I have a requirement to change the transistor/MOSFET switch state at two different voltages.

Below is a small graphical representation, 3.3 V (red) to 4.5 V (green).

enter image description here

The switch should be open when the voltage is between red line and green.

Once the voltage reaches green the switch should be closed as long as it has not reached the red line level. Once the voltage had reached the red line the switch should immediately open and should not close unless it has gone back to the green level.

I was thinking of a Schmitt trigger with a transistor but could not get it. Voltage can vary between 3.3 V (red) and 4.5 V (green).


I managed to get what is needed with the following circuit, it is powered 4 V power supply.

Following is the schematic:

enter image description here

I could not get the upper and lower threshold voltages. Currently I am getting 1.5 V to 1.8 V as threshold.

Any suggestion for change in resistor values to shift the threshold to desired value between 3.3v and 3.9v (upper)?

  • \$\begingroup\$ What do you mean by "could not get it"? \$\endgroup\$
    – Wesley Lee
    Commented Sep 13, 2017 at 5:56
  • \$\begingroup\$ If you want to do a Schmitt trigger with transistors only, it's the same circuit as that of a 2-Transistor flip-flop. The magic is in a shared emitter resistor. \$\endgroup\$
    – Janka
    Commented Sep 13, 2017 at 9:53
  • \$\begingroup\$ With transistor I am not getting a sharp edges. Connected led to test but the led goes dim to bright not completely off. \$\endgroup\$ Commented Sep 13, 2017 at 12:25

2 Answers 2


The usual "jellybean" solution to these kinds of questions uses a comparator configured with positive feedback, to form a schmitt trigger with very clearly defined switching thresholds:


simulate this circuit – Schematic created using CircuitLab

That will produce a response something like:

enter image description here

Calculation of the resistances needed to achieve your specific thresholds is not difficult, but also not trivial. The values I show here are very close to your 3.3V and 4.5V thresholds.

Given loose tolerances and restricted selection of E12 (or even E24) resistor values, you'll probably require some way of fine tuning the thresholds. The gap between lower and upper threshold potentials (hysteresis) is controlled by the ratio of R3 and R5. The potential about which hysteresis is centered is controlled by the ratio of R2 and R1.

You may use potentiometers to vary these ratios:


simulate this circuit


First let's fix your diagram.

enter image description here

Figure 1. Required schmitt trigger operation. Output turns high when green level exceeded. Output turns low when less than red level.

There are many Schmitt trigger calculators available online such as one by Random Science Tools. Note that there are inverting and non-inverting versions. The inverting

  • \$\begingroup\$ Have something thing for transistor, common comparator chips need a vcc minimum 5v \$\endgroup\$ Commented Sep 13, 2017 at 13:31
  • \$\begingroup\$ I am afraid that is not a proper English sentence so I do not understand your comment. If English is not your first language then please add this to your profile so we can make allowances. Please try to make your comment more clearly. \$\endgroup\$
    – Transistor
    Commented Sep 13, 2017 at 14:05
  • \$\begingroup\$ sorry for that. I like the link you suggested, which shows resistor value selection for comparator IC. Most IC has Vcc > 5v. My Vcc is limited to 4 v or lesser. I am using transistor for the design. CMOS comparator are quite expensive also not easily available at my place. I manage to design the requires circuit today but the threshold voltage is between 1.5v to 1.8v. How do i shift the threshold volts higher. i have used transistor schmitt trigger circuit. \$\endgroup\$ Commented Sep 14, 2017 at 3:18
  • \$\begingroup\$ Your schematic looks as though it came from JohnHearfield.com. If you read the article he shows calculations and additional stages. Note that you are missing capital letters on many of your sentences in your original question and comments and then add them randomly on words such as "Green". Also it's 'V' for volt, etc. All of this makes it difficult to read. \$\endgroup\$
    – Transistor
    Commented Sep 14, 2017 at 11:13
  • \$\begingroup\$ English is not my first language sorry for that. \$\endgroup\$ Commented Sep 14, 2017 at 14:24

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