On Thu, May 23, 2002 at 12:07:02AM -0400, Rick C. wrote:
> Let me try to give you a straight answer. All that is needed is an op amp,

Yes.

> voltage reference,

Yes.

> and a voltage divider.

And yes.

> A piezo buzzer can be used as the alarm.
>
> Use an LM336-2.5 as a reference to the + input to the op amp.
> It is powered from
> the +12 volt battery source.

The only discussion here is the choice of part. Common parts are almost
always better. A 7805 (or even better a 78L05 if handy) could do the
job as well. No need for a specialized precision part.

> Use any op amp. LM741, LM1458, etc. Plus to +12 volt battery source, - to
> ground.

> Piezo buzzer from op amp output to +12 volt battery source.

Yup.

> A voltage divider from the +12 volt battery source to ground. The divider made
> up of two resistors and a 1K0 trimpot.

Exactly.

> +12 volt battery source through 8K2 resistor to end of pot. Other end of pot
> through 2K2 resistor to ground. Wiper of pot to negative input of op amp.
> Set trimpot to threshold where you want the alarm to sound.
> If there is too much hysteresis, place a feedback resistor from the op amp
> output to the - input if the op amp. Value should be around M10.
> Total current draw will be about 5 mils when off. A CMOS op amp will reduce this
> significantly.
>
> I'm just doing this from memory so you might have to fudge the values slightly
> but in theory, it will work.

Excellent work. I finally all made sense to me when Roman explained that the
control voltage (2.5V in your case, 5V in mine) doesn't have to be in the
range of the voltage to be measured.

BAJ

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