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Old 03-09-2013, 07:57 AM #1
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Question Switching laser on/off with transistor

Hello,

after some time reading here, I have finally made a laser driver based on the LM317. I followed tutorial by rog8811: Laser driver - It can be done. It works very well, but I would like to replace the on/off switch by a transistor, so I can send a signal for turning on/off.

My question is: Which option is better, NPN or PNP? I was thinking on the TIP120-TIP127 family. But I'm not sure if a NPN switching the returning ground is a good solution or if a PNP switching the voltage IN is better.

So, what is you opinion?

Thanks a lot in advance.


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Old 03-09-2013, 08:12 AM #2
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Default Re: Switching laser on/off with transistor

Either will work, NPN's tend to be more common, and thus cheaper. It depends on your application. For example, if you were building this into a host where the host itself was ground, it'd be more convenient to use a PNP and switch the highside instead of trying to isolate the module from the host.
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Old 03-09-2013, 08:34 AM #3
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Default Re: Switching laser on/off with transistor

Hello,

Thanks for your answer!

If I connect like in the picture, is there somenthing wrong?
I have tested with leds and it binks as expected, but I'm not sure if I'm making in the bad way.

Thanks again.
Regards.
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Old 03-09-2013, 08:47 AM #4
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Default Re: Switching laser on/off with transistor

If that's a PNP transistor, yes, but if it's an NPN, no, it needs to go on the ground lead. So connect the LM317 ground to the collector, and ground to the emitter.

If you use a PNP, keep in mind that because it has no ground reference, in order to turn it on, you need to allow current to flow out of the base pin. This means that the base pin needs to be pulled (roughly) around 0.7V below the + rail voltage for it to turn on.

If you use a NPN, it's much easier to interface to things like microcontrollers.
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Old 03-09-2013, 10:09 AM #5
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Default Re: Switching laser on/off with transistor

If you haven't already built the driver, you can omit the LM317 altogether and just use transistors directly like this. One thing nice about the circuit in the link is that you don't need that large voltage drop across the LM317.
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Old 03-09-2013, 03:34 PM #6
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Default Re: Switching laser on/off with transistor

Hello,

thank you booth for you explanations.

I have already made driver based on the LM317, so cannot use the solution by Bionic-Badger. But it was interesting for a future.

Finally I'm working with the circuit in the picture. I think that now is as teached by Mr Things.

I can finally switch on and off the laser with the arduino.

Thanks again.
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Old 03-10-2013, 08:19 PM #7
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Default Re: Switching laser on/off with transistor

Hello,

I finished the driver.

I share here the final result. except the case, that is not finished.

The jumper allows to run in tow modes:
1. Arduino controlled (will work always in this mode).
2. always ON.

Regards.

By the way, this is my first circuit!!!
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