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Is there a difference between a Volume Pedal and a Control Voltage "expression" pedal?

I know that they are 2 difference USES, but are the pedals functionally/technically different? Or are they the same type of pedal used for 2 different functions?

So can I use a Volume pedal as a CV "expression" pedal to control parameters on my synth in real time (such as modifying filter sweep or envelopes), for example?

To the best of my knowledge, a volume pedal and an expression pedal are the same hardware. The only other major ones I can think of are Sustain Pedals and Switch Pedals.

a good sustain pedal should have 'half-pedal' capacity, which means that pedal responses your foot pressure with continuous volume change, while a 'switch pedal' only functions as a on/off switch without sensibility to sustain volume curve.

Frozen one,

The difference depends on the synth you are using. For instance, my Hammond XK-2 uses a specially designed CV expression pedal, that has a special multi-pin connector. It costs over $100 retail, while a Yamaha FC7 pedal with its 1/4" plug connector costs around $25 street.

Typically, pedals like the FC7 are a variable resistor device and control small current levels. Pedals that are true CV expression pedals like that for my XK or one off an old B3 are engineered for higher currents and usually have additional electrical components. But I'm not looking at a schematic as I write this so my answer is more professional opinion than fact. I'll bet you can consult Captain Foldback and get an authoritative answer if you like.

http://www.captain-foldback.com/

On the other hand my Yamaha midi keyboard uses the FC7 input for either the volume control or expression control, and the small change in current coming from the pedal wire is converted to a digital value in the synth. A correspond MIDI control event is generated, either CC# 7 for volume or CC# 11 for expression. The electrical characteristics of the two signals are identical but the resulting digital messages are different, configurable from a synth's setup menu.

Hope that helps. The moral of the story is, don't plug a simple pedal like the FC7 into a port meant for a CV Expression pedal, or you'll smell smoke (and give the poor organist a hotfoot!)

Thank you all for your input and help. From what I'm understanding then, in theory, you could get 2 identical expression pedals--plug one into the CV port and the other into the Volume port, set them each to function accordingly and use them that way. So one of those hypothetical pedals could work as either a CV expression or as a volume depending on which port you plugged it into. If I'm on the right track, them perhaps my pedal shopping just got a bit easier.

FYI, the board that I need a pedal for has the 1/4" in jacks not the the multi-pin connection.

Speaking of multi-pin, are there MIDI pedals as well then?

No, the multipin CV pedals are not midi, but input directly into the circuitry, in the Hammond, probably feeding the grid that controls the preamp output.

Yes, the 1/4" jacks are both probably midi, but check out the specs for the particular keyboard before plugging in. If they are, they are low power devices that should go nowhere near a CV circuit. Instead, they feed an A/D circuit, maybe a simple voltage divider, that converts a current related to the pedal's position into values between 0-127.

But check the specs.