Simple Discrete Circuit for Dual-Range Voltage Window Detection (0-1V, 4-5V)

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How can I detect whether an analog voltage is between 0–1 V or 4–5 V using as few discrete components as possible?

The practical answer is to use two comparators or op-amps as comparators rather than trying to build it from a few transistors and diodes. A simple two-op-amp comparator approach was suggested directly, and the LM139/LM239/LM339 family was recommended as a better fit if you want a dedicated comparator IC [#21684568][#21684581] Two transistors with resistor dividers can sort of do it, but the switching thresholds depend heavily on the individual transistor characteristics, so it is not very precise [#21684571] Using a reverse 3.3 V zener for the upper threshold and a forward diode for the lower threshold was also considered, but it was concluded that low-voltage zeners and series diodes do not have a sharp enough turn-on point for reliable thresholds [#21684572][#21684573][#21684574] The LM2900 quad Norton amplifier was specifically called out as needing special current-input resistor networks, so it is not a drop-in ordinary op-amp comparator solution [#21684579][#21684581]

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#1 21684566 09 Jun 2020 19:55

Graham Rounce Graham Rounce

Anonymous

Post #1
21684566 09 Jun 2020 19:55

Hi -
I'd like a very simple (because I'll be using lots of them) circuit that has a voltage (out of an op-amp) as an input, and two outputs, both normally low. One output goes high if the input is between 0v and 1v, and the other one goes high if the input is between 4v and 5v. (It won't matter if those highs and lows are reversed.)

I'd rather not use another op-amp, as I've just bought a load of one-in-a-package MCP601s and I'd have to kick myself if I really should have got two-in-a-package MCP602s.Is there an ingenious minimum of discrete components I can use to paper over my shortsightedness?
Thank-you!

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#2 21684567 09 Jun 2020 21:09

Giovanni Di Maria Giovanni Di Maria

Anonymous

Post #2
21684567 09 Jun 2020 21:09

Hi.I post here a pseudo-code with a MCU.
I'm thinking about the circuit with discrete compenets...........

while(TRUE) {if(PORB.0 >=0 && PORTB.0<=1) PORTB.1=1; PORTB.2=0;
if(PORB.0 >=4 && PORTB.0<=5)
PORTB.1=0;
PORTB.2=1;}

I'm thinking about the circuit with discrete compenets...........
RegardsGiovanmi Di Maria
#3 21684568 09 Jun 2020 21:31

Giovanni Di Maria Giovanni Di Maria

Anonymous

Post #3
21684568 09 Jun 2020 21:31

Ok.I have designed a circuit with two opamps as comparers.I have used opamp LT1001 but you can use your opamps.I send you in attachment the pictures of the schematic and the two plots at two ouputs.Best regardsGiovanni Di Maria
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#4 21684569 09 Jun 2020 21:34

Graham Rounce Graham Rounce

Anonymous

Post #4
21684569 09 Jun 2020 21:34

Thanks! I was going to MCU the whole thing but as it turned out it wouldn't save much hardware, and hardware is v cheap these days... Breath bated for the discrete ver
#5 21684570 09 Jun 2020 21:38

Giovanni Di Maria Giovanni Di Maria

Anonymous

Post #5
21684570 09 Jun 2020 21:38

Today young designers can only use MCUs :-)Giovanni
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#6 21684571 09 Jun 2020 23:08

Elizabeth Simon Elizabeth Simon

Anonymous

Post #6
21684571 09 Jun 2020 23:08

If you aren't too picky about the thresholds, you may be able to do this with two transistors. One P channel with source tied to the supply and one N channel with source tied to ground. Feed the gates with a resistor divider and take the outputs from the drains.The problem is that setting the thresholds is dependent on the particulars of each transistor.Much easier to use two op-amps as Giovanni did.
#7 21684572 10 Jun 2020 00:49

Graham Rounce Graham Rounce

Anonymous

Post #7
21684572 10 Jun 2020 00:49

Yes, that's kind of where my thoughts are heading, except using npn and pnp bipolars, the npn with a reverse 3.3v zener in series with the base, and the pnp with a forward silicon diode in series (effectively a 0.6v zener).With the 0.6 v across the base-emitters, I'm *hoping* for switchovers at 3.9v and 1.2v. Lots of resistors, unfort, but anyway...
I'll report back!Thanks,
#8 21684573 11 Jun 2020 04:09

Elizabeth Simon Elizabeth Simon

Anonymous

Post #8
21684573 11 Jun 2020 04:09

Sounds like a good approach.I think the base-emitter voltage on a BJT is a bit better defined than the gate threshold of a MOSFET.
Pay close attention to the zener diode specs. The breakdown voltage isn't always as well-defined as you might like at lower current levels. See my article on zener diodes for more information
https://www.eeweb.com/profile/elizabeth-simon/articles/how-to-read-data-sheets-zener-diodes
#9 21684574 11 Jun 2020 20:45

Graham Rounce Graham Rounce

Anonymous

Post #9
21684574 11 Jun 2020 20:45

Right, thanks. I now agree! Zeners under 5v and bunches of forward-diodes in series don't have a sharp turn-on point.. I've installed LTSpice, which also agrees. (As far as it's able, that is - there aren't any zeners below 4.7v with the one I installed.**)I've now ordered another load of op-amps, quad this time. Any offers for 60 x MCP601s?**Btw, Is there a library for LTSpice of "common components you've actually heard of"?? There's a zillion obscure god-knows-whats, but no zeners under 4.7v, no MCP601,2,3or4, no LM2900or3900... the non-list seems endless! Unless it's just me, of course.
#10 21684575 11 Jun 2020 22:01

Elizabeth Simon Elizabeth Simon

Anonymous

Post #10
21684575 11 Jun 2020 22:01

LTSpice will read standard (text based) spice models. Go to a manufacturer's web site for the component you are looking for and look for spice models. Most transistors, diodes and op-amps have models available. They are often on the product page but sometimes you have to look a bit harder.I've also had some luck with a Google search for "<component> model"
#11 21684576 11 Jun 2020 22:18

Graham Rounce Graham Rounce

Anonymous

Post #11
21684576 11 Jun 2020 22:18

I've found a model for the LM2900, which seems ok, not that I tested it to breaking point.It all seems a bit of a faffy procedure, though. You'd think by this time there'd be an option to search the internet for models and directly download them into the library. Still, it's easy to criticise. The whole thing is rather awesome.
#12 21684577 13 Jun 2020 02:39

PeterTraneus Anderson PeterTraneus Anderson

Anonymous

Post #12
21684577 13 Jun 2020 02:39

A resistor between base and emitter on a BJT will give higher diode current, giving more-stable diode characteristics.
#13 21684578 14 Jun 2020 02:03

Graham Rounce Graham Rounce

Anonymous

Post #13
21684578 14 Jun 2020 02:03

Thanks Peter, but I decided to go the easy way. Unfortunately...Here is the circuit so far.

The left half works, with point A resting at about 2.5v, and going up to 4.8v or down to 0.2v, depending on the polarity of a magnet presented to the Hall sensor.The right half also works, at least on LTSpice. If the voltage at point B is over about 4v one output goes high. If B is less than a volt, the other output goes high.But when A and B are joined, the voltage there droops, is reluctant to go high and doesn't go far low either.As the 2900 has 1M input resistance, I thought changing the resistors around the 602 to something lower, like the values in brackets, might help, but it doesn't.
I really hate having to ask for help every little step of the way, but....??Thank-you!
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#14 21684579 14 Jun 2020 06:11

PeterTraneus Anderson PeterTraneus Anderson

Anonymous

Post #14
21684579 14 Jun 2020 06:11

The LM2900 quad Norton amplifier has unique inputs. I recommend you read the datasheet and application notes very carefully, as the usual opamp circuits do not work with the LM2900, and can even damage the inputs. "Norton" indicates that each amplifier has differential CURRENT inputs, not the usual differential voltage inputs. Notice that the amplifier symbol shows a vertical arrow between the inputs on the triangle. The common-mode input voltage range is zilch, as the LM2900 internally sets the input voltage at a diode drop above the negative supply rail. The inputs like being driven through resistor networks, and hate being driven by voltage sources.
#15 21684580 14 Jun 2020 13:57

Graham Rounce Graham Rounce

Anonymous

Post #15
21684580 14 Jun 2020 13:57

Haha, thanks! Any offers, anyone, on 60 of them?PS - You live and learn, albeit painfully slowly in my case. (A pity my forgetfulness is still improving daily! :)The datasheet doesn't mention its current-centric operation until way down in the Applications section. I imagined "Norton" as in the motorcycle - powerful and fast, not an almost completely different kind of creature!Lol, sob.
#16 21684581 14 Jun 2020 23:46

PeterTraneus Anderson PeterTraneus Anderson

Anonymous

Post #16
21684581 14 Jun 2020 23:46

"Norton" comes from Norton's Theorem, where a current source is placed in parallel with a resistor, to do what Thevenin's Theorem does with a voltage source in series with a resistor.https://en.wikipedia.org/wiki/Thevenin_circuit has a comparison of the two approaches.www.ti.com/litv/pdf/snoa653 "AN-72 The LM3900: A New Current-Differencing Quad of Plus or Minus Input Amplifiers" describes how to use the LM2900 inputs. You can use LM2900's for your comparators if you rearrange the resistors and their values to suit the LM2900's inputs.Check the LM139 LM239 LM339 quad comparators, as they may be better suited to your circuit.National Semiconductor (now part of Texas Instruments) produced a lot of marvelous analog chips starting in the early 1970s. The LM2900 dates to 1972, 48 years ago!
#17 21684582 23 Jul 2020 15:49

rashmika mandanna rashmika mandanna

Anonymous

Post #17
21684582 23 Jul 2020 15:49

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Topic summary

✨ The discussion focuses on designing a simple discrete circuit for dual-range voltage window detection with two outputs indicating when an input voltage (from an op-amp) is within 0-1V or 4-5V ranges. The original poster prefers to avoid using additional op-amps due to prior purchases of single-package MCP601 devices. Suggestions include using two op-amps as comparators, as implemented with LT1001 and LT602 devices, or employing transistor-based solutions with PNP and NPN BJTs combined with zener and silicon diodes to set threshold voltages. However, transistor threshold variability and zener diode voltage instability at low currents complicate this approach. The LM2900 quad Norton amplifier was tested but found unsuitable without careful consideration of its current-differencing input stage and unique input characteristics, which differ from standard voltage-input op-amps. Users recommend consulting datasheets and application notes for LM2900 and considering LM139/LM239/LM339 quad comparators as better alternatives. Spice simulation challenges were noted due to limited availability of low-voltage zener and specific op-amp models like MCP601 in LTSpice libraries. Overall, the consensus favors using dual op-amp comparator circuits for reliable window detection over discrete transistor or unconventional amplifier solutions.
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FAQ

TL;DR: LM2900 dates to 1972; “Check the LM139 LM239 LM339 quad comparators.” Build the 0–1 V and 4–5 V detector with two standard comparators, not Norton amps. [Elektroda, Anonymous, post #21684581]

Why it matters: This FAQ helps designers quickly choose reliable, low-cost parts and avoid LM2900 input pitfalls for dual-range window detection.

Quick-Facts

Two op-amps/comparators can implement the 0–1 V and 4–5 V windows with minimal parts. [Elektroda, Giovanni Di Maria, post #21684568]
LM2900/LM3900 use Norton current-differencing inputs; avoid direct voltage-drive. [Elektroda, Anonymous, post #21684579]
Sub‑5 V Zeners have soft turn‑on at low current; thresholds drift. [Elektroda, Elizabeth Simon, post #21684573]
LTspice accepts vendor SPICE models; download from component makers. [Elektroda, Anonymous, post #21684575]
LM339 is a quad comparator that suits voltage window tasks well. [Elektroda, Anonymous, post #21684581]

Quick Facts

Two op-amps/comparators can implement the 0–1 V and 4–5 V windows with minimal parts. [Elektroda, Giovanni Di Maria, post #21684568]
LM2900/LM3900 use Norton current-differencing inputs; avoid direct voltage-drive. [Elektroda, Anonymous, post #21684579]
Sub‑5 V Zeners have soft turn‑on at low current; thresholds drift. [Elektroda, Elizabeth Simon, post #21684573]
LTspice accepts vendor SPICE models; download from component makers. [Elektroda, Anonymous, post #21684575]
LM339 is a quad comparator that suits voltage window tasks well. [Elektroda, Anonymous, post #21684581]

What’s the simplest way to detect 0–1 V and 4–5 V windows?

Use two comparators set as window detectors. One triggers between 0–1 V, the other between 4–5 V. This approach is stable and easy to scale. It beats transistor-only tricks for repeatable thresholds. “Much easier to use two op-amps.” [Elektroda, Anonymous, post #21684571]

Can I do it with just a couple of transistors or MOSFETs?

Yes, but thresholds vary with device parameters, temperature, and Vth spread. That makes calibration painful across many units. Designers in the thread recommended moving to comparator-based windows for consistency and fewer surprises. [Elektroda, Anonymous, post #21684571]

Why did my LM2900/LM3900 window comparator misbehave?

LM2900 is a Norton amplifier with current inputs. It sets input nodes near a diode drop above ground. Driving it from a low-impedance voltage source causes errors and may damage inputs. Use resistor networks or choose a voltage comparator. [Elektroda, Anonymous, post #21684579]

What does “Norton amplifier” actually mean here?

It follows Norton’s Theorem: current source behavior with resistive networks. The LM2900/LM3900 compare input currents rather than voltages. Designers suggested LM339-family comparators for voltage windows instead. [Elektroda, Anonymous, post #21684581]

Which comparator should I pick for dual windows?

Pick a quad comparator like LM339/LM239/LM139. You get four channels per package, so two handle your two windows, leaving two spares. “Check the LM139 LM239 LM339 quad comparators.” [Elektroda, Anonymous, post #21684581]

How do I build the two windows with standard comparators?

Set comparator A thresholds near 0 V and 1 V using a divider and reference.
Set comparator B thresholds near 4 V and 5 V similarly.
OR each window’s in/out logic to two outputs as needed. Two-op-amp examples were shared and simulated successfully. [Elektroda, Giovanni Di Maria, post #21684568]

Can I just program an MCU instead of adding comparators?

Yes. Sample the input and assert two digital outputs for the windows. The posted pseudo‑code shows the logic structure. Hardware may not reduce, but flexibility improves. [Elektroda, Giovanni Di Maria, post #21684567]

Are Zeners a good way to set 1.2 V and 3.9 V thresholds?

Not for precision at low current. Sub‑5 V Zeners and diode stacks have soft knees. Their breakdown varies with current and temperature. Expect drift and fuzzy switching without careful biasing. [Elektroda, Elizabeth Simon, post #21684573]

My LTspice result looked great, but the hardware is mushy—why?

A model set may lack accurate low‑voltage Zeners, and idealized diodes hide softness. Real parts show rounded knees and higher tolerance. The OP saw this and switched to quad op‑amps. [Elektroda, Anonymous, post #21684574]

How do I add real vendor parts to LTspice fast?

Download SPICE models from manufacturer pages and include them in LTspice. Many op‑amps, diodes, and transistors provide ready models. A quick “part + model” search helps. [Elektroda, Anonymous, post #21684575]

Any small tweak to stabilize a BJT-as-diode threshold?

Add a resistor from base to emitter. It raises diode current and sharpens the effective knee, improving repeatability slightly. This still trails a comparator for accuracy. [Elektroda, Anonymous, post #21684577]

Is there a risk if I connect a voltage source straight to LM2900 inputs?

Yes. The inputs prefer current drive through resistors. Direct voltage drive violates its input structure and can damage inputs. Route through resistive networks or choose LM339 instead. [Elektroda, Anonymous, post #21684579]

What did the original designer finally choose?

They moved away from Zeners and Norton amps, and ordered quad op‑amps instead. Simulations and experience led to the simpler, more reliable comparator path. [Elektroda, Anonymous, post #21684574]

Does this approach scale for many channels on a board?

Yes. A quad comparator gives four channels per IC, reducing cost and space. That’s 4 inputs per package, useful when duplicating windows across sensors. [Elektroda, Anonymous, post #21684581]

What if my signal source is a Hall sensor like in the thread?

Buffer or condition the Hall output, then feed the comparators. The OP’s node toggled ~0.2–4.8 V with magnet polarity, suiting window detection well. [Elektroda, Anonymous, post #21684578]