[cynwoody]

A timer would overcome the problem of getting the radio signal through the steel pressure cooker.

Not necessary. Position the electronics outside the cooker and run a wire through a small hole drilled in the cooker, the other side of the circuit being the cooker itself. When the RC receives the proper signal, it sends an electric current through the wire, detonating the charge.

[bgill]

For days, people have been talking drilling holes in pressure cookers. There’s no need. Just rig a wire through the steam vent.

http://www.doityourself.com/stry/recommendcanners

[Momto2]

From the Daily Mail : "The homemade pressure cooker bombs used in the deadly Boston Marathon explosions were triggered by a remote detonator of the kind used in remote-control toys, U.S. officials have revealed. The officials said investigators found pieces of the remote-control equipment among the debris and were analyzing them."

So if the device exploded somehow at the scene - how were the brothers not injured from this explosion - I assume they were holding the device to trigger it?