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Old 03-10-2008, 09:23 PM #1
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Default Q about microcontroller PWM to Red Laser

Hi,

To cut to the chase:

I need to control the brightness of a Red Laser Diode from a Microcontroller.
What is the best frequency and duty cycle range to drive a Red Laser Diode from a Microcontroller?
Has anyone got any circuit diagrams for PWM driving? Can I get away with PWM from Micro and a 47uf Cap?
Can you PWM drive a red laser diode at 6v+ with a lower duty cycle (normally run off 3v)? (ie. using two CR2032's or two CR2016's)
Has anyone run a laser diode of a Joule Thief?
(I have some cheap $2 Red laser pointers I can smoke test before I use the $40 Laser sight.)

Background Info:

I've been playing with ATMEL ATTINY13 programmable microcontrollers and I had a ?great? idea last night.
I play Airsoft and I use Red Dot Sights on my guns that work by reflecting a low power laser off a specially coated lens.
They usually have 10-12 "brightness" settings for various ambient lighting conditions.

The big problem is that they are REALLY easy to
a) leave the power on
b) knock the power on
This results in a lot of dead CR2032 and LR44 batteries which are also difficult/annoying to change :'(

High end Commercial models aimed at military tend to have nice features like 50,000 hours battery life, run off AA's and auto shut-off.
Obviously they have high end optics and components to achieve that kind of efficiently, this comes with a nice price tag ($300us+)

I have three main idea's to replicate these features to upgrade an existing cheap $30US Red dot sight:
a) Add a micro controller to implement PWM brightness control and Auto shut off ...
(Micro uses <0.1uAH in power down mode, thats 200years on a CR2032 batt.)

b) Use Photo detector (X to micro) to auto adjust the brightness level.

c1) Look at using a Joule Thief to run the whole thing off low voltage source, ie. A single AA or AAA battery.
or
c2) Use two CR2032's or CR2016's and run the thing off 5v with Voltage Regulator.

Final note: I want to try and keep the part count as low as possible.
Ie.
Batteries.
2x push buttons
2x pull up resistors
1x ATTINY micro (Using DIP8 but may go SMD )
1x Transistor ? SMD ? (for PWM the LD)
1x 47uf Cap (Voltage smoothing)
1x Laser Diode

Joule thief would obviously require more parts.....


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Old 03-11-2008, 02:34 AM #2
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Default Re: Q about microcontroller PWM to Red Laser

Just an idea, I have heard you can get voltage regulators with an enable pin, if you set one up as a current source and hook your pwm pin to the enable pin, that might work.
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Old 03-11-2008, 12:50 PM #3
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Default Re: Q about microcontroller PWM to Red Laser

if your laser has a buit in driver you can just pwm it from a transistor. it probably doesn't matter what freq you use. higher will have less visible flicker. maybe like >1KHz
and use trial and error to determine duty cycle. so long as your LD has a driver you can't blow it.

everything else sounds good. don't know about using a jule thief though. i have seen small step up converters from single AA's, controlled by the uC that it is powering, using a button to start it up (because the uC can't operate from 1.5V) and it happens to use an attiny13
look here; http://spritesmods.com/?art=ucboost
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Old 03-11-2008, 05:26 PM #4
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Default Re: Q about microcontroller PWM to Red Laser

Are you using PicBASIC or C code? It would be easy to do with a capacitor and transistor. I would recommend a ddl circuit and a digital pot in lieu of the resistor or turn pot if the laser doesn't have a driver.
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Old 03-11-2008, 08:34 PM #5
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Default Re: Q about microcontroller PWM to Red Laser

Thanks guys,

goldfingerfif: I'm using Bascom basic for AVR Atmel processor. I could use C but it just seems cumbersome and doesn't offer any major advantages.

woop: I have a better circuit than that ... its a modified joule thief / solar light, uses feedback from the inductor to switch the coil rather than having to use PWM from the Mcu. Can't find the link right now, but its basicly a solar light circuit with a voltage regulator on it.

I'm actually starting to suspect that it's a regular LED not a laser diode
I pulled the red dot assembly apart and basicly the brightness adjustment is an array of SMD resistors (as I expected), but the "red dot" diode is embedded in plastic and has a foil sticker with a pin hole in it.
I carefully pealed back the foil and turned on the diode and it appears a bright uniform red. It doesn't "sparkle" like my red laser diodes. Also the lead coming from the Diode it a regular lead like on a LED and the wire is soldiered to it ... no room for a PCB or metal housing ...
Does this scream LED?
Is there any definite (non distructive) way to tell ?

If its a LED it makes me a lot more comfortable to start PWM the $%^ out of it.
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Old 03-13-2008, 07:15 PM #6
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Default Re: Q about microcontroller PWM to Red Laser

goldfiner its funny you said that I built a circuit to do this with a basic stamp the pic2 I used the picbasic from parrellax that you get with the basic stamp kitand it works fine and easy to program I used the lm317 driver board and feed that with my mod board to regulate output I used the out and put commands with timing loops to get the varible voltage it works but I it can be a little slow if you look at the output with a scope maybe a faster chip would work even better but those dam stamps are so easy to work with I am new here and have gotten lots of info and help so if anyone wants to see the cicuit or code just ask maybe if others want to do the same thing we can build a project out if it
just my .002
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