TTL-controlled Laser Diode Driver. Updated schematics.

TTL-controlled Laser Diode Driver. Updated schematics. TTL-Controlled Laser Diode Driver TTL-Controlled Laser Diode Driver

During the course of my DIY laser cutter project I needed a laser diode driver that I could control with a CNC software, such as EMC2. I’ve already made an attempt to build one based on Linear Technology’s LT1121 voltage regulator with Enable input but the driver design that came out was not exactly successful :(
So, I took another stub at it …

I have to admit, it was a pretty silly move to try designing an analog circuit based on Linear Technology device and not use their own free LTSpice circuit simulation software. I was at some point deterred by the fact that it’s a Windows software but turns out it runs just fine under Wine and it was a rather easy simulation at that.
Here is the Spice model of the laser diode driver I made. I’ve been using the resulting driver pretty extensively now and it seems to be working pretty well.

This Spice model shows that the driver should start working at approx 4.8V on the Input pin. I’ve found that at such low input voltage the current across the laser diode comes up less than what’s calculated by the software. It could be that the laser diodes I used (mostly harvested from broken DVD writers) don’t have the same exact parameters as the SLD1121VS used in the model – that was the only laser diode model I could find on the Net.

I am actually using this driver with input voltage of approx +5V due to my desire to use a simple open frame +5V power supply I had available for the entire CNC cutter, stepper driver, logic, interfaces and the laser diode – all of it from the same supply voltage. It kind of works fine but at +5V input I can only get 120-125 mA currents across the laser diode. It’s just not enough for the LT1121 to bring the reference voltage up to the required +3.75V (see the datasheet). For some reason the LTSpice model does not show that. But if you raise the input voltage to +9V, you should have no problem with getting the currents you receive from the software model.

I’m going to hold off publishing the Eagle files until the boards I ordered from Olimex come and I had a chance to test them – last time I published a bad design board was rather embarrassing. So check back here in a couple of weeks – I should have both the boards for those that don’t want make their own and the Eagle files for those that do.


13 Responses to “TTL-controlled Laser Diode Driver. Updated schematics.”

  • [...] Laser Diode Driver Based on LT1121 Voltage Regulator – schematic and PCB May 5th, 2011 | Author: admin Laser diode driver based on LT1121. PCB version 1.2, populated. 01-12-2012 Update This post is kept alive just to hold the old comments – please see the corrected circuit design here: [...]

  • [...] of my belated updates. It has been about 3 months now since I’ve received the boards for the updated version of the laser diode driver from Olimex. However, I haven’t had a chance to work on any project that required a laser [...]

  • Mario Alberto Castaño Ospina:

    Hello and good afternoon! .. Very good project, I can you please help me with the diagram to the circuit board upgraded your laser burner? .. Thanks

  • Mario Castaño:

    Hello and good afternoon! … Then have printed electronic circuit as excellent project, I have come to an unexpected end and I can not find the integrated LT1121 CN8 or any replacement that is sold commercially here in my city. Visit the website: replacement and can not find any. I need your great cooperation to end this project as it is of very great interest to my work.

    Thank you very much.

  • Mario Alberto Castaño:

    Very good afternoon! …. After looking at different stores have not found the LT1121 and still I have my project, I wanted in Colombia and that component is not commercial and do not sell it. Please let me know I can use another built to replace the one that is in your project?. I appreciate your feedback immensely. Blessings from Colombia.

    • Hi Mario,

      I am sorry I have to admit, I don’t know of any pin-by-pin replacement of LT1121 . As I already lamented about in my blog post :) I am very disappointed myself: when I started developing the driver in 2009, they were perhaps the most widely spread voltage regulator IC with the shutdown feature, which was crucially important to my own prokect. Right now I am having problems finding them myself (for less than $5 apiece). I have some quantity of their cousins LT1129 which appear to be similar in function albeight different package (5-leads TO-220). If you are not trying to reproduce the board, and only the schematics, LT1129 may be a good replacement. They are still available online around the Web at about $0.80 apiece and I’m thinking about making another board for this IC in the future, but I don’t have it yet.

    • Thanks Raul, but no, that one is a preset 3.3V but for the current regulator to work we need the adjustable version of it. The part number is LT1121 CN8, I think. It’s been awhile since I last looked for it though. Perhaps Linear has it now, too. Cheers!

  • Geoff:

    How would I extend the driving power of this circuit? I want to drive a 1.7 amp blue laser.

    Would adding a FET to the output work?

  • You seem to be trying to implement a constant current supply but the LT1121 doesn’t have a constant current mode. As is, you have a 3.75V regulator followed by a 10K load resistor and a series (current limiting) resistor with a value that is the parallel of R1 and R2.

    The ubiquitous LM317 in current limit configuration would seem a better choice, or just a single transistor current source circuit. is a calculator for figuring the resistor to use with an LM317 and also shows how to wire it. For about 130mA, it’s 10ohms in parallel with 270ohms (total value 9.64ohms) and the exact current is 129.6mA. For 125mA it’s just 10ohms.
    To add a PWM control to the LM317, connect your microcontroller I/O through a 1K resistor to the adjust pin and for “on”, set the pin to input, for “off”, set the pin to high and output. The “off” state current will not turn completely off, but will be significantly reduced. This simplistic PWM only works with Red and IR, a more complete PWM would need to be used for other (higher voltage) colors.

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