wiring up laser pointer directly to usb power
A tear-down of a red laser (no LED) shows a very simple circuit (left):



simulate this circuit &ndash; Schematic created using CircuitLab
 Note that the metal shell connects to laser anode, and is at the most positive voltage. The inner &quot;springy&quot; contact connects the battery negative terminal (through the series resistor and momentary switch) to laser cathode.
If you substitute 5V USB power source, an extra external series resistor to the &quot;springy&quot; contact is all that's required to keep laser current in the 20mA ballpark (right).
We often use USB DC connections with the assumption that the negative end is ground, and the positive end will be &quot;hot&quot;. Not so with these cheap lasers - the shell connection should not be grounded.