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Using Arduino => General Electronics => Topic started by: kerimil on Oct 11, 2013, 05:21 am

Title: How to TTL a laser?
Post by: kerimil on Oct 11, 2013, 05:21 am
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
Title: Re: How to TTL a laser?
Post by: cmiyc on Oct 11, 2013, 05:51 am
What do you mean by TTL?
Title: Re: How to TTL a laser?
Post by: kerimil on Oct 11, 2013, 05:59 am
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
Title: Re: How to TTL a laser?
Post by: cjdelphi on Oct 11, 2013, 06:23 am
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?
Title: Re: How to TTL a laser?
Post by: cjdelphi on Oct 11, 2013, 06:27 am


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..

Title: Re: How to TTL a laser?
Post by: cjdelphi on Oct 11, 2013, 06:35 am
Check out the modification....

I swapped them around, otherwise the transistor would waste power.
Title: Re: How to TTL a laser?
Post by: kerimil on Oct 11, 2013, 07:02 am
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
Title: Re: How to TTL a laser?
Post by: 123Splat on Oct 11, 2013, 04:47 pm
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!!
Title: Re: How to TTL a laser?
Post by: kerimil on Oct 11, 2013, 05:10 pm
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
Title: Re: How to TTL a laser?
Post by: cjdelphi on Oct 11, 2013, 06:23 pm

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...
Title: Re: How to TTL a laser?
Post by: 123Splat on Oct 11, 2013, 06:40 pm
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.
Title: Re: How to TTL a laser?
Post by: 123Splat on Oct 11, 2013, 06:48 pm
cjd,
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.
Title: Re: How to TTL a laser?
Post by: cjdelphi on Oct 11, 2013, 07:47 pm
In that circuit i see a constant 5v regulator and a constant current of 300ma.....

So what are you saying?
Title: Re: How to TTL a laser?
Post by: MarkT on Oct 11, 2013, 08:04 pm

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...
Title: Re: How to TTL a laser?
Post by: tylernt on Oct 11, 2013, 08:16 pm
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.
Title: Re: How to TTL a laser?
Post by: 123Splat on Oct 11, 2013, 09:07 pm

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

So what are you saying?

In the circuit, as drawn, I see a 7805 linear voltage regulator being fed by a 1.5V cell.  But assuming that the poster meant that the source voltage was, infact 7V or above, as required by the regulator, the regulator will supply 5V out, as long as the supply can keep input at, or over, 7V; at a MAXIMUM of 1A draw, as long the supply can supply 1A or more.  The output is only regulated to 5V at a max of 1A.  if the input current (source battery depleation, for example) drops below 1A, so does the output current.  One Amp in, through 10 Ohms (disregarding losses in the transistor) is 500mA, not 300mA. What I am saying is read the datasheet.

MarkT,
"But a series resistor is one perfectly good way to get constant current drive..." for a first year E.T. student, or maybe more accurately said, Until the resistor heats up and changes value (remember what lousey temp coefficients resistors have?).

I do a lot of playing around LASER diodes at various wavelengths, and the first rule (after use your goggles) is ALWAYS use a constant driver driver circuit.  Then you worry about things like heatsink requirements...
You CAN depend on a resistor for your current control. if you want to (they call it 'Kip Kay'ing'), but you are gonna loose more diodes to over-current-burn-outs than you will to ANY other cause.

You do it however you want. Listen, or not, read the datasheet, or not  I did my part. See ya!

Title: Re: How to TTL a laser?
Post by: Grumpy_Mike on Oct 11, 2013, 09:16 pm
There is a lot of rubbish in this thread but fortunately 123Splat is speaking sense.
Quote
You do it however you want. Listen, or not, read the datasheet, or not  I did my part.

Thanks.

If you want more info about driving lasers see:-
http://www.repairfaq.org/sam/laserdps.htm (http://www.repairfaq.org/sam/laserdps.htm)
Title: Re: How to TTL a laser?
Post by: kerimil on Oct 12, 2013, 08:46 am
Thx for your input - will look into it. Though I think it's a more profound problem for those who run everything at max rated current.  It's naive to assume that there is some magic threshold below which everything is fine and then 2mA above it you've got a burnt diode. Not that I am saying that you aren't right - quite the opposite I think current regulating is such a big deal because of running everything at max rated current
Title: Re: How to TTL a laser?
Post by: cjdelphi on Oct 12, 2013, 09:54 am


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

So what are you saying?

In the circuit, as drawn, I see a 7805 linear voltage regulator being fed by a 1.5V cell.  But assuming that the poster meant that the source voltage was, infact 7V or above, as required by the regulator, the regulator will supply 5V out, as long as the supply can keep input at, or over, 7V; at a MAXIMUM of 1A draw, as long the supply can supply 1A or more.  The output is only regulated to 5V at a max of 1A.  if the input current (source battery depleation, for example) drops below 1A, so does the output current.  One Amp in, through 10 Ohms (disregarding losses in the transistor) is 500mA, not 300mA. What I am saying is read the datasheet.


MarkT,
"But a series resistor is one perfectly good way to get constant current drive..." for a first year E.T. student, or maybe more accurately said, Until the resistor heats up and changes value (remember what lousey temp coefficients resistors have?).

I do a lot of playing around LASER diodes at various wavelengths, and the first rule (after use your goggles) is ALWAYS use a constant driver driver circuit.  Then you worry about things like heatsink requirements...
You CAN depend on a resistor for your current control. if you want to (they call it 'Kip Kay'ing'), but you are gonna loose more diodes to over-current-burn-outs than you will to ANY other cause.

You do it however you want. Listen, or not, read the datasheet, or not  I did my part. See ya!




I was taking into acount the forwarf voltage drop of a typical 2v diode.
Title: Re: How to TTL a laser?
Post by: kerimil on Oct 12, 2013, 10:56 am
Awesome now I am lost... forward voltage drop has to be taken into consideration, right ? so I should choose resistor value based on:

(supply voltage - forward voltage drop of the LD) / LD planned current

...precisely what I wrote earlier in the thread.

What's more, while temperature has effect on value of resistors I found that the temperature coefficient of most resistors is 0.0050% per 1 deg C. So in other words if the temperature increases by 100 deg C the change in resistance will be 0.5% so way below tolerance of most resistors.

so... who is right ??
Title: Re: How to TTL a laser?
Post by: Grumpy_Mike on Oct 12, 2013, 12:53 pm
Laser diodes are a lot more complex than LEDs. The forward voltage drop changes with temperature, that is why a resistor will not cut it. Read the link I posted.
Title: Re: How to TTL a laser?
Post by: MarkT on Oct 12, 2013, 01:29 pm
It does if you have enough voltage drop (12V supply to blue laser for instance),
although its crude and wasteful of power, its good enough as a constant current
source - you make sure you aren't pushing the diode too hard of course...

Ideally a DC-DC converter + constant current driver would be employed, indeed,
but thats extra cost and complexity and the budget went on the laser diode and a 12V
supply was already there...
Title: Re: How to TTL a laser?
Post by: runaway_pancake on Oct 12, 2013, 03:32 pm
Why not link to some data regarding this "laser module" of yours?
To some it's about a loose "laser diode", I guess, while others figure it's a prepped ready-to-go "module" (like for "pointers") or something.

Title: Re: How to TTL a laser?
Post by: kerimil on Oct 23, 2013, 04:29 am
Now here is another question related to this topic. It might be a bit stupid but anyway...
Can I use a multimeter to check polarity of a laser diode ? (using resistance setting)

Can I check it's voltage drop using a "Diode check" function that some multimeters have ?
If, yes I assume I might as well test it by measuring voltage across it when more current is passed through it - as described here ->
Title: Re: How to TTL a laser?
Post by: Grumpy_Mike on Oct 23, 2013, 09:16 am
Quote
Can I use a multimeter to check polarity of a laser diode

Probbley, I do this to test LEDs especially surface mount ones.

The diagram is the way to measure the volts drop.
Title: Re: How to TTL a laser?
Post by: kerimil on Oct 23, 2013, 12:09 pm
Thx. Yeah the pic shows an alternative method because diode check function supposedly provides voltage drop value of the diode, but because there's not much current flowing it is a bit off.
Title: Re: How to TTL a laser?
Post by: Grumpy_Mike on Oct 23, 2013, 03:02 pm
When I test an LED with the resistance or continuity function of a meter the LED glows very dimly when the red meter wire is on the anode and the black is on the cathode.
Title: Re: How to TTL a laser?
Post by: 123Splat on Oct 24, 2013, 08:42 pm
If you are talking LASER diode in a TO type can, looking at the rear (bottom), you will notice that there are three notches in the rim (Base) of the can, two V shaped and one Rectangular.  If you hold the can with the square or rectangular notch to the top (12 o'clock position) and the V shaped notches at 9 o'clock and 3 o'clock, you will see three or four pins: pin #1 at 9o'clock, #2 at 12 o'clock (attached to the case), #3 at 3 o'clock, and, if there is one, #4 at 6 o'clock.

USUALLY, for I.R. diodes (also what is used in 'Green DPSS modules), the case (and pin#2) are positive. For reds,blues and most violets, the case is negative.  Pin #1 will usually be negative for I.R. and POSITIVE for the rest.  Observe current and static electricty precautions. Don't look into the light.......
Title: Re: How to TTL a laser?
Post by: 123Splat on Oct 24, 2013, 08:44 pm
Also, not usually a very good idea to use the multimeter in Ohms setting to check LASERs. Could cause current damage.
Title: Re: How to TTL a laser?
Post by: polymorph on Oct 24, 2013, 08:45 pm
A bare LASER diode typically has a built-in photodiode for power feedback. So make sure you are actually measuring the LASER diode itself.

However, if it has the controller circuit board on it, you can't just check it with a continuity meter.
Title: Re: How to TTL a laser?
Post by: polymorph on Oct 24, 2013, 08:47 pm
123Splat, I think the idea of an Ohmmeter damaging diodes is a holdover from the old analog meter days. In the lowest resistance setting, a higher current flowed through them, and the old low current Ge diodes and transistors could be damaged. So the recommendation was to use the next higher Ohm setting.

That has not been an issue for modern DMMs. However, I will say that I cannot speak for all the piece of cXXp cheapo DMMs out there.
Title: Re: How to TTL a laser?
Post by: 123Splat on Oct 25, 2013, 04:04 pm
Polymorph,
Look at the avatar,,,, how modern do you think I am?......

Good point, but I tend to go on the safe side with LASER diodes, most of the ones I play with are about $50 a whack.  Smoke hurts a little more than on a $1 or less transistor...

Side note:  Most manufacturers stopped building in the photodiode a couple of years ago.  unless you are getting an older unit, or scavenging out of old equipment, you will no longer find pin 3 or 4 connected to the photodiode (use external optical limiting nowdays).
Title: Re: How to TTL a laser?
Post by: polymorph on Oct 25, 2013, 04:33 pm
Safe side - good point. I'm thinking of the $5 LASER pointers.

I didn't know they were leaving out the photodiodes, now.

As for modern - the first LASER diode I owned was $27 in the late '70s, a fortune, and required something like 2 or 3A to reach LASER threshold but could only sustain a very short pulse. So an SCR would dump a charged capacitor into it. If it didn't reach the lasing threshold, almost the entire capacitor energy would go to heat and blow the diode. Absolutely forbidden to use a VOM's ohmmeter scale to test it.

I only fired it a few times, then made a mistake and didn't let the capacitor charge high enough. Crack, no more LASER.
Title: Re: How to TTL a laser?
Post by: cjdelphi on Oct 26, 2013, 02:21 am
First laser was 1995 for me, and my last powerful one was a 200mw greenie,  burnt bags lit matches, then after almost losing my right eye,  i tried to increase its power and killed it $150 put me off lasers for a while, switched to high powered flashlights . .

I do love uv lasers though and green glow in the dark paper, draw by light.
Title: Re: How to TTL a laser?
Post by: kerimil on Nov 07, 2013, 06:28 am
ohh if anyone is curious - yes, you can find polarity of a LD with a multimeter

Quote
USUALLY, for I.R. diodes (also what is used in 'Green DPSS modules), the case (and pin#2) are positive. For reds,blues and most violets, the case is negative.  Pin #1 will usually be negative for I.R. and POSITIVE for the rest.  Observe current and static electricty precautions. Don't look into the light.......
on the ones I have the case is negative and the 9 o'clock pin is positive...
BTW they're kind of scary to be honest - cut through black paper but are totally invisible to the naked eye
luckily I've got a cheap webcam that pics it up well. I am not even in the same room when I power it on. 
Title: Re: How to TTL a laser?
Post by: PoppyAnn on Jan 06, 2015, 02:30 pm
Hi to all if anyone is still monitoring this page any longer,

I have just come across the same problem (i think) i have a 3W laser diode which i want to use on a cnc machine i am building the main use for the machine is as a 3D printer but i want to add either a laser engraver or laser cutter by fitting a second head to it.

at the moment I am playing about with a 1W diode and have a laser driver/power supply which states it can be controlled by a TTL input operating at 0= off 5v = on i just do not know what a TTL input is i remember from my distant past that TTL = Transistor Transistor Logic but it is so far back in my past i forgot what it is, is it just a 5 volt signal which is switched on and off at different speeds or is it 0-5 volt adjusted in between the 0 to 5 volt to get different outputs from the laser?

this is the driver/power supply i am using :-

http://www.aliexpress.com/item/Power-Supply-Driver-f-808nm-850nm-980nm-IR-Infrared-Laser-Diode-TTL-200mw-1000mw/696788740.html

also if anyone can answer is the power supply just a current voltage regulator or does it switch at different frequencies as i want to start using a 3W diode which i have on order but i was wondering if i could just use a constant current/voltage regulator set to the correct requirement to do the same job as i have a couple of these that i was thinking of using:-

http://www.aliexpress.com/store/product/DC-5V-32V-to-0-8V-30V-5A-Buck-Converter-Constant-voltage-constant-current-LED-Driver/215739_1587654141.html

as these appear to give the correct current and voltage is there any way i can fit something like a solid state relay to control switching it on and off?

I dont have problems with building things like 3D printers rc quadcopters etc but my knowledge of electronics and programing is sadly lacking and at my age it is getting harder and harder to learn new things even with my having lots of time due to not working due to disability caused in the army so plenty of time + little knowledge = lots of blue smoke when trying new things but it passes the time.

any help gratefully received,

regards Poppy Ann.
Title: Re: How to TTL a laser?
Post by: raschemmel on Jan 06, 2015, 04:15 pm
Laser Diode circuit -1 (http://www.instructables.com/id/Arduino-Laser-Engraver/step5/Laser-Diode-Driver/)

Laser Diode Circuit -2 (http://forum.arduino.cc/index.php?topic=21852.0)
Title: Re: How to TTL a laser?
Post by: Shanjaq on Oct 26, 2015, 11:04 am
Does this look like a decent TTL Laser Driver?  It uses an op-amp to servo the current using the base-emitter threshold voltage of an NPN transistor as a reference, relative to a sense-resistor under the main transistor and diode.  Very simple, just 3 common parts and some resistors:

TTL Laser Driver (http://www.falstad.com/circuit/circuitjs.html?cct=$+1+0.000005+0.13386567243530942+60+5+50%0AO+176+336+240+336+0%0Ar+128+384+128+432+0+10000%0Aa+208+400+304+400+1+5+0+1000000%0Ar+208+416+208+448+0+10000%0Aw+352+416+352+448+0%0Aw+352+448+208+448+0%0Ar+352+448+352+496+0+5%0Aw+208+384+176+384+0%0Aw+176+384+176+336+0%0A162+352+320+352+384+1+2.1024259+1+0+0%0AR+352+320+352+288+0+0+40+9+0+0+0.5%0Aw+176+384+128+384+0%0Aw+128+432+128+464+0%0Aw+128+496+128+464+0%0AL+80+384+32+384+0+1+false+5+0%0Ag+352+496+352+512+0%0At+208+448+176+448+0+1+-0.000012670083773258511+0.5728313383722027+100%0Aw+176+432+176+384+0%0Aw+176+464+176+496+0%0Aw+128+496+176+496+0%0Aw+176+496+352+496+0%0Ar+128+384+80+384+0+4700%0At+320+400+352+400+0+1+-5.772514172152878+0.6941770389508432+100%0Aw+304+400+320+400+0%0Ao+0+2+0+290+0.8949657474523425+9.765625000000002e-255+0+-1%0Ao+9+1+0+289+1.093625362391506+0.17498005798264096+1+-1%0Ao+9+1+1+291+0.3125+0.00009765625+1+-1%0A)

I = Vbe / R

Vbe for NPN (check datasheet, commonly between 0.66 and 0.7) is ~572 in this sim, so that makes:
114.4 Ma = 0.572 / 5
Title: Re: How to TTL a laser?
Post by: raschemmel on Oct 26, 2015, 01:35 pm
What does "out" represent ? Shouldn't the sense ressitor be tapped at the end not connected to ground instead of on the collector of that transistor ? (I guess I'm missing something)
Title: Re: How to TTL a laser?
Post by: Shanjaq on Oct 26, 2015, 07:00 pm
Oh yes, I forgot to remove that scope probe labeled "Out", for which you can see the trace when running the simulator from the link (right-click the probe and select "view in scope".)  It's basically showing that the control transistor is working to balance the sensed voltage to the internal reference voltage (~0.572V in this case) created by its base-emitter junction.  You'll need to vary this according to the Vbe value/range found in the datasheet for your particular small-signal transistor.

The sense resistor is nearest to ground so it's always relative to the reference voltage, even when the power transistor is in cutoff.  This places it on the Emitter of an NPN, which is technically high-side switching but it's not really that high; just above the reference voltage yet still pulling the LED/Laser down from its positive rail.
Title: Re: How to TTL a laser?
Post by: raschemmel on Oct 26, 2015, 07:33 pm
I didn't see any connection out from the sense resistor.
Title: Re: How to TTL a laser?
Post by: Shanjaq on Oct 26, 2015, 07:47 pm
Is the sim link loading an incomplete schematic?  Falstad has been known to do that on occasion..  If so, the image attachment should suffice, it's the 5-ohm resistor under the Power transistor to the far right (selected to pull up to the reference voltage ~114mA,) which is connected to the Base of the Control transistor that pulls the non-inverting input of the op-amp down when actively regulating.
Title: Re: How to TTL a laser?
Post by: raschemmel on Oct 26, 2015, 08:01 pm
Yes, I SEE the sense resistor. I'm saying that if were a real circuit , more than likely you would have a wire coming off that going to the analog input pin to measure the current.  I can see the
voltage drop across the sense resistor is used for the drive circuit.
Title: Re: How to TTL a laser?
Post by: Shanjaq on Oct 26, 2015, 08:13 pm
Oh!  I see what you're getting at.  The circuit posted earlier has a static current setting based on the value of the sense resistor and the Vbe threshold of a small-signal transistor.  The op-amp is what uses the current-sense input internal to the circuit, not an external microcontroller.  (I would not trust the resolution of a uC's ADC or latency/stability of its software to servo the current fast enough and with enough precision to survive any unforeseen changes in supply voltage or thermal resistive changes in the diode, but that's a personal preference since I only have the older Arduinos & some super-slow 8-bit Atmel's..)
Title: Re: How to TTL a laser?
Post by: raschemmel on Oct 26, 2015, 10:13 pm
Comparator
Vo = 1 if V+ > V -
Vo = 0 if V + < V-

For H = 5V,
V+ = 3.4V (due to voltage divider)
If current through R = 0.140 A, Vbe = 0.7V, and V+ = 0
thus the comparator output is driving the led driver transistor between 0.5  and 0.7 V.
If the the feedback transistor turns on completely, V+ will be pull below V- , shutting down
down the output of the comparator.

At least that's how I see it. If I am wrong, please correct me.
Title: Re: How to TTL a laser?
Post by: Shanjaq on Oct 26, 2015, 10:38 pm
Comparator
Vo = 1 if V+ > V -
Vo = 0 if V + < V-

For H = 5V,
V+ = 3.4V (due to voltage divider)
If current through R = 0.140 A, Vbe = 0.7V, and V+ = 0
thus the comparator output is driving the led driver transistor between 0.5  and 0.7 V.
If the the feedback transistor turns on completely, V+ will be pull below V- , shutting down
down the output of the comparator.

At least that's how I see it. If I am wrong, please correct me.
That sounds about right.  So since a state in which the 5-ohm sense-resistor is passing 140mA would pull it higher than the Vbe reference voltage, this should cause the control transistor to turn on, which pulls V+ down, thus causing the op-amp to turn down/off the LED driver transistor.  Once the current across the sense-resistor drops to ~114mA, the control transistor should begin to turn off, which would cause the op-amp to begin turning the LED driver transistor on, thus it's a feedback-stabilized control loop.

I = Vbe / R

Since the op-amp is intended to be hooked up to the 5v logic supply, its output could potentially swing from 0 to 5v depending on the base currents required to pass the desired collector current through the LED driver transistor.  The LED power supply can be any voltage that won't dissipate too much power through the driver transistor, something like 9-24V seems reasonable.