HOw to pulse a samll pen laser 650nm < 5mw

I use a transistor driven by ttl or cmos logic. Most pen lasers use 3 batteries at 1.5 volrs each so using 5volts (ttl logic voltage) will damage laser. I usually put a couple of diodes in series with the transistor and the laser.

+5 volts---diode---diode---+-laser---collector of npn transistor. common ground---emitter of npn transistor. ttl or cmos or 555 timer------1K resistor-------base of npn transistor

Dave

circuit that will do it.

Check out the this explanation of an experiemtn I did at 1996:

--
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/

Dave's idea was to simply drive the laser with a switched voltage through a series resistor. This will work, but typical lasers have a rather small current region between the lasing threshold and the maximum allowed light output, so you're likely to either get less than ideal operation, or damage the laser.

Tomi's idea (ref above) was to switch the laser module, but he found this greatly limited the maximum switching speed. Still, if speed is not important to you, Tomi's idea will work fine.

My idea has been to create a custom circuit to switch the laser, but this has led to a small pile of rather complicated circuits. That's because even cheap lasers often use light-level feedback with a sensor mounted inside the laser. This sensor puts out a current of 30 to 100uA at the maximum operating condition of the laser, and the laser-driver includes a servo using this current. Because this servo is usually not very fast (certainly not as fast as the laser is capable of being modulated), switching it on can result in a slow laser turnon. One solution I have used is to have the servo switch from ON to HOLD during the off times, so it waits at the operating point while the laser is off, and quickly re- establishes the proper current level when the laser is turned on. As an element of extra complexity, the high-power laser current source which the servo drives can also be kept on, but its current diverted away from the laser during shutoff. This trick further enhances the switching speed, even as complexity grows.

An alternate idea is to make a very fast servo, and feed an extra current into the sensor node, tricking it into thinking the laser is putting out lots of light even as it shuts the laser off. This simple approach is OK for medium-speed applications.

Finally, for very high-speed laser modulation, maintain a constant dc current, and simply add an ac-coupled high-frequency current in parallel. I used this approach to go to 1200MHz with a surplus red Hitachi DVD laser. Some will remember the discussion here on s.e.d. where I described the setup with a 47-ohm SMD series resistor right at the laser, terminating the modulation-signal transmission line, and the microwave bias-T to inject the current. There was evidence my optical receiver caused the 1200MHz limit, rather than the laser.

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 Thanks,
    - Win