If the light emitted by the laser pen exceeds a certain level, it will cause damage to the human eye even if it is reflected directly or through a bright object and enters the human eye, even for a short period of time. Therefore, most countries have laser safety requirements that limit the maximum emission level. This design example describes a laser driver that works even when a 1.5V battery is discharged to 1V. Two current-controlled transistors improve reliability in low-power conditions and allow for excessive laser current. With glow. 
Using this DC-DC step-up boost circuit and two current-controlled transistors, the laser can be safely powered by an almost discharged battery.
As shown, transistors Q1, Q2, and Q3 form a negative impedance that can be roughly expressed as Z ≈ - β (VDD - VBE) / RF. Assuming that all transistors have the same current gain (β), VBE is the base shot voltage of all transistors. Feedback is provided by RF, and R1 is used to bias control the Q1 collector current. The inductor L1 and the parasitic capacitance form a resonant circuit that oscillates due to the negative impedance, so that when the battery is 1V, a peak-to-peak value of about 3.5V is generated at the collector of Q1. Schottky diodes D1 and C1 form a half-wave rectifier that supplies 3V to the cathode of the laser; VDD is 1V, thus providing a 4V operating range that exceeds the laser threshold.
Q5 and Q6 control the laser current. The photodiode built into the laser component is used to monitor the brightness of the laser and to provide a negative feedback through Q4, biasing Q5 and Q6 to the appropriate collector current to ensure the desired laser brightness is stable. The Q5 and Q6 pairs are connected in series, so if one of them fails and is short-circuited, the other can still maintain the laser current at a safe level. The probability of two transistors failing at the same time is much lower than the probability of a single transistor failing.
Editor's Note: Due to variations in laser and photodiode efficiency, R7 may need to be adjusted to ensure that the laser output is within safety specifications.
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