Laser Driver Circuit for Keychains

The project describes the operation of a cheap keychain lasers that are being sold in low price.

In theory, most common laser heads contains two semiconductors inside which includes a laser diode (LD) and a photodiode (PD). Forward biasing will be done on the laser diode while its cathode (LDC) will connect to a driver transistor and/or network. This will regulated the LD current on the photodiode current known as the feedback network. On the other hand, reverse biasing is done on photodiode while its anode (PDA) will feed a driver regulator in order for the control to give a feedback signal for the LD driver.

The feedback is considered as optional since part of the laser beam goes backwards to reach the photodiode junction. There are three pins that the laser diode head has been labeled and these are the Laser Diode Cathode (LDC), Photo Diode Anode (PDA), and the common Positive Terminal (COM+). The two semiconductors inside the laser diode head are used to regulate the laser diode current with an external feedback loop.

A tiny PCB is found behind the laser diode head that has SMD device mounted on it.