Laser Diode Driver with TTL Control. PCB V2.0 is ready.
July 8th, 2012 | 
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Laser Diode Driver with TTL control. PCB V2.0, populated
TTL-controlled laser driver board V2 on a breadboard
I haven’t installed the potentiometer (R1, space indicated on the board) because the project I’m using it for, the DVD CNC laser cutter, does not really require much current adjustment, which is what R1 is doing. I’m powering all parts of the CNC laser cutter, including the laser diode driver from a +5V power supply and it just happens that the 2 Ohm resistor R2 gives me just the right current, approximately 150mA, needed for the red laser diode I’ve harvested from a DVD burner drive. If you did really need current adjustment, the power supply would have to be raised to about +9V , the R2 would have to be 10 Ohm or just about and the 100 ohm R1 potentiometer would need to be installed.
Elabz Laser Diode Driver Board V2
It may just be my luck or it may be a temporary situation, but when I started to design this driver back in late 2010, the LT1121 power regulator chip was very common and very cheap, about $0.40 each, delivered from China. It was one of the most important reasons I based the design on LT1121, actually. It took me a while to get a final usable design and have the boards manufactured, so fast forward to July 2012 – the LT1121 chip is nowhere to be found for less than $5 EACH! I still have 3 or 4 of these chips left laying around the bench, and I’m obviously on the lookout for a good source. If anyone knows a good source, I would appreciate a tip!
And last but not least: the Eagle board and schematics files are here:
Laser Diode Driver V2 Eagle schematics and board files
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Hi!
I have a question about the schematic. If I read it correctly LT1121 sets 3.75V on PIN2 which goes across R2 and LD in series. However everything I read strongly suggests that you should never drive a laser diode with voltage. Am i missing something here?
It is true, the voltage regulator will try to raise the voltage on its output until the voltage on the ADJ pin (Pin 2 on LT1121) equals a pre-set value, for LI1121 it’s 3.75V. In this schematic ( http://elabz.com/wp-content/uploads/2012/01/Elabz_TTL_Controlled_Laser_Driver.png ) the output (Pin 1) and ADJ (Pin 2) are shorted and the only path to ground is through the R2 (and, if installed, R1) and the laser diode. The voltage across a red laser diode is always 2.5V and therefore the rest of the 1.25V difference has to come from the current passing through the laser diode and R2. In other words, the regulator does not raise voltage across the laser diode, which would be bad for the diode. It raises the voltage across the R2 resistor until the sum of the voltages on R2 and LD becomes equal 3.75V. If you use higher value R2, the voltage will rise at smaller current and so the current through the LD will be less.
Check out the LTSpice model for the driver here: http://elabz.com/wp-content/uploads/2012/01/current_source.asc You can play around with input voltages to see that the current across the laser diode is constant from about 5V to all the way up to the upper limit of LT1121 – 30V
Thanks for stopping by my blog!
Digi-Key has them for $3/ea, or $2/ea for 25.
If you saw these somewhere for $0.40, they almost certainly weren’t genuine Linear parts — Linear (and Analog Devices, for that matter) don’t make such inexpensive parts… 1/2
The ones you saw may have been something like the LT1118 that other companies make pin-compatibles of for 10-25% of the original’s cost.
good luck!
-e
$1.83 direct from Linear or $1.50 in quantity of 1K
a little question: I am using Multisim to simulate your circuit and what load should I use?
When I use a resistor as load the current is always around 6mA. Should I change a load and what it should be?
Thanks for stopping by, Tony. I’ve posted an LTSpice model of the driver some time ago, check it out: http://elabz.com/ttl-controlled-laser-diode-driver/ It includes the full model of the laser diode as output (including the brightness feedback photodiode, not used in this driver, just in case you need it in a different driver design). Hope you can use it in Multisim or re-create it in Multisim – it’s a very simple circuit.
So many thanks!