wrote:   [reply @ pratik]
    • good reperesentation

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