BTW, at one point, I rigged a timer for a portable CD player. That ended up being our "timer" so that older DS knew that when the music was over, he was welcome to join younger DS and I if he chose to leave his current activity. Sometimes the straight timer was an anxiety-producer for older DS because he turned into a clock-watcher or he'd say the timer noise startled him.
On another note, we did something similar for his naps when he was 2. I got an alarm clock that played "white noise" (e.g., forest/ocean/brook sounds) for an hour. DS understood that he was to stay in bed until the "bugs stopped making noise" then if he was awake, he could get up. It helped us a lot -- stopped him asking whether it was time to get up yet and also helped him calm down and actually sleep.
