Purpose of visible green LED on a green laser pointer driver - Page 1
Forum section: electronics / beginners

author: josecanuc
author_rank: Contributor
timestamp: July 21, 2019, 02:37:36 am
content: I was given a cheap green laser pointer as part of a conference's swag kit. These seem to be quite ubiquitous for "cheap Chinese green laser pointer".I tried to work out, as best I could, the schematic of the driver circuit, as shown.I haven't yet reverse engineered how it works, but I gather that there is a phototransistor that's detecting the output power of the laser diode and using that to modulate the power output to the 5mW specified on the box. (And in fact, if I give it input voltages from 2.1V to 4.0V, the power drawn is exactly 5mW in every case.)What struck me initially was that there's a standard visible-light green LED on the circuit, which would normally be encased in the metal case/tube of the laser pointer. What is the purpose of this LED? Is it to provide a cheap voltage reference per the forward voltage of the LED?

author: josecanuc
author_rank: Contributor
timestamp: July 21, 2019, 02:44:27 am
content: Just double-checking the schematic and I see a couple of mistakes, which make me re-think the polarity of the laser diode. But the main question is about the purpose of the non-laser LED.

author: ledtester
author_rank: Super Contributor
timestamp: July 21, 2019, 04:43:09 am
content: It might be related to monitoring the laser diode. This video has a tear-down of what's in a laser diode assembly:Some assemblies use a photo-diode to detect the amount of light actually being emitted by the laser.

author: Dabbot
author_rank: Regular Contributor
timestamp: July 21, 2019, 05:36:14 am
content: This schematic doesn't add up. Q2 is going to short everything after the phototransistor via its base-emitter.

author: mikerj
author_rank: Super Contributor
timestamp: July 21, 2019, 12:09:14 pm
content: Quote from: Dabbot on July 21, 2019, 05:36:14 amThere are significant errors in the schematic, the entire circuit including the laser diode can only draw current through the feedback phototransistor which is definitely not correct.

author: josecanuc
author_rank: Contributor
timestamp: July 21, 2019, 01:36:57 pm
content: I am most likely misinterpreting the 3-lead laser diode package -- the side with the single-pin is connected to the brass body of the diode-lens assembly. I quickly looked up a few laser diode pinouts and most have a 3rd lead listed as N/C or case-connected. I assumed that meant it wasn't connected to any of the actual laser diode, which may not be right.I also made assumptions on the transistors given the markings on them. "491" seems to be an NPN power transistorhttps://www.diodes.com/assets/Datasheets/FMMT491.pdf, and "2X" seems to be an NPN silicon switching transistorhttps://www.onsemi.com/pub/Collateral/MMBT4401LT1-D.PDF.Does anyone have any advice on deciphering the laser diode pinout? I am not sure if the diode module has a feedback phototransistor, because there is a separate device that appears to be such a thing, mounted in the brass holder -- that's what's connected to the black and red wires.

author: exe
author_rank: Unknown
timestamp: July 22, 2019, 06:34:48 pm
content: May be it's a way to bias the bjt?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 22, 2019, 07:11:47 pm
content: Does it light when running? If so maybe just an emission indicator, for when used in a different case where it is visible

author: chris_leyson
author_rank: Super Contributor
timestamp: July 22, 2019, 07:44:38 pm
content: From the PLT5-510 Osram laser diode data sheet:-Pin 1: LD CathodePin 2: LD Anode, PD Cathode (case)Pin 3: PD AnodeYour schematic is wrong firstly because of the pin out and secondly the phototransistor should be drawn as a photodiode. EDIT: The green LED might be used to as a battery indicator and it is also probably being used a stable voltage reference.

author: josecanuc
author_rank: Contributor
timestamp: July 22, 2019, 09:42:56 pm
content: So, based on the hints you folks gave about photodiode and such, I double-checked the schematic from the board traces and fixed mistakes, as well as changed the laser diode and phototransistor.I found that the 3-pin laser diode was not one with a N/C pin (by measuring with the diode-test function of my multimeter), and most of the datasheets I found showed the laser diode combined with photodiode in same package. But this module also has an external device that is glued above the LD with a partial mirrored glass at 45deg. This has a black and a red wire, which now makes more sense with a photodiode vs a phototransistor (I assumed those terms were interchangeable.)I guess the external PD in parallel with the internal PD increases the sensitivity of the arrangement?I see that the PDs drive Q3 to draw current away from Q2, so that more light results in less LD current.Yes, the visible-light LED lights up during operation. I suppose it could be an emission indicator, but it feels like that would be a wasted penny or two on these cheap giveaway devices. Would a LED be lower cost than a zener diode?Another question: What would R1/C2 be doing? Would I likely be correct to think that the R4/R5 group would be the setpoint for the power regulation, and that increasing the resistance to greater than 12kohm would increase the output power? Or would lowering R3 be most likely the regulation setpoint? I tend to think R4/R5 because having 2 series resistors looks like a development iteration of this design would have a fixed and variable resistance there, and they dialed in the final product specs, and replaced the variable with a fixed.

author: mariush
author_rank: Super Contributor
timestamp: July 22, 2019, 09:51:24 pm
content: Maybe they reused the circuit board in multiple products. Maybe a particular OEM product had a tiny view port that showed the led (or some light ring around the laser pointer) while powered on.

author: KL27x
author_rank: Super Contributor
timestamp: July 22, 2019, 10:50:40 pm
content: ^+2. I feel like it's most likely not used as a voltage reference, because there's no node across it that doesn't also include a series resistor?

author: chris_leyson
author_rank: Super Contributor
timestamp: July 23, 2019, 12:51:31 am
content: Thanks josecanuc for posting corrected schematic, that makes a lot more sense.LED D1 doesn't serve any purpose apart from being a battery indicator. and now for the verboseQ2 drives the laser diode and is biased on by R3. When the laser diode reaches it's threshold current and starts to emit light then the photodiode produces photocurrent that flows out of the anode and into Q3 base. Q3 collector then shunts Q2 base drive and provides negative feedback. The laser diode operating current and output power is controlled by the photodiode current because the photodiode detects the amount of light emitted from the laser diode rear facet.In your schematic R4 and R5 set the required photo current to set the laser power. Back of envelope calculation goes like this, say you need 0.7V bias voltage to get Q3 to conduct, then you need about 60uA photodiode current flowing through R4 R5. The data sheet says 150uA typical photodiode current at 10mW output power, so at 60uA the laser output would be about 4mW if you trust the data sheet.Is it eye safe ?EDIT: R1 C1 provide a snubber or a transient suppressor RC network cross the laser diode.