Our Graco Pack 'n Play came with a little vibrating motor unit (a scaled up version of what wiggles your smart phone) that provides some movement and some white-noise to help Moriarty sleep. The problem is, the silly thing runs on D-batteries, and turns off after 30 minutes. So after many instances of Moriarty sleeping peacefully for 30 minutes, then waking up when the viberation shut itself off, we decided to hack it.
We got an AC power adaptor to replace the D battery, and we decided to add a simple on/off switch to control the thing.
I'll spare you the details of the voltages, and soldering hassles. Long story short: it works now. We turn it on, put Moriarty in, and it helps him sleep until he's hungry, or needs a diaper pit-stop.
Here is the hacked version: