[Sycraft]

I have a question for a friend working on a project for school involving an alcohol sensor, which provides a 0 to 18mV voltage difference between two output wires (varies with alcohol). Is it possible to increase the voltage between these 2 wires with a simple op-amp design? He would like the amplified range to have a gain resulting in a 0 to 5 or 3V range. The 0 to 18mV voltage difference is accross the two wires, and does not involve being tied to ground, so He is not quite sure how this would interface with a standard op-amp. This sounds like a simple op-amp design problem. Thanks for any info, advice, or links. Thanks!

 

[AndrewB]

Check the data sheets at National Semiconductors site.

I'm thinking LM-series comparators

 

[amb]

An opamp will work fine. In fact you could power your opamp with a single supply (as opposed to split supplies) and connect one of the sensor's leads to "ground". Set the gain appropriately and you'll have the output voltage range you want.

 

[rickcr42]

Even simpler :

A single stage transistor or jfet amplifier.Any basic electronics page will have the diagram and formula and the whole thing could be built for about a buck


Opamps/comparators are way overkill if all you need to do is up a non critical voltage.I.C.s are meant for wide bandwidth circuits and that is not only not required here but would add complexity also not neccesary

 

[amb]

Quote:

Originally Posted by rickcr42 Even simpler : A single stage transistor or jfet amplifier.Any basic electronics page will have the diagram and formula and the whole thing could be built for about a buck Opamps/comparators are way overkill if all you need to do is up a non critical voltage.I.C.s are meant for wide bandwidth circuits and that is not only not required here but would add complexity also not neccesary

While that's theoretically true, in terms of complexity and cost I think it's a wash in this case. You could build a transistor or jfet circuit with a couple of biasing/feedback resistors and get an amp for cheap, but then a 741-type opamp and two feedback resistors is all you need in the opamp solution as well, and that could also be under a buck. There is not much difference in space requirement either. Sure, the 741 IC has a whole bunch of transistors inside, but who cares. In terms of cost and real parts count they are about the same.

 

[rickcr42]

you serious man ? you really would use a part with eight to fourteen pins to solder and touchy layout to avoid oscilations which are always on the verge with high bandwidth opamps,a part with thousands of transistors on the chip over a basic electronic building block - the transistor amplifier,which is the heart of an opamp anyway ?

three pins to solder,3-5 resistors to set operating points,a cap or two for bandwidth and DONE man ! And can be assembled right on the jack pins.direct soldering without using a perf board or any other mount.

That is how i would do it anyway.Simple and basic.No more than what i need to do the job but that is me

different strokes man

 

[amb]

Quote:

Originally Posted by rickcr42 you serious man ? you really would use a part with eight to fourteen pins to solder and touchy layout to avoid oscilations which are always on the verge with high bandwidth opamps,a part with thousands of transistors on the chip over a basic electronic building block - the transistor amplifier,which is the heart of an opamp anyway ?

The 741 and "high bandwidth" in the same breath? Thousands of transistors? Not even close. Have a look at the datasheet (TI version, National version, Fairchild version), I counted only 22, 20 and 24 in each of the three versions. This is about the most benign opamp for general purpose use. At $0.42 a piece from Digikey, almost as cheap as any single transistor. Not good for audio, though.

Quote:

three pins to solder,3-5 resistors to set operating points,a cap or two for bandwidth and DONE man ! And can be assembled right on the jack pins.direct soldering without using a perf board or any other mount.

I'll grant you that you can't easily solder a DIP package onto a jack directly without a perfboard, but it's been done

. Whether it's a transistor and a handful of resistors or a DIP with two resistors, doing it without a board is just messy no matter how you cut it.

 

[rickcr42]

Quote:

Whether it's a transistor and a handful of resistors or a DIP with two resistors, doing it without a board is just messy no matter how you cut it.

three pins is not a messy proposition by any stretch and I have 'dead bugged' many circuits that have served me for years reliably.
The basic buidling blocks of electronics are something many here fail to see the use of and would rather just plug in an opamp even when the less complex is called for such as voltage follower,led drivers,input buffers,and even gain stages.
as always you can do what you want,you will anyway,but i answered the question asked with what i personally would do and I use opamps as a last resort in any circuit if i can get there with a simple single stage transistor or mosfet or if i want to get really complex a complimentry pair which is still some 800 transistors short of the almighty opamp but twice as flexable