2 Beam Optical Pulse Generator

[WOZZY-2010]

Greetings,

I developed this pulse generator to create an Optical High Resolution Timer and Speed Measuring device. It uses two laser beams as gates.

http://lh5.ggpht.com/_vEhjpGGusOY/S2o89lEQm4I/AAAAAAAAAKI/ADzYhWnWo2I/2%20Beam%20Pulse%20Generator.jpg
2-BEAM OPTICAL PULSE GENERATOR
(download high resolution PDF file below)

I've been having a great deal of fun with it and thought I'd share it with you all.
I have no way of measuring the optical lag, however the circuit itself is quite fast and produces a clean output signal. Any optical lag should cancel, as the start and stop signals are treated the same way. Here's a scope shot that shows the response.

http://lh5.ggpht.com/_vEhjpGGusOY/S2o89me86dI/AAAAAAAAAKM/zKQRwrFRBxk/Scope.JPG

It takes two individual OSRAM optical sensor outputs and produces a single pulse that rises on the rising edge of the start sensor, and falls on the rising edge of the stop sensor. The OSRAM sensors are very nice, they are sensitive to all optical wavelengths and have a built in Schmitt trigger.

This circuit which uses a Set-Reset Latch built from feeding back two NAND gates on the 74F00. This allows me to bring the signal into a single PIC pin. I then can use timer-1 to determine the pulse width. Here's an image of the prototype circuit:

http://lh3.ggpht.com/_vEhjpGGusOY/S2o89dqreEI/AAAAAAAAAKE/okXmM2X7VaM/2%20Beam%20Optical%20Pulse%20-%20Small.JPG

Anyway, I've learned so much from this forum that I thought It was time to reciprocate a little. Please feel free to comment on the circuit design, as I'm always learning.

The rest of the project is going pretty well, and now I'm down to just a few tweaks to optimize the code to allow for error corrections and to maximize precision. (I'll post the full code and PIC portion of this project soon)

Thanks to all the great educators, patient teachers and generally really smart folks on PBP Forums.

Bob

[WOZZY-2010]

Notes on 2-Beam Optical Pulse Generator Circuit.

The cross wired 74F00 NAND Gate forms a Set-Reset Latch due to feedback from the opposing output.
The output signal goes high when the first sensor is activated.
The output remains high until the second sensor is activated.
It is then reset, and ready for next cycle, no arm or reset button is necessary.
The first sensor needs to be clear, before the second sensor is activated.
Note that direction does matter, the circuit will not work if the sensors are activated in reverse order.

This circuit can be quickly converted to a single sensor pulse generator, by fixing the Second sensor in active state, or shorting its output to ground. The Pulse Out will then follow the First Sensor only.

OSRAM SFH-5840 is Active LOW, used in this circuit (Due to availability)
OSRAM SFH-5841 is active HIGH, could simplify circuit by not requiring the inverters.
The SFH-584X is sensitive to broad spectrum visible wavelengths, nearly any visible light source can be used.

74F14 Hex Inverter includes Schmitt Trigger.
74F04 Hex Inverter could be substituted because the SFH-5850 has a very clean output.

If using remotely mounted sensors, remote mount the Indicator LEDs nearby to facilitate the optical alignment.
(Probably really only need one LED per sensor)

This circuit can be used as a latching pulse generator for nearly any TTL or 5V sensor or input in lieu of the SFH-584X.

Average Speed = d/t
The average speed through the two gates.
d = distance between photo gates.
t = time output pulse is high.


--Bob

[mackrackit]

Hi WOZZY,

Thanks for sharing this. It give me some ideas...

[mtripoli]

Thanks for sharing. If this was some project where you had to use flip-flops or something then I get it. However, (and I know I'll take crap for this), I have to ask; couldn't all of the above been done with just one PIC (pick your type and flavor)?

[WOZZY-2010]

mtripoli,

You're probably right.

However the 74FXX chips cost less than $0.20 each.
Their switching speed is phenomenal...on the order of 3nS
My PIC timer project has a measurement resolution of 200nS.

Do you think that the PIC could react this fast?

This circuit kind of evolved from an earlier circuit that could control the Set and Reset function of a precision bench-top frequency counter / timer that I have. That controller needed 2 S-R latches. one of which I also use to control PIC timer for comparison

I can use the bench-top unit to determine error corrections for the PIC timer

The PIC timer uses CCP1 and TIMER1 instant interrupts to count clock pulses from a single pulse on a single pin.
The core engine of the was put together by Bruce, Based on Darrel's instant interrupts.
See: http://www.picbasic.co.uk/forum/showpost.php?p=23401&postcount=3

Thanks for the suggestion.
Bob W.

[mackrackit]

I like this for a couple of reasons.

Most of the logic is done outside of the PIC. The latching of one till the other is triggered and re-setting when the second is triggered seems like some code load would be taken from the PIC.

A one PIC pin solution. Often we find that more I/Os are needed, leading to multiple PICs or larger PICs just for a pin or two. Off the top I would probably use two PIC pins if I were doing this PIC only.

Many ways to do anything. This project is a good reminder that a MCU is not always needed or can be complimented with external circuitry.

Now if I could just remember how to make LEDs blink without a MCU :)

[WOZZY-2010]

I posted the Code and PIC schematic part of this project in Code Examples Here:
http://www.picbasic.co.uk/forum/showthread.php?t=12634

-- Bob

[marvelaugusts]

Is this true, An optical top hat pulse generator controlled by solitons. The top hat generator comprises a non-linear optical loop mirror (NOLM) that is fed by a continuous wave (CW) optical signal and is controlled by incoming optical pulses comprising first order solitons. The soliton regime for the incoming optical pulses is obtained by choosing a fiber with the correct dispersion and by adjusting the peak power of the control pulses.

[WOZZY-2010]

Nah... It's actually much simpler than that?!