Can you help design this circuit .....

What fun. If you aren't too picky about the voltage it turns on, you can use a silly circuit like this:

It turns on when the input is near Vbe of Q2, which is generally between

0.65 and 0.7V.

The schmitt trigger gives you a (tiny) bit of hysteresis. The oscillator is inhibited until the input circuit turns on. The power stage gives you a reasonably loud 600Hz whine. Use a 100 ohm rheostat in series with the speaker to limit the output. A bigger cap than 10uF will allow more of the power to come through. If you us an electrolytic, put the - side towards the speaker.

The LEDs are turned on and off by complimentary transistors, tied to the output of the schmitt trigger.

You can make it more accurate w.r.t. input by using a differential amp on the input, along with a reference. However, you may not care.

--
Regards,
   Robert Monsen

"Your Highness, I have no need of this hypothesis."
     - Pierre Laplace (1749-1827), to Napoleon,
        on why his works on celestial mechanics make no mention of God.

You're not going to have enough bias current for the '385 at Vdd=5V when the 1K reference pot consumes 1.24mA. And the LED currents range over ~(12-2)/(5-2) or > 3:1 ratio with Vdd. So some adjustments will have to made for Vdd to operate over the specified 5-12V range. Maybe you could re-arrange the reference ckt to produce both a Vthreshold for the comparison and a current source for the LEDs like so ( simplified): View in a fixed-width font such as Courier.

Thanks John ..... :-)

At the risk of becoming a nuisance to you Electronic Pros, I pose yet another so called "simple" question..... From what point in the circuit and how would you provide the operating power to drive a basic fixed frequency ( approx. 600 Hz ) oscillator that drives a (very) small

0.2 watt speaker (and includes a user volume control) such that this oscillator comes ON in tandem with the Green LED above? It wouldn't matter if the negative rail of this oscillator was and output stage was above the "GND" potential. Again, low component count and cost. would be an important consideration.

TIA

My point is, this is a worldwide forum where people have different backgrounds, languages and abilities. I try to make things clear when I post or write a technical article. Its very easy to be to close to what you write. It makes perfect sense to you but not to someone casually reading it. No need to get all bent out of shape. I am starting to loose my eyesight and it is getting easier to misread something, or take it the wrong way because of the wording.

I notice you snipped the part about BYTE. I had three copies of issue number one, and two were still in the mailing wrappers. I gave the extras away and filled in a few missing holes in my collection. I liked the magazine when it first came out. A lot of surplus hardware hacking and some basic digital designs to study. I have see post from some of the authors on the newsgroups over the years. Later on it was mostly ads for different brands of computers disguised as reviews.

--
Beware of those who post from srvinet.com!

Michael A. Terrell
Central Florida

can

And you know what -- your circuit here is probably closer to what the OP actually needs, too.

Thanks Chris

Thank you Chris ......

/Racer

Hi Fred ! Wow! .... The circuit is becoming quite complex with each improvement....... :-p Perhaps I could use a Siemens "TCA105" 6 pin dil IC to acheive the same result ..... no? Thanks, /Racer

I don't see why not. Where is that input coming from and how critical is that 0.658V threshold? Is it from a temperature sensing thermistor and what is the impedance of the circuit in which the signal originates?

--
Here's something that ought to do everything you asked for, plus
taking care of the variation in the supply voltage.  The only problem
might be that the hysteresis is asymmetrical.  About 15mV when the
input goes higher than 0.658V and about 5mV when it goes lower than
0.658V.  The speed-up capacitor, C2, will help, though.

news:m58121hqp6lq6e4qgv95u0rpe07r3kmd2d@4ax.com

--
Yes, thank you.

Personal pages, courtesy comcast.

-- Regards, Robert Monsen

"Your Highness, I have no need of this hypothesis." - Pierre Laplace (1749-1827), to Napoleon, on why his works on celestial mechanics make no mention of God.

That makes no sense.

--
Pull-up and symmetry fix here:

news:deq121t2u5b754tctmk3f8ov8dm4n4vh4g@4ax.com

You need to use R10 to pull-up to 3.3V and run the OC output direct to the base.

This has nothing to do with your command of English- it has to do a simple alarm that goes off one way or the other at a 0.66V threhold and somehow is supposed to be general purpose and a bunch of blah-blah stuff about manufacturing tolerance. None of this makes any sense, general purpose at fixed threshold and hysteresis is oxymoronic.

...... snip .....

Actually, the circuit is supposed to be used as a standard independent unit to which could be interfaced various kinds of output from a wide range of sensors for temperature, RPM, light intensity, level, voltage, current, power, etc etc .... I think you get the picture. The threshold of 0.658V ( at very low current as the trigger to minimise loading and ensure the voltage was slightly above the forward voltage of most transistors ) was provided so this unit, when finally fabricated, would be able to function even when tight tolerances are required. tq =====================================================

Dear All

I don't know who here sent me the e-mail advising that I take a look at the Siemens " TCA105 " as a low cost solution (but minus the audio stage ) - Whoever it was, I'd like to say a BIG THANK YOU!!

I have tried to source the data and application sheet but managed to get just one ...... hope the chip has not been discontinued.

Feedback on your views or advice about links on the use of this Siemens" TCA105" IC would be appreciated.

Thank you everyone who contributed with their expertise.

/Racer

...... snip .....

Actually, the circuit is supposed to be used as a standard independent unit to which could be interfaced various kinds of output from a wide range of sensors for temperature, RPM, light intensity, level, voltage, current, power, etc etc .... I think you get the picture. The threshold of 0.658V ( at very low current as the trigger to minimise loading and ensure the voltage was slightly above the forward voltage of most transistors ) was provided so this unit, when finally fabricated, would be able to function even when tight tolerances are required. tq =====================================================

Dear All

I don't know who here sent me the e-mail advising that I take a look at the Siemens " TCA105 " as a low cost solution (but minus the audio stage ) - Whoever it was, I'd like to say a BIG THANK YOU!!

I have tried to source the data and application sheet but managed to get just one ...... hope the chip has not been discontinued.

Feedback on your views or advice about links on the use of this Siemens" TCA105" IC would be appreciated.

Thank you everyone who contributed with their expertise.

/Racer

Thank you Robert ! ..... :-) Is the above link your web site?

/Racer

Well Fred, with my poor command of English and an even poorer appreciation of electronics, that's the best I can do ....... :-)

/Racer