Tuesday, May 31, 2016

Red-Blue Laser Show Teardown

I wanted some moderately powerful lasers for a new project I am prototyping. Last week we saw a red-green laser show teardown; this week we'll be looking at a more expensive red-blue unit.

NOTE: again, laser safety regulations are there for a reason! This is all fun and games until someone literally loses an eye. Don't be the dope who does damage like this!

So it looks like an inexpensive way to get modestly powerful laser modules is to extract them from cheap stage lighting effects. Here's one that has both red and blue laser modules that I found on Amazon for $70. (Blue lasers are considerably more expensive than red or green for whatever reason.)

So here's the unit with the top off. This is designed for outdoor use so it's in a nice cast metal enclosure, with a silicone-gasketed front panel and waterproof 12VDC power entry. There's no control panel but it comes with a remote. From the front you can see the two laser modules mounted in a heatsink and the edge of the PCB. The round wheel is one of the diffraction gratings: two more are glued to the back of each transparent exit window on the front panel.

Dangling off to the side is a passive IR motion sensor. I think this is designed to turn the unit off if someone walks in front of it. It did not work very well -- likely because the IR window was full of the opaque RTV silicone used to cement the detector into the front panel.

Top popped

Extracted showing PCB, heatsink, laser
modules, and motor

Unscrewing the interior unit we can pull it out to look at the top of the PCB. Visible is a bridge rectifier on the input (so it will work with any polarity), as well as the edge of the RF receiver board (the curly white wire is the antenna), as well as some plugs and bypass capacitors. Also visible but somewhat obscured are two PT4115 constant-current drive circuits, one for each laser. This is considerably more sophisticated than the current-limiting resistors we saw last week in the red-green unit.

Top of PCB

Here's the back of the circuit board with the heatsink liberated. Some interesting stuff here. First of all the motor is not a DC motor but a geared stepper! Not sure this is cheaper, but it certainly seems to be better than the DC motors used elsewhere as it can go arbitrarily slowly, if not smoothly (unlike DC motors which stall below a certain speed). It's being driven by the IC on the left, the good old ULN2003 Darlington array. The chip in the center is some kind of microcontroller, flavor unknown. (The typical chip obfuscation is to grind off the IC numbers on the top, but this one simply doesn't have any. Zoom into the flickr image and take a closer look.) Top right is analog circuitry for the PIR sensor -- these need some kind of analog differential amplifier as typically they sense the difference between the signal from two sensor sides in the same package. There's also two linear voltage regulators: a 5V 78M05 for the stepper motor, and a 3.3V AMS1117 for the microcontroller.

Extracted and powered on

Here are all the parts exploded. The blue laser has some kind of secondary driver inside the black shrink-wrap. I will guess that the board is designed for a red-green laser combo and the ble laser is an add-on plugged into the green laser constant current circuit. There are also two kinds of heat sensors glued into the heatsink -- a thermistor and what I am guessing is a thermal fuse in case things get hot in that thermal enclosure. The red laser has a heatsink jacket which limits the electrical and heat conduction to the heatsink so I'm guessing all that thermal stuff is for the blue laser.

Cheapo red-blue laser show teardown

The blue laser ran cool to the touch even when on continuously for an hour, but whatever was in the heatshrinked driver was getting uncomfortably hot. Removing the shrinkwrap revealed some kind of op-amp + transistor circuit: I'm guessing this is a current limiting circuit that uses the transistor in the linear region. For CW use I will heatsink this little board, and add some heatsink compound to the blue laser.

It's curious that this linear regulator was on top of the more efficient PT4115 buck limiter. Best guess here is that the main board was designed for a red and a green laser (note the red and green connector plugs!), and the additional limiter board allowed the blue laser to be substituted in without any other modifications.

Because all I really cared about was using the lasers, I removed the mystery microcontroller and broke out the enable lines to the laser drive circuits (on the red and blue bodge wires). These go to the "DIM" inputs on the respective PT4115s through a 1K resistor and allow me to dim the lasers individually by using a 5V PWM signal (like from an Arduino).

Hacked circuit board

Here's the inside of the remote. Note the A23 12V battery which are fairly rare and mostly used for these little cheapo transmitters and cigarette lighters. The transmitter IC is a PT2264 which has handy data sheets available; unfortunately the receiver was some off-brand part and not the matching PT2294 (which seems to be Taiwanese and unavailable in the US). While it would have been cool to salvage the transmitter/receiver pair for reuse, I put the receiver serial data output on the scope but it was crappy enough I did not feel like reverse-engineering it. Into the parts bin!

Inside the remote control

So two gorgeous, moderately powerful laser modules with dimmable drivers and a bonus well-made outdoor enclosure.

← Prev: Red-Green Laser Show Teardown Next: Remembering the Cambridge Computer Laboratory

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