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Topic: How to TTL a laser? (Read 35881 times) previous topic - next topic


Oct 11, 2013, 05:21 am Last Edit: Oct 11, 2013, 05:27 am by kerimil Reason: 1
So I want to TTL a laser to modulate it (36-38khz to be more precise) . The thing is I just want to be sure there isn't something I am missing.

Is it OK to just put a transistor on the output side ?
Check the schematic

Also what if I use a switching regulator such as LM2596?? would a similar circuit work as well or is there somethign different about them that I am missing ?

The laser has a capacitor across it's terminals - sure I got to get rid of it, but what could I do to protect it from surges then ??

ohh damn it forgot


What do you mean by TTL?
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switch it on an off at a high frequency - sort of like PWMing an LED
not sure why they reffer to it as TTL probably because control is usually achieved by 0 and 5V signals


Oct 11, 2013, 06:23 am Last Edit: Oct 11, 2013, 06:28 am by cjdelphi Reason: 1
TTL is transistor to transistor logic... (well for me anyway)

ok, i've never done this, i'm going from memory from what i've *read* here,  by using a divider you can get that clock speed apparently, or something near it, you could use PWM out to a transistor via a 1k resistor, have that go to the base of a npn, how powerful is it?... if it's a 100watt green laser, you'll be looking at mosfets, if it's a small 5mw laser, you could simply use a low ohm resistor, it really depends on what you have to power it..

How to protect it, again what's the current? many different methods, but i'm a little confused, at 32khz the capacitor would not be "buffering" it would start behaving more like a wire i'm trying to work out what it's for what kind of laser is this? solid state? or a gas laser?


That would work, but it's better to put the laser below Q1 so the laser gets a full 5v along with the resistor, and the transistor does not work as hard..


Check out the modification....

I swapped them around, otherwise the transistor would waste power.


Oct 11, 2013, 07:02 am Last Edit: Oct 11, 2013, 07:13 am by kerimil Reason: 1
It's a solid state laser - IR to be more precise - 808nm. You can pretty much guess what I want to achieve - IR + 38khz modulation ;-)

ohh yeah and I pretty much forked up the circuit since I am going to use much higher value resistor to limit output power. I posted 10 ohm as that's what I currently use with my other laser (a red one - 250mW output I got no problems with powering it). The IR laser is 150mW - I am going to feed it slightly above the threshold level  (minimal needed power for it to properly 'light up' so to speak) since I don't need power

Thx for help


I.R. LASER diode needs to be fed 2.2 volts, not 5 volts.  You need a driver (constant current source) instead of that 5volt linear regulator, if you are talking a bare diode (not a module, where the driver is included).  If you just want to drive it at 38KHz, you could use a NE555 oscillator to drive a MOSFET switching power to the LASER driver circuit.  Since I'm betting that you want to use the LASER in I.R. Commo/remote-ctl., You might want to look into sourcing a driver for 808nm LASER diode, really cheap, and most already have provision for modulation (if it comes with a button switch, you remove the switch and replace it with a MOSFET(2N7000 I think) 'S' to + side, 'D' to return side, modulation signal to 'G' through a 1k resistor).  If that's too much for you, try looking at LASERPOINTERFORUMS.COM, and do some searching and reading.  Good luck and BE CAREFULL!!


I plan to use arduino as the source of 38khz modulation and use it to generate IR signals using IRremote library.

Now as far as voltage goes... isn't it that 2.2V is just voltage drop and I've got to use the follwoing formula:

Resistance = (V supplied- Vdrop)/laser power


I.R. LASER diode needs to be fed 2.2 volts, not 5 volts.  You need a driver (constant current source) instead of that 5volt linear regulator.

If you look at the circuit, he IS using a constant current source (and constant voltage) WITH that 5v linear regulator.

Secondly, 5v or 2.2v, big whoop, a PWM switching method is preferred over linear, already been stated...


You only need to supply the LASER diode with the Vf (drop of 2.2Vdc, you are correct), not 5V. BUT, you NEED to regulate the current supplied to be sure 1) you stay over the Threshold current for lasing for that diode, 2) you stay below the max current for the device, and 3) you maintain a consistiant On siginal for discrimination in comm and remote-ctl recvr ckts (digital modulation, as in TTL modulation (at TTL voltage levels) is full on and full off; analog modulation varies to some points between the two).  
So,,, you dont use a voltage regulation in the driver circuit, you use current regulation.

A boost regulator (switching regulator) will work, but watch out for switching transients (spikes) if you are using a moderate to low power LD (i think one way to suppress this issue is to design with as large a cap as you can, and use something like a 10uF cap across the LD, you'll need to read up to check that value, using foggy memory).

Some of us over on LPF have experimented with using 808nm LD's for night vision illum and I.R. remoting.  If you are thinking of using an un-collimated diode for greater siginal dispersion (wider field exposure to the recvr), you'll find that high output I.R. LEDs work much better.  If you are planning on collimated output, you'll find that you can get by with a lower power diode, for the same distance, but emitter/recvr alignment is a bitch.  Again, good luck with the project and BE CAREFULL with the LD's (you won't know how much exposure you are getting from the output and reflections, etc.


sorry for the doubble post, was writing the above diatribe while you posted,,,

I did look at the circuit.  He is using the 7805 in it's Voltage regulation configuration, look at the datasheet.  in that configuration, you only reduce incomming voltage to the regulated limit of 5Vdc, as long as the input voltage is above 5Volts pluss the 2volt drop-out.... The current is limited to a maximum of 1A draw before the regulator starts to shut-down.  if the current supply drops on input, it drops on output also...  In the current regulation configuration the 7805 type linear regulator will attempt to keep the current output at the same value, as long as input current is above the set value and Vin is above 5V + dropout...  see the datasheet.


In that circuit i see a constant 5v regulator and a constant current of 300ma.....

So what are you saying?


I.R. LASER diode needs to be fed 2.2 volts, not 5 volts.  You need a driver (constant current source)

But a series resistor is one perfectly good way to get constant current drive...
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


Good luck and BE CAREFULL!!
Indeed. From what I understand, visible light laser are less dangerous because we tend to avert our eyes so exposure it limited to a fraction of a second. But we can't see IR, so damage can occur before you even know what happened.

According to repairfaq.ece.drexel.edu/sam/lasersaf.htm "A 100 mW laser diode operating on battery power can blow a hole in your retina"

Hopefully the OP is using his 150mW IR laser in controlled conditions and with protective eyewear. Waving this thing around in public to turn off TVs and such sounds like a good way to blind innocent people.

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