I am new to this. I want to make a circuit to measure the voltage from (0 ~12 V), which can have 2 op-amps as comparator to give the output positive, when the voltage goes from 8 V to 12 V and give output negative when the voltage goes from 0 V to 4 V. The reference voltage should be 6 V (+/-2 V). Please suggest me the basic circuit using 2 op-amps.

  • Input voltage: 0 to 12 V
  • Reference voltage is 6 V
  • Higher limit : 8 V
  • Lower limit : 4 V

Means, when I increase the input voltage from 6 V, it should not show any change until it crosses 8 V and when its more than 8 V, it should show HIGH VOLTAGE. Similarly, like when i down the input voltage from 6 V, it should not show any change in output until it cross the 4 V and when its lower than 4 V. it should show LOW VOLTAGE.


Table 1. Function table for OP to edit.

V_IN        V_OUT
----------- ----------
0 - 4 V     Negative
4 - 8 V     0 V
8 - 12 V    Positive

Please Refer This Cicuit

Please Refer this Circuit and suggest me the corrections, if any. Thanks

  • 1
    \$\begingroup\$ Your deleted schematic shows an AC input. Is this correct? If so, does "4 volts" mean mean pk-pk, RMS, or average rectified? Or something else? And how much accuracy do you need? How many power supplies are you allowed for the op amps? You show a single 12 supply. Is +/- 12 allowed? What accuracy is required? \$\endgroup\$ – WhatRoughBeast Aug 9 '18 at 11:25
  • \$\begingroup\$ Hi, I just added the circuit. please check that. \$\endgroup\$ – Emlinux Aug 10 '18 at 6:01
  • \$\begingroup\$ @Emlinux: I've fixed the formatting in your question. Note 'V' for volt. (Capitals matter.) You can often save a lot of words with a table to define operation. I've added one for you with my understanding of your requirements. Please review and edit to fix any errors. You seem to be requiring a negative voltage output but your schematic shows a positive-only supply. Where is the negative voltage supposed to come from? \$\endgroup\$ – Transistor Aug 10 '18 at 7:39
  • \$\begingroup\$ My comment copied from your deleted "answer": Note that you could make your schematic a lot more readable by using ground symbols close to each grounded point, eliminate all the ground wiring and the same with the V+ connections. Try and eliminate kinks in wiring such as above R4 and the loop under D1. \$\endgroup\$ – Transistor Aug 10 '18 at 16:34
  • \$\begingroup\$ Your circuit runs off a single +12 supply. How do you expect to get a negative output (per your table)? \$\endgroup\$ – WhatRoughBeast Aug 11 '18 at 1:22

Take a look at the following document:


Also, I am not quite sure, why you need two opamps to do the job. Why not just use one comparator?

  • \$\begingroup\$ Thanks Saqib. I am attaching here with my design, please refer it let me know your suggestions please. Thanks. \$\endgroup\$ – Emlinux Aug 9 '18 at 3:21
  • \$\begingroup\$ I don't see any attachment. \$\endgroup\$ – Saqib Shah Aug 9 '18 at 3:25

What you talking about is a Schmitt trigger

You may just pick one as an ic or turn an op amp into Schmitt trigger using positive feedback and then scale input using voltage divider


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