I have composed a modern classical piece of music which has sections for violins, cellos, some horns and percussion. The problem is that I did all of that using an electric guitar, so I actually don't know how it really sounds played by instruments I wrote it for. So I would like to know how I can get my guitar to sound like a violin and cello? I think there should be an effect to achieve that sound, but I've been unable to find it.
very simple answer...
I've had one for 30 years, there's nothing quite like it, but it is a technique in & of itself.
You can do the standard 'never-ending note' by simply holding it over a string & sliding/hammering up & down the fretboard, but with a little practise you can make it sound like violin/cello spiccato by banging the string with the e-bow to give the note attack - something that cannot be done with volume knob/swell pedal technique alone.
OK, I got the thumbs-up, I'll share my embarrassment... 15second example from a 30-year-old track trying to use an E-Bow as a 'cello'. I sincerely apologise for the drums, & for my stunning lack of technique ;-)
The important part of a violin sound is a gentle attack at the start of each note. Some players use a volume or swell pedal to achieve this: the note is played just as the pedal swells the volume in. Others use the volume pot on the guitar. Strats and Teles are quite easy to do this on, as the knob is close to where the string is picked. Again, the string is played just as the little finger rotates the volume knob from 0 to 10. A bit of distortion can make it sound more like a violin, but that needs to be applied carefully.
Some effects processors have a setting which does the swell for you.Boss is one.
If you can play the parts on the unwound high E string, you can use a fiddle bow, as Jimmy Page did. Just remember to rosin the bow and use a cheap one as the guitar strings are hard on the horsehair.
Another alternative is the Electro Harmonix SuperEgo which will allow you to adjust the attack (Gliss) as well as the sustain ("Speed");
Yet another alternative is the Electro Harmonix POG2 which allows you to control the attack, adjust the octaves and the low pass filter. I use this one to get a nice arco cello and arco bass sound. The POG2 is one of the cleanest octave pedals I've ever worked with. Very few glitches.
With e.g. a Roland GR-55 guitar synth and special pickup GK-3 you can get semi-convincing violin and cello sounds. Using midi you can connect to a e.g. a DAW and likely get vastly superior results through sound libraries. Not the cheapest solution, but probably closest to the real thing. Could be considered cheating though...
I'd not bother. Bowing creates a special waveform that is quite different from that of a guitar, being more like a sawtooth (because it basically results from a stick and slide pattern on the bow hair: the violin is built to suck off and transmit much more sound energy than a guitar, so plucked notes don't have significant sustain). That's actually not easy to create from the input of a guitar without tracking the actual pitch because it amounts to something like using overdrive and suppressing the even-numbered harmonics afterwards. If you have something reliably tracking pitch, you are likely better off just feeding a violin synth via Midi.
However, what makes a violin sound actually live is expressiveness particularly in long notes. That means that you should find some pedals that give you subtle ways to make long notes live. Also you should play with a guitar type giving you long-sustained notes by default. So solid-body, light gauge, soft play.
If you are going to do more composition, it makes sense to employ keyboards. String sections are usually done somewhat convincingly on most keyboards these days. Solo play, however, is hard to do because of the expressiveness angle. It might be worth working with a reedless accordion as input device since the bellows pressure gives you the kind of intuitive continuous expression input that bowing has.
If you do not have significant prior keyboard exposure and want to give that combination a try, I recommend going for a chromatic button accordion keyboard (regarding the choice of button system: reedless accordions usually allow you to switch. All systems, C/B/G-Griff are pretty much equivalent but the used acoustic instrument markets in various countries tend to heavily favor one kind, so it makes sense to pick a system that will make it easier for you to get the kind of acoustic instrument you prefer).
You'll get quite more range into your right hand and a nicer size factor. Also the controls are "naturally spaced". Basically, you are playing a 20-string tap guitar with 4 frets and tuned in minor thirds. No C major scale standing out.
If you are not going to use that kind of setup often enough to learn a different instrument type, you might get along with some kind of Midi guitar (never tried one of those myself). I don't think that you'll arrive at concert-level quality with that, but if your main focus is "composer", others are going to play the instruments anyway in concert.
However, I think that for composing you'll be better off with a "proper" keyboard instrument in the long run. For example, the chord voicing of various chords on a guitar is untypical for most other instrument types. Keyboard chords are more "standard".