How do I make an Emacs keybinding to set a variable?

(defun set-my-var (value)
  "set my var"
  (interactive)
  (defvar my-var value
    "a variable of mine")
)

(global-set-key "C-v" 'set-my-var)

call-interactively: Wrong number of arguments: (lambda (value) "set my var"
(interactive) (defvar my-var value "a variable of mine")), 0

(defun set-my-var (value)
  "Revised version by Charlie Martin"
  (interactive "p")
  (setq my-var value))

(defun set-my-var (value)
  "Revised version by Charlie Martin"
  (interactive "p")
  (setq my-var value)) ; do ^J to evaluate the defun form, showing return in the buffer.
set-my-var

(global-set-key "C-v" 'set-my-var)
set-my-var
 ;Another ^J

;;; Now, what's the value after ^U^V?
my-var
4
;;; Now ^U 8 ^V
my-var
8
;;; now ^U^U^V
my-var
16