Laser pointer circuit puzzle

[Dave92F1]

I have a bunch of these cheap red laser pointers, and am trying to understand how the circuit works. (And see if I can do anything fun with them, like modulate the beam or increase the power.)



The board has a laser pointer and white LED light. It's powered by two CR2016 coin cells in series (so 6 volts).

Both sides of the board are shown. I did my best to figure out the circuit, and came up with this (ignoring the LED circuit):



Q1, the SOT-23 part, is marked "HL6". I unsoldered one and connected it to my handy-dandy component identifier box, which says it's a NPN transistor (not too surprising).

But I don't understand how the circuit works. How does Q1 regulate the current to the laser diode?

And most mystifying to me - what is the "DZ" component doing? It's a little glass device. At first I thought it was a diode (if it is, I still don't know what it's for), but I unsoldered one and checked it, and my component identifier box says it's a pair of diodes in parallel, as shown. With vastly different forward voltages (about 0.7 and 2.7 volts).

If I short around it, the laser doesn't work. What's "DZ" for? Is it really a double diode?

How does this work?



 

[mikeselectricstuff]

2.7v zener diode.

[Dave92F1]

2.7v zener diode.

Thanks! Why is it there? What's it doing in the circuit?

[ConKbot]

2.7v is a reference voltage, the pot divides it to a new reference voltage (vref), and the transistor maintains vref-Vbe across the 68 ohm resistor. With a constant voltage across the resistor, that means a constant current must be flowing across it too. That makes a constant current sink, so the laser diode is ruining at a constant current too.

[Dave92F1]

2.7v is a reference voltage, the pot divides it to a new reference voltage (vref), and the transistor maintains vref-Vbe across the 68 ohm resistor. With a constant voltage across the resistor, that means a constant current must be flowing across it too. That makes a constant current sink, so the laser diode is ruining at a constant current too.

The pot is only connected on one side, so how does it divide anything?

[SeanB]

Base current of the transistor provides a voltage drop across the variable resistor. Not going to make a very good constant current source, as the current will vary with gain of the transistor ( which a good current source should compensate for) but here it just has to keep the current in the laser diode somewhere between the threshold current below which it is merely a LED, and between a current which burns out the LED active region.

If you want to modulate the diode current the unconnected end of the pot is a convenient lace to place a second NPN transistor, collector to pot, emitter to the common which is the junction of DZ and R3, and use the base to modulate the laser. Turn on the second transistor ( via a base resistor) and you will turn off the laser diode.

[dannyf]

How does this work?

The diodes (zener) establishes a reference voltage to drive the npn's base. together with the 20k pot and the emitter resistor, it determines the current through the base, and thus the current through the emitter / collector / laser diode.

[LeonV]

I'm surprised i hadn't seen this here yet: