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ZL1ANM > DESIGN 25.03.06 15:38l 50 Lines 1878 Bytes #999 (0) @ WW
BID : ZL1ANM135
Read: GUEST
Subj: Re: Logic gate IC question
Path: DB0FHN<DB0FOR<DB0SIF<DB0EA<DB0ACC<DB0PKE<DB0RES<ON0AR<ZL2BAU<ZL1AB
Sent: 060325/0850Z @:ZL1AB.#11.NZL.OC #:38394 [AUCKLAND] FBB7.00i $:ZL1ANM135
From: ZL1ANM@ZL1AB.#11.NZL.OC
To : DESIGN@WW
>I have a question about using commonly available logic IC's.
>
>What I need to know is what voltage window can they reliably be used at ?
>
>Example:
>
>I require an input to work at a certain voltage (let's pretend it's 5v).
>
>At 4.9v I need it to be off - but will a comon logic IC be that accurate,
>or will I have to accept a bit of a grey are, if so what sort of window
>of operation should I expect ?
>
>I've never had to design anything using logic before in the real world,
>except only in theory.
>
>- Analogue Andy -
>
Use an op-amp in a comparator configuration. Even the humble '741 will
do this for you, but its certainly not the best choice. The LM-358 is
pretty good for non-critical situations. How to make a comparator:
Let's assume your requirement is that when the input rises above +4.9
volts, the comparator's output will switch abruptly from low (0V) to
high (near the op-amp supply voltage, example 5-15 volts).
Bias the -ve op-amp terminal to +4.9V by using a resistive divider.
Regulate the supply to the divider if you need superior accuracy and/or
stability. Connect the +ve op-amp terminal to the output of your circuit.
When the latter's output exceeds +4.9V by a millivolt or so, the gain of
the op-amp is so great (15,000) that its output will abruptly swing high,
and can be used to drive some other device such as a LED or buzzer, via
a normal transistor if necessary. Use good decoupling, on account of the
high gain of the op-amp.
Most op-amps work well in this application but have relatively low input
impedance (couple of thousand ohms). If you require minimal loading, use
an FET op-amp, such as the TL-071. They have input impedances of 10 Meg
or more, and circuit configuration is otherwise identical.
73 de Neil ZL1ANM
NBF 2.08
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