desperate need of help for op amps

[theChris]

do op amps work? I seriously have had a bad experience.

i wanted to build two circuts:
1.) non-inverting comparator
2.) inverting comparator

i got the inverting comparator to somewhat work.
the noninverting comparator doesn't work.
i can't get the inverting one to work now
the noninventing one now works...
also, i built a circut that changes states based upon the position of your hand. i move my hand to the switch to turn it off, but didn't have to touch the switch to turn it off... after a little bit more experimenting, the circut would change states if i moved my hand to certain positions. i found that odd.

so how do you make a working comparator circut? none of the textbook examples work for me.

op amp is tl082cp and no bypass cap. powered by battery, signal comes from a second battery.

[jean-paul]

do op amps work?

You came to the right place. Some of us pose the same question when opamps are used for audio.

Maybe it is a good idea to mention the resistor values and attaching a drawing ( or the schematic you used ) will help too for any answers.

[daatkins]

Yes, op amps work!

Inverting and non-inverting comparitors are exactly the same except you swap the + and - inputs (not power inputs!). You apply a reference to one input and your signal to be compared on the other. If the reference is on the - and input on the + then the output will be positive if the i/p is higher than the ref. The opposite is true if you swap the inputs over.

Note that some opamps don't like having 0v applied to an input if they aren't powered by a split rail.

Nice one,
David.

[daatkins]

I'd just clarify my last point: some op-amps don't like the inputs going outside a certain range wrt the power-supply rails.

For a TL082, the common mode input range is specified as +15v to -11v (typ) WHEN OPERATING FROM +/- 15v RAILS. Extrapolating, it looks like the inverting input should not be taken below about 4v if you're running off +15v and GND.

And a schematic would be nice.

Nice one,
David.

[theChris]

the schematic is practially what you would see in a text book.
one battery powers the op amp. the positive terminal of the battery goes to a spdt switch then to the Vcc (i think) of the opamp, and the negative terminal to the Vee (agian, i may have that backwards.)

the output1 terminal runs to a variable resistor, which then runs to a LED then to the negative terminal of the battery.

a second battery is also used. this battery's poitive terminal attaches to a push button switch, then from there to the non-invering input of the opamp. the negative terminal hooks to the inverting input.

i flip the power switch the led turns on. i press the switch and the led remains on. i remove all wires but the output and the led stays on.

the inverting comparator now has teh led on, and pressing the swtich slighly dims it.

[theChris]

should i go with a different op amp, like a dual 741? that was the op amp i fired...

[ThingyNess]

http://www-s.ti.com/sc/psheets/slod006b/slod006b.pdf

Long, but excellent and comprehensive.

Try to make sure the metal chassis/body of the switch is grounded to prevent it from picking up any stray RF and such.

Also, make sure your power supply decoupling is solid, and that both of the inputs to the opamp have some resistance to ground (IE, not floating), at all times, as the very high input impedance of an opamp makes it somewhat susceptible to picking up RF at the inputs.

[theChris]

ok, i went to a quad 741 (mc4741cp) that i had laying around.

hooked up the non-inverting and inverting op amps. inverting works. non inverting stays on regardless of signal...

[theChris]

ok, i got the thing to work, but instead of using the compartor listed, i went with a feedback resistor. in any case, the test works.

so to apply it to my project i need to know somehting about grounds. the parellel port on my computer has a number of "ground" connectors for the signals, and the power supply has the same "ground". would shorting a wire between these two "grounds" cause any problems, or should i use a resistor to be safe?