Can you help design this circuit .....

Sounds like a homework problem

When you are done designing this circuit, you'll find that it isusually NOT a good idea to use the same up and down threshold.

Instead you'll want to add a dead band with slight to moderate hysteresis. This can be done by feedback from the comparator level to raise the input threshold on an upevent and lower it on a downevent.

--
Many thanks,

Don Lancaster
Synergetics   3860 West First Street  Box 809  Thatcher, AZ 85552
voice: (928)428-4073 email: don@tinaja.com

Please visit my GURU's LAIR web site at http://www.tinaja.com

Dear All, Does anyone have an idea for a low voltage threshold circuit that switches from its Low (Off) to High (On) state (or vice versa) upon an input voltage reaching a fixed predetermined low voltage level of .... say 0.658 volts? The design must disregard all input voltage variations (DC and/or AC) until the threshold ( or higher ) voltage of

0.658 volts is reached and at this point, switch On and remain On until the input voltage drops below 0.658 volts. The output will be resistive and switch between zero volt and the supply voltage ( about 4.5 volts to 12 volts ) I think one could call the circuit a "low voltage threshold switch". The simpler the design and lower the component count, the better. Ideas ..... anyone? ............. :-)

Trying to tell someone about hysterisis when they don't know about comparators is somewhat counter-productive.

Until we know what Racer is actually trying to do, more detailed advice is probably wasted - and in any event if Racer has a real talent for electronics, that sort of information could be mined from the LM193 datasheet, whose URL I posted.

The next time you want tell somebody about hysterisis, think about being more explicit in describing how the (positive) feedback is actually done - a phrase like "a high value resistor between the comparator output and the non-inverting input" might not go amiss.

------------ Bill Sloman, Nijmegen

LM393

0.685V

collector

pin

us a

more

been

isusually

hysteresis.

Hi, Racer. Let's recap the bidding. You want to switch a digital output (logic 1, 0) at around 0.658V analog input voltage. It's been recommended that you use a voltage reference and a pot to get your voltage reference, and that you use a comparator with hysteresis to avoid "chatter" or multiple logic transistions as your signal passes through the reference voltage. You now want to have something inexpensive which will light one of two LEDs depending on the input voltage. How does "free" sound?

The LM393 is a dual comparator, and you can use the other comparator to drive the other LED. This might be what you need (view in fixed font or M$ Notepad):

VCC VCC VCC + + + | | | 4.7K| Red | Green| .-. V ~ V ~ | | V(in) ___ - ~ - ~ | | o-|___|-o---. 1/2 LM393 | | '-' 10K | | | | | --- | VCC .-. .-. | .01uF--- | + | |1K 1K| | o---------. | | | | | | | | | === | |\| '-' '-' | 10K| GND '-----|-\ | | LM385| .-. ___ | >---o | /-/ NC | |

Chris Carlen has suggested a simple way of dealing with the LEDs.

I usually respond to direct e-mails - my e-mail address is real. I don't know what Don Lancaster does.

--------- Bill Sloman, Nijmegen

You're welcome. Business is slow in these parts, so I'll sometimes have a chance to answer in a little depth.

I like the "dummies" books, and sometimes recommend them to customers who aren't familiar with Excel or something, but newbies aren't "dummies". We were all there once.

If you want a good introduction to electronics, just Google sci.electronics.basics or sci.electronics.design for a number of threads on books and magazines for newbies. In particular, I would recommend the books and magazine articles by the author of one of the responses, Don Lancaster. In addition to writing the CMOS, TTL and RTL cookbooks, he's been a feature writer for several electronics magazines over the years. I remember buying the CMOS cookbook when it first came out. Answering your post, I was thinking how cool it would have been back in the days when "Kansas City Standard" 300 baud modems ruled the earth to actually have Mr. Lancaster pop up to answer a newsgroup post on the "internets" back then. The amazing thing is that 95% of what's in the CMOS Cookbook is still relevant, and you can still learn a lot about electronics from it.

Most everything Mr. Lancaster has written is available from his website, either free or for purchase. I still keep a copy of his Active Filter Cookbook in a desk drawer.

Browse around -- this is one of the best electronics newbie sites out there.

Good luck Chris

--
A few thoughts:

Assuming a common pot with a rotation of 270° and 1.23V across it, its
sensitivity would be about 4.56mV/°, so it might be difficult to set
it precisely to 0.685V, that LSD corresponding to roughly 1°.

Also, with 1 megohm of feedback the hysteresis will be about 150mV,
not 15mV.

An LM393 has a saturation voltage of 700mV max with only 4mA of
collector current, so with 10 mA of collector current in the red
comparator, it's conceivable that the saturation voltage could be
higher than the reference, making it impossible for the green LED to
ever turn off.

Also, I'm not a big fan of putting hysteresis on the reference, so
here's how I'd do it:

news:prcp115sbgjnp0385l8n9d35i8t1d9vn05@4ax.com

Yes, thnx Don .... the hysteresis band will be set to...... say

10 - 15 mV.

To Bill Sloman..... thanks for your pointers and links.

Now, on to the next question ........ ;-p What simple circuit would you consider using to interface the output of the comparator to switch one Led or the other depending on whether the output was Low or High. At no point should the two Leds be lit even at low luminance? It would have to be one or the other. A low powered mechanical relay would do the trick but what about some simple semiconductor arrangement. Again, low component count and cost is a factor.

Thanks again Don and Bill for you for your responses.

Would an e-mail to consult with you personally be replied?

_____________________________________________

its

Mr. Fields is correct on all counts. I'd assume you'd want a 10-turn pot to tweak your voltage. The resistor ratio for the hysteresis was obiously incorrect -- modified below. Also, the output open collector transistor of the LM393 is weak, and tends to have a high saturation voltage (logic low level) when it's on. You can remedy that by using transistors to drive the LEDs like this (view in fixed font or M$ Notepad):

` VCC VCC ` VCC + + ` + | | ` | .-. .-. ` 4.7K| | | | | ` .-. | | | | ` | | V(in) ___ VCC '-' '-' ` | | o-|___|-o---. + | | ` '-' 10K | | | Red| Green| ` | --- | VCC .-. V ~ V~ ` | .01uF--- | + | |10K - ~ -~ ` o---------. | | | | | | | ` | | === | |\| '-' | | ` | 1K| GND '-----|-\ | ___ |/ VCC | ` LM385| .-. ___ | >--o-|___|-|2N3904 + | ` /-/ NC | | | | ` ^ ---o | | 2.2K | |/| | | .-. | ` | '-' | === | === 10K| | | ` | | | GND | GND | | 2N3904| ` === | | ___ | |\ '-' | ` GND === o----|___|-o-------------|-\ | ___ |/ ` GND | 2.2M | >-o-|___|-| ` | .-------------|+/ 10K |>

` | | |/ | ` '----------' === ` GND `created by Andy´s ASCII-Circuit v1.24.140803 Beta

This is just for reference. Thanks for the spot, Mr. Fields.

Chris

That figures. He's well known in the US.

--
Beware of those who suffer from delusions of adequacy!

Michael A. Terrell
Central Florida

---

+V----+------+----------------+----+---+----+ | | | | | | [R] | [R] | | [R] | [R] +--[R]---+ | [R] | | | | | | [LED] | | [LED] [POT]--+---+-------|-\ | | +------|-/ | | >--+ [R] | +--|+/ | |K | | | [REF] [R] | | | | | GND>--+------+---------------------+---+

-- John Fields

Hello Chris, thank you for responding.

Yes

True again.....

Spot On !

Huh? ..... i lost you there !? ...... :P

I really don't know how to thank you for taking so much time and trouble to explain ..... Yours is an elegant solution if it works - It meets all the requirements of simple circuitry, low component count, low cost and not requiring a +/- VCC supply...... in a word, ...... perfect !! Perhaps you shoul;d consider writing a book for dummies like myself ! ........ :-)

Thanks so much Chris .......

/Racer

Umm.... perhaps it is! ....... :-)

So why don't _you_ go ahead and design a UV laser? ...... jus joking ...... hehehe

Hello Chris, thank you for responding.

Yes

True again.....

Spot On !

Huh? ..... i lost you there !? ...... :P

I really don't know how to thank you for taking so much time and trouble to explain ..... Yours is an elegant solution if it works - It meets all the requirements of simple circuitry, low component count, low cost and not requiring a +/- VCC supply...... in a word, ...... perfect !! Perhaps you shoul;d consider writing a book for dummies like myself ! ........ :-)

Thanks so much Chris .......

/Racer

Mind (re)posting the article here or e-mailing it to me? thnx! /Racer

This is a prime example of text-chopping - what I was saying was that I don't know if Don Lancaster answers private e-mails.

Since I was a foundation subscriber to Byte (thanks to Dan Fylstra who got interested in my wife - some years before we finally married - when she was doing at post-doc at MIT and Dan was involved in setting up Byte) I've known about Don Lancaster for more than 25 years. His Gurugrams might suggest that he'd see a private e-mail as an opportunity to get some consulting, but his books are popular enough that he might get so many private e-mails that this wouldn't be practical.

You seem to be suffering from such a delusion.

----------- Bill Sloman, Nijmegen

It sounds like the LM555 might do the job for you. It can be hooked up with a couple of resistors and a cap to oscillate at around 600Hz, and it can easily drive an 0.2 watt speaker.

The control would be through the reset pin (pin 4). When it's low, it don't go. So you can connect the 555 pin 4 to the collector of the transistor that's driving your _RED_ LED, and then do something like this (view in fixed font or M$ Notepad):

` From Transistor Collector ` >---------------------. ` | ` VCC VCC | ` + + | ` | | | ` .-. | | ` | | | | ` 4.7K| | | | ` '-' .---o----o-----. ` | | 8 4 | ` | | Reset | ` o-----o7 | ` | | | 22uF ` .-. | | +|| ___ ` | | | 3o---||--|___|--. ` 10K| | | LM555 | || 270 ohm| ` '-' | | | ` | .--o6 | .-. __ /| ` | | | | 1K| |

that I

know

Here is the text in context.

"> Would an e-mail to consult with you personally be replied?

Chris Carlen has suggested a simple way of dealing with the LEDs.

I usually respond to direct e-mails - my e-mail address is real. I don't know what Don Lancaster does. "

My wife (who is an expert on language - which is what she was doing at MIT) tells me that I should have written "I don't know whether Don Lancaster does" if I'd wanted to avoid wilful misundertandings, but this imposes a binary choice on a what is really a rather wide range of possibilities.

------- Bill Sloman, Nijmegen