Each contained a plastic explosives charge designed to detonate when the devices were switched on.
Ah, see that's where you went wrong. You have to wait until they've all been deployed and are in use before you detonate them. Look at the Israeli pager attack. It was very effective, because there was a large dwell time between delivery and detonation.
Of course, the pager attack was despicable because it wasn't targeted, and almost certainly harmed protected noncombatants. This is more targeted, because it's given directly to combatants, and there's much less chance of innocent bystanders. But booby traps are still a war crime regardless.