Updated 2014-01-28 01:02:09 by uniquename

From a news:comp.lang.tcl posting by Roger E Critchlow on Aug 17, 1999 --

Create a window, $w, with an analog dial of $diameter pixels, background color $bg, foreground color $fg, and middleground color $mg. The dial will update the frequency stored in the global named $var between minimum value $min and maximum value $max. Grab the dial anywhere with the left mouse button and spin, the marker that spins is only there to give you some rotational feedback. The concentric regions marked on the dial give different update speeds depending on which region you grab with the mouse. The mouse is tracked anywhere on the screen as long as the button remains depressed, so you can get nice fine adjustments by pulling the mouse way outside the dial.
 proc dial {w var min max diameter bg fg mg} {
    # create the dial canvas
    canvas $w -width $diameter -height $diameter -bg $bg \
        -highlightthickness 0
    # create a data array
    upvar \#0 $w data
    # cleanup the data array
    bind $w <Destroy> [list unset $w]
    # save arguments
    foreach v {w var min max diameter bg fg mg} {
        set data($v) [set $v]
    # compute some values
    set data(x0) [expr {$diameter/2}]
    set data(y0) [expr {$diameter/2}]
    set data(r0) [expr {($diameter-10)/2}]
    set data(dr) [expr {$data(r0)/5}]
    set data(mark-theta) 250
    set data(2pi) [expr {2*atan2(0,-1)}]
    # create the dial disk
    $w create oval 5 5 [expr {$diameter-5}] [expr {$diameter-5}] \
        -fill $fg -outline $mg -width 4 \
        -tags dial
    # mark the resolution radii with concentric circles
    foreach d {2 3 4} {
        set r [expr {$d*$data(dr)}]
        $w create oval \
            [expr {$data(x0)-$r}] [expr {$data(y0)-$r}] \
            [expr {$data(x0)+$r}] [expr {$data(y0)+$r}] \
            -fill {} -outline $mg -width 4 \
            -tags dial
    # draw a rotating marker
    $w create line $data(x0) $data(y0) $data(x0) 5 \
        -width 4 -fill $mg -arrow last \
        -tags {dial mark}
    # press binding
    $w bind dial <ButtonPress-1> {dial-press %W %x %y}
    # motion binding
    $w bind dial <B1-Motion> {dial-motion %W %x %y}
    # return our window
    return $w
 proc dial-press {w x y} {
    # bind to data array
    upvar \#0 $w data
    # determine radius and theta
    set x [expr {$x-$data(x0)}]
    set y [expr {$data(y0)-$y}]
    set data(r) [expr {sqrt($x*$x+$y*$y)}]
    set data(theta) [expr {int(1000*atan2($y,$x)/$data(2pi))}]
    # compute frequency update for this radius
    # this assumes the 1000 counts/rev done below
    switch [expr {int(($data(r0)-$data(r))/$data(dr))}] {
        0 {
            set data(kHz/count) 0.01;   # 10 kHz/rev
        1 {
            set data(kHz/count) 0.1;    # 100 kHz/rev
        2 {
            set data(kHz/count) 1;      # 1 MHz/rev
        default {
            set data(kHz/count) 10;     # 10 MHz/rev
 proc dial-motion {w x y} {
    # bind to data array
    upvar \#0 $w data
    # compute dtheta
    set x [expr {$x-$data(x0)}]
    set y [expr {$data(y0)-$y}]
    set dtheta [expr {(int(1000*atan2($y,$x)/$data(2pi))-$data(theta))%1000}]
    if {$dtheta > 500} { set dtheta [expr {$dtheta-1000}] }
    # update theta
    set data(theta) [expr {($data(theta)+$dtheta)%1000}]
    # update marker
    set data(mark-theta) [expr {$data(mark-theta)+$dtheta}]
    set t [expr {$data(2pi)*$data(mark-theta)/1000}]
    $w coords mark $data(x0) $data(y0) \
        [expr {$data(r0)*cos($t)+$data(x0)}] \
        [expr {$data(y0)-$data(r0)*sin($t)}]
    # update frequency
    upvar \#0 $data(var) frequency
    set f [expr {$frequency-$dtheta*$data(kHz/count)}]
    if {$f < $data(min)} {
        set f $data(min)
    } elseif {$f > $data(max)} {
        set f $data(max)
    # update frequency variable
    set frequency [format %12.2f $f]

 # Here's some demo code:
 set bg \#4f4f4f
 set fg grey
 set mg white
 set frequency 15000.00
 . configure -bg $bg
 pack [frame .f -bg $bg] -side top -fill x
 pack [label .f.l -font {-size 32} -width 9 -anchor e \
        -textvar frequency -bg $bg -fg $mg] -side left
 pack [label .f.hz -font {-size 32} -text "kHz" -bg $bg -fg $mg] -side  left
 pack [dial .d frequency 0.00 30000.0 200 $bg $fg $mg] -side top

uniquename 2014jan27

For those who do not have the facilities or time to implement the code above, here is an image of the dial and needle that are drawn on a Tk 'canvas' --- which is packed below a 'frame' widget containing a couple of 'label' widgets.

You can drag the needle either clockwise or counter-clockwise --- and the displayed number either increases or decreases, respectively.

This dial-and-needle could be 'hooked up' to a function that animates the dial according to a time variable. The Tcl 'after $millisecs' command could be used to advance the time variable at a suitable rate --- in particular, to keep the process of needle-updating from proceeding so fast that it uses CPU-cycles at a ridiculous, CPU-temperature-elevating rate.