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Old 08-02-2011, 04:44 PM #1
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Question TTL capable driver based on 2 LM317(2 DDL drivers) possible?

Hey! Check picture of circuit next post

Im thinking in doing a RGB projector, while first im going to get all the diodes working, but as I am thinking in enabling TTL modulation for at least the red and blue lasers (the green would be tricky, due to the fancy optics).

Ive been thinking if anyone (although i searched for it, and came with nothing) tried to do a "dual ddl" driver, just so he/she could get more juice into one of those 445 diodes. I've seen dual flexes, but iirc, no double ddl's.

Why am i asking this?? There is a really simple way of getting the TTL 5V voltage higher (to ur battery/PSU voltage) using a darlington array (we used them in a microcontrollers lab with that exact purpose: 5V in and at out=PSU, 0V in @out=0V) and the datasheet of those ULN2804 can be found here.

At least thats what i think they were doing...i dont know much about electronics...im a big n00b in that field, but i was thinking in having a DDL circuit feeding the bias (threshold) current, and then another DDL circuit delivering the rest of current when turned on/off.

Those ULN2804 are "much cheaper" than a LM317 and it has 8 ins/outs, and if u need more current, u can split ur signal into 2 and use 2outs in series to have 2x load current maximum.

Before even knowing if this is possible my first problem would be "isolating" both DDL circuits from each other, if needed. So that current would only be going to the diode, and not to the other DDL circuit.


I will even take this boat and ask about using a LPC-815 with a DDL driver, as well as a 445 LD (separate driver), and as i would be using a wall-plug AC/DC converter, they would have same '-' (gnd). I read something about problems with different voltage casings, but in this case i dunno if the 445 case has any influence or if not, or the case of them having different cases, which would cause a problem using same ground for the DDL circuits (thats what it seemed from what i read...there would be a problem). And how does the white fusion kit works, if the whole thing is made of Aluminium, the greens module casing is case positive and the reddies is case negative...or are they insulated in any way??

And last question, as more of a safe side then question. Im still thinking if i would lase my reddies both with same current (so series) or if in separate (im doing a dual PBS combined LPC-815). To use a DDL and both LD's connected in series to the DDL i would just need to feed the LM317 with at least ~2x3v(each LPC voltage drop)+1.25V+3V(from LM117)=~10.25V, if iirc everything from that circuit.

Oh! Just last thing, as to the capacitor value (throughout the designs in the internet, that capacitor is in the 10 milifarad range) would start cutting frequencies around the kHz, am i wrong? for TTL modulation, at around 10kHz that would be a problem...and it would have to be pushed to the uF region or even lower, no? Do you reckon voltage spikes to be of higher frequencies than just a few us??, cuz thats in the MHz range and we could keep it off with lets say 10uF...anyone please corect me if im completely wrong...

thanks for reading, and hope i was clear enough.

EDIT
Edited a few errors i had


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Old 08-08-2011, 10:16 AM #2
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Default Re: TTL capable driver based on 2 LM317(2 DDL drivers) possible?



no one??

EDIT:
i decided to add a picture, as it seems either my description is completely not understandable, or people are only able to read circuits (which is a good thing!)

(noticed now i mislabeled the LM317...current comes out of Vout, not Vadj, so just think of Vout and Vadj as switched)
So, i would calculate R2 to give lets say 20mA for laser threshold, and in case of a LPC-815, R1 to 200~300mA. Maybe use some diodes where i said to ensure correct current flow, and cap would be to take out ripples...when TTL signal comes at Vin in Darlington array (forgot to add the Vin of darlington array in the pic), it makes Vout=Vcc. If Vin=0V, then Vout=0V. So i would feed the LM317 with usable voltage.

Do you guys think this would work?? is LM317 capable of fast switches...also the DA?? i dont know where to find that in the spec sheets...(spec sheet of darlington array in previous post)
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Old 08-09-2011, 09:04 AM #3
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Default Re: TTL capable driver based on 2 LM317(2 DDL drivers) possible?

and now i have to bump this again to the front! im so sorry for this! its not my intention, but i really want to know, and if i dont bump it, this will just go further down, and down and down...

so
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Old 08-09-2011, 09:44 AM #4
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Default Re: TTL capable driver based on 2 LM317(2 DDL drivers) possible?

hm some where in the past i had a schematic that showed you could turn a lm317 into ttl by grounding the adj pin.
give me some time to find that.
found that post
Building a driver.
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Old 08-09-2011, 10:03 AM #5
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Default Re: TTL capable driver based on 2 LM317(2 DDL drivers) possible?

Quote:
Originally Posted by kiyoukan View Post
hm some where in the past i had a schematic that showed you could turn a lm317 into ttl by grounding the adj pin.
give me some time to find that.
found that post
Building a driver.
After reading it, i noticed its a TTL based solely on "full on"/"full off", in the case of my (maybe completely useless) schematic you would keep LD at threshold, and when u had signal, you would get it on. No signal=off.
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Old 08-09-2011, 10:05 AM #6
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Default Re: TTL capable driver based on 2 LM317(2 DDL drivers) possible?

i dont think that matter if its at 0v or right before the lase thresh hold. the time it takes to go back to lasing is instant.
but that one there was the simplest way to get a ttl lm317.
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Old 08-09-2011, 11:20 AM #7
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Default Re: TTL capable driver based on 2 LM317(2 DDL drivers) possible?

well, its the difference between a few Hz and KHz modulation...if u wanna go to higher freqs you need faster laser response...of course for like 500Hz you do not need to get the laser on threshold, u can simply shut it off...while lets say to 2kHz maybe u do, or the line will look "fady" (instead of a solid line, u got a ramp in brightness)
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Old 08-09-2011, 02:41 PM #8
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Default Re: TTL capable driver based on 2 LM317(2 DDL drivers) possible?

I'm using this schematic to modulate my red 250mw and so far, so good. It includes a resistor that you would experiment with to determine it's max value to set the "off" current, and the other resistor you can calculate (see the schematic) to determine the Max "on" current.

It uses two 2N2222A transistors, but any good NPN that can handle the current requirements you need and has a known Vbe (which you need to calculate the resistor for Imax) should work. Shoot for a transistor with a Vbe around 0.7v.

This circuit works for me up to 10kHz, but I can't imagine why it wouldn't work beyond that if necessary.

FWIW, I'm driving this circuit with a Parallax Propeller chip which I use to produce independently programmable frequency and duty cycle pulses. The output of the Propeller is around 3.3vdc, going into that first transistor's base.

The two transistors in the configuration shown work as a constant-current regulator - as current starts to flow in the top transistor, it starts to bias the lower transistor, which then starts to short out the current at the base of the first transistor, limiting the current flow through the diode to a level set by the resistor.

I've checked this circuit on my meter for voltage and current and it seems really nice. On my scope the waveform passed is a faithful reproduction of the input waveform. Note that since I do occasionally need to pass higher frequencies, I do not have a capacitor across LD1.

It should also be noted that I run this circuit off of batteries only (4 x AA) so there's very little chance of V+ unexpectedly exceeding 6 volts, but even if it did - it should not spike the output.

Let me know if this helps - or if anyone sees any issues I hadn't considered here.

Dave
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