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532 green laser pointer mod

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presse55

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Nov 29, 2018
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Hello group,

Totally new to the forum, and have limited electronics & laser competence; thanks in advance for reading this thread!

I'm an amateur astronomer, and wish to modify a cheap green laser pointer to help orient my telescope more conveniently.
The mods involve:
  • removing button
  • removing batteries
  • powering from a circuit on which the button is mounted
This way I can have the laser mounted on the scope, and turn it on or off at will, without actually having to touch the scope.

The specs for the pointer I obtained here are:
  • Wavelength: 532nm
  • Visible Color: Green
  • Body Material: Aviation Aluminum
  • Powered by: AAA*2 (batteries not included)
  • Activation: Pushbutton
  • Light distance: Over 500 meters
  • Spot size: Minimum Spot Diameter ≤φ1.0mm; Spot Diameter at 15m <Φ18mm
  • Operating Voltage: DC = 3.0V
  • Power: 5mw
  • Size: 17cm x 1.5cm (length x body diameter)
  • Weight: 50g
laser pointer 2.jpg

laser pointer 1.jpg

Here is my first stab at a circuit to accomplish what I want.
It basically consists of:
  • a LM317 to step voltage from 5.5V to 4.0V;
  • a 2N2222A (npn) transistor to switch on or off using a SPST (or eventually an Arduino digital IO pin)
  • the laser module itself, with its control circuit as in the thumbnails above;
  • a trim pot (in test) to limit current;
schematic.jpg

So far, this doesn't work :confused:

I retested the laser by itself, and it works fine.
The voltage across the laser leads is 2 volts, and varies only slightly when I turn the trim pot screw.
When I replace the laser module by a LED, the circuit does work as expected.

So, where am I going wrong? Something tells me I'm not setting the transistor up properly.

Any suggestions to go about it any better way?

Thanks a million in advance!

Ron
Montreal
 
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You just want to mount that bare module on a scope and have a separate switch away from the scope your using??
and why can't you use the laser mounts that are offered for telescopes for just that purpose?
 
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The end objective is to mount the bare module on the scope, and to have an Arduino IO pin, away from the laser/scope, turn the laser on/off. Hence the transistor.
So for now, till the Arduino side is done, I'm working with a physical switch to turn the transistor on/off.

As for your second question, I'm part of an 'amateur telescope maker' group.;)

Hope I'm making sense, here!!
 
Welcome :) ,You know you could remove the spring i remember my first time soldering did the same thing right on the spring :sneaky:
 
OK, ditch the 100 ohm pot. Then get a different transistor. The 2N2222 has too high a Vce @ the 200-300mA your module wants. That should give you the result you're after.
 
You can dispense with the circuit to lower the +5 volts in. This driver can work anywhere from +5 volts to +3 volts in, so it is unnecessary. You already have a tactile switch on the driver board. Why reinvent the wheel? Just use the contacts for the tactile switch to setup your off board switch.
 
You're basically limiting the current to the pump diode at four locations - the pot, the regulator, the pass BJT, and the module driver. They will all fight each other, with the lowest limit winning in the end. Since the lm317 needs in excess of 2.5V drop under many conditions to properly regulate its output, that is likely your bottleneck since you're only allowing for 1V.

Get rid of everything, and switch 5V to the laser module directly. Monitor the temperature. If anything on the driver board gets too hot to touch, add a standard diode or three as needed in series to drop the voltage.
 
I don't know how much the OP knows so I must caution when your laser is on and aimed skyward there must not be any aircraft, it's a really big deal so take zero chances.
 
It's working - removed the pot, increased voltage input on LM317.
Thanks a million for all your feedback.
My lab is in the basement of our home - I've read up quite a bit, and take recommended precautions seriously. Aircraft are safe, this former pilot is glad to report.

I'll post a corrected schematic, along with measured voltages and current tomorrow morning

Thanks folks!
 
RedCowboy said:
I don't know how much the OP knows so I must caution when your laser is on and aimed skyward there must not be any aircraft, it's a really big deal so take zero chances.
Click to expand...
Let's be precise so no one is unduly scared. It's a 5 mW laser which is pretty much invisible seen from any wide angle so pointing it skyward isn't a concern. Accidentally hitting an aircraft isn't a worry too, just deliberate targeting is and unlawful. A mistake might happen when viewing a low angle celestial object but probably not likely since there would not be persistent targeting.
 
Hello,

I got my project working as intended.
  • increased input to LM317 to 7V
  • output of LM317 is 4V
  • decreased Rb (base of NPN transistor) to 500 ohms
Pressing the button activates the laser. I measured 310 mA, and a drop of 3.6V across the laser.

This is good enough for now, though I read that a MOSFET would probably be better. Also, could consider configuring LM317 as constant current, as opposed to constant voltage.

Thanks for your comments.

Ron

schematic updated.jpg
 
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That circuit gives you no current regulation. You need to use the adjust pin on the LM317 as your output to achieve current regulation. The way you have this setup, it will regulate the voltage, but that is not what you want for a laser diode. They decrease in resistance with heat causing them to pull more current.

If this is using the same 532nm module in the OP, then it has a driver and I still don't understand why you felt the need to increase the voltage to the LM317 to get 4 volts out when the +5 volts you were using before was fine the way it was.
 
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