The hour glass can be toggled on and off by hitting it with a mana burst, which probably is easiest to do with a pulse spreader that activates when the comparator signal exceeds a certain amount and again when the value drops below another value. To prevent that from happening too often (which may be prone to failing eventually), you probably want to not just use a simple comparator, but a hysteresis circuit. (The x-1..x+1 design is probably good enough for your purpose.)