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These are great free flashlights. The LED's on these lights are regulated only by a couple resistors. By changing the voltage from 4.5V (1.5v x 3) to 12V, you're proportionally increasing the current to the LED's They will be brighter, but won't last very long. So be sure to check the LED current and change the resistors accordingly.I did a similar mod for the inside of my milling machine cabinet. But I used a 5V adapter, instead of 12V. No need to change the resistors for this mod, as the current is only increased by 10%.
Just so you know... What you've made here produces a stereogram. There's nothing holographic about the process. Holography is an interference process from laser light.