Using the GD/GN507 as an example, the “mechanical” or “GN” version of the paddle has two pairs of metal contacts which are “closed” by moving the levers. They’re like the contacts on a straight key or most any other paddle or bug. The closing of the contacts also stops the motion of the lever, providing a crisp tactile feedback to the operator.
In the “optical” or “GD” versions of the paddle, the contacts are replaced by optical sensors. There is a fixed sensor consisting of an LED transmitter and receiver, with a continuous beam of light passing between them. When you move the lever, a small “flag” on the lever moves into the beam and interrupts it, and a keying transistor completes the keying circuit just as a contact closure would on a normal key.
The disadvantages of the optical contacts are that (1) the lever motion
is stopped by a cushioned post, resulting in a “soft” feel as compared
with mechanical contacts, (2) the optical sensors require a power
supply, and (3) the flag on the lever must move a minimum distance to trip
the sensor, so you can’t set the extremely close “contact spacing” that
many ops prefer.
The advantages of the optical sensors are (1) the soft feel, which is
actually preferred by some operators, (2) the “contacts” never need to be
cleaned, and (3) the keying action is silent.
In the case of the “bugs,” both the semiautomaic and fully automatic versions, the optical versions offer a further advantage– ease of adjustment. With a traditional bug you will need to readjust the pitch and dwell of the dot contacts if you change the speed by more more than a few WPM. Optical sensors eliminate the physical contacts, and so there is nothing to adjust when you change speed. Also there is no “drag” on the pendulum, so there is no need to adjust the “throw” of the lever.