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Old 02-15-2008, 06:48 PM #1
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Default TTL / PWM experiment.

Hi there,

Rather new here, but here's something that a few freinds and I have been thinking about doing.

We want to test some communications over laser. Initally, we are going to modulate a laser at about 800Hz, 70% Duty cycle. I wrote a quick program for a PIC16 series microcontroller, and the TTL / Pulse Width Modulation works very well.

Now, on lab lasers that are marked as TTL compatible, what exactly does the laser want to see on it's TTL input? Does it want to see a TTL-compatible pulse train? I tried finding some data sheets / documentation to no avail.

The idea is to have a photodetector on the other end, and some supporting components. We were planning on having the photodetector drive a MOSFET, which in turn will drive a speaker. When the detector is hit by the pulsed laser, it should output a corresponding 800Hz tone on the speaker. I'm oversimplifying this, I know

So for cliffnotes, what type of signal does the LASER want to see on it's TTL line?

If we get this elementary comms. project going, we were going to try to get into this a little deeper, and encoding some voice using PWM.


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Old 02-16-2008, 05:03 PM #2
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Default Re: TTL / PWM experiment.

It depends on the driver obviously... they mostly have a blanking input that turns the laser off when pulled to ground, open connection or +5v being 'on' (internal pullup resistor i reckon).

As for TTL compatiblity, sometimes the low has to be quite low, below half a volt or so. It'd make sense to use a discrete transistor to pull it to ground, which will give you a nice inverted input that requires little current to operate (also cmos compatible).
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