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Optical rev counter

Hi guys,
I been given an electronic project to make an optical rev counter
for a RC airplane. If anyone has any experience with makeing one i'd
really appreciate any help or links to information or schematics.
I really can't seem to find any information about this on the net.
 
P

Phil Allison

Jan 1, 1970
0
**Groper Alert !!

I been given an electronic project to make an optical rev counter
for a RC airplane.


** What the **** for ?

You can but them for almost peanuts.

Wanna make your own DVD player or GSM phone too ?




......... Phil
 
A

Anthony Fremont

Jan 1, 1970
0
Hi guys,
I been given an electronic project to make an optical rev counter
for a RC airplane. If anyone has any experience with makeing one i'd
really appreciate any help or links to information or schematics.
I really can't seem to find any information about this on the net.

The standard way seems to be to bounce infra-red light off the prop and
count the pulses coming back over a defined period of time and then dividing
the pulses by the number of blades on the prop (2,3 or 4 usually). Since
most RC engines run at 20,000RPM or less, you have to be able to count
pulses at the rate of up to neary 1500 pulses per second for a four blade
prop.

You could also take an all analog approach by cleaning up the pulses and
turning each into a fixed width. You would then low-pass filter this signal
using an RC circuit to create an analog voltage proportionate to the pulse
rate and use this to swing an analog meter.

What are your accuracy requirements? What kind of display or output device
did you want to use?
 
D

DylanJ

Jan 1, 1970
0
The standard way seems to be to bounce infra-red light off the prop and
count the pulses coming back over a defined period of time and then dividing
the pulses by the number of blades on the prop (2,3 or 4 usually). Since
most RC engines run at 20,000RPM or less, you have to be able to count
pulses at the rate of up to neary 1500 pulses per second for a four blade
prop.

You could also take an all analog approach by cleaning up the pulses and
turning each into a fixed width. You would then low-pass filter this signal
using an RC circuit to create an analog voltage proportionate to the pulse
rate and use this to swing an analog meter.

What are your accuracy requirements? What kind of display or output device
did you want to use?

Thanks man, the accuracy I was asked for was +- 5%, and I was thinking
of using a LCD display, the displays not too much of a problem though.
 
R

Rich Grise

Jan 1, 1970
0
Hi guys,
I been given an electronic project to make an optical rev counter
for a RC airplane. If anyone has any experience with makeing one i'd
really appreciate any help or links to information or schematics.
I really can't seem to find any information about this on the net.

Do you want to buy or build? A google for "optical tachometer" with
the quotes gives "about 20,100 hits".

If you're building it, just get a reflective opto sensor and count
pulses. :)

Good Luck!
Rich
 
R

Robert Roland

Jan 1, 1970
0
I been given an electronic project to make an optical rev counter
for a RC airplane. If anyone has any experience with makeing one i'd
really appreciate any help or links to information or schematics.

I have designed one that works very well, but I am not going to ruin
your assignment by giving you the complete solution. I'll tell you
what I did, though, so you have something to start with:

The optical pickup turned out to be easier than expected: A photo
transistor with a suitable amplifier and filter was all that was
needed. No need for active lighting. Ambient light is plenty.

Nowadays, I'd simply use a micro controller to read the pulses and
drive the display. Back then, I didn't have the knowledge to do micro
controllers.

I used three decade counters, each connected to an LCD driver with
latch. The counters would count propeller pulses directly off the
photo transistor amplifier. A crystal driven timer (a counter which
resets itself when it reaches a certain number) would then latch the
counter values into the LCD drivers and reset the counters at a
certain interval, depending on how many blades the propeller had. The
crystal would also drive the LCD drivers, but off a low frequency in
the timer frequency divider.

Good luck.
 
P

Phil Allison

Jan 1, 1970
0
** Groper Fool Alert

Thanks man, the accuracy I was asked for was +- 5%, and I was thinking
of using a LCD display, the displays not too much of a problem though.


** ROTFLMAO

No problem with LED displays ......

Except for the act you cannot read one in bright daylight !




........ Phil
 
A

Anthony Fremont

Jan 1, 1970
0
Phil said:
** Groper Fool Alert

We all know who you are Phil, no introduction is necessary.
** ROTFLMAO

No problem with LED displays ......

Except for the act you cannot read one in bright daylight !

Try reading it again. LCD Are you using an LED display on your
computer?
 
A

Anthony Fremont

Jan 1, 1970
0
DylanJ said:
Thanks man, the accuracy I was asked for was +- 5%, and I was thinking
of using a LCD display, the displays not too much of a problem though.

I don't know how hard that would be to obtain using an all analog method,
but getting that accuracy with a micro should be easy. The first problem to
solve is how to get pulses from the prop. Another poster pointed out (from
personal experience) that a light source was not necessary. He says that
enough ambient light will be reflected from the prop to do it. At this
point, I would start experimenting with a phototransistor and an op-amp.
Something like this might be a good starting point:
http://www.coilgun.eclipse.co.uk/optical_trigger_module.html

I just quickly googled up that page, I have no idea if the circuit works or
has fast enough response time for your needs, but it is a good example of
amplifying and detecting pulses from a phototransistor. The output of the
op-amp could be fed to a LM-393 comparator to further clean up the pulses so
they can be fed into digital logic of some sort if you like. Personally
this is the approach I would take if it were my project.

Or, as I said before, you could take those pulses and use them to trigger a
one-shot timer to set the pulse width to a consitent size and then integrate
these to come up with a voltage proportionate to the pulse rate. I would
think this could be calibrated to within 5%, just.
 
R

Robert Roland

Jan 1, 1970
0
Another poster pointed out (from
personal experience) that a light source was not necessary. He says that
enough ambient light will be reflected from the prop to do it.

I never put a scope on the counter while measuring an actual
propeller, but I strongly believe that the effect is opposite. The
propeller is actually blocking some of the ambient light when it
passes in front of the photo transistor.
Something like this might be a good starting point:
http://www.coilgun.eclipse.co.uk/optical_trigger_module.html

Yes, that looks like a good start (omit the light source, though). It
has a problem though: The potentiometer essentially adjusts
comensation for ambient light level. Since the ambient light varies
greatly (this is a handheld device), you'd drive yourself nuts
twiddling the knob while trying to hold perfectly still.

I solved the problem by taking a low pass filter (simple R/C type) and
putting the output of that into the other input of the op amp.
The output of the
op-amp could be fed to a LM-393 comparator to further clean up the pulses so
they can be fed into digital logic of some sort if you like.

I even used a Schmitt-trigger here. It turned out I also had to put a
low pass filter between the first op-amp and the Schmitt-trigger,
because the photo transistor would pick up the white lettering on the
black propeller and cause incorrect and unstable readings.
 
A

Anthony Fremont

Jan 1, 1970
0
Robert said:
I never put a scope on the counter while measuring an actual
propeller, but I strongly believe that the effect is opposite. The
propeller is actually blocking some of the ambient light when it
passes in front of the photo transistor.

Sorry about that, it is me that added the reflected part to what you said.
Sorry again. That does make sense.
Yes, that looks like a good start (omit the light source, though). It
has a problem though: The potentiometer essentially adjusts
comensation for ambient light level. Since the ambient light varies
greatly (this is a handheld device), you'd drive yourself nuts
twiddling the knob while trying to hold perfectly still.

I solved the problem by taking a low pass filter (simple R/C type) and
putting the output of that into the other input of the op amp.


I even used a Schmitt-trigger here. It turned out I also had to put a
low pass filter between the first op-amp and the Schmitt-trigger,
because the photo transistor would pick up the white lettering on the
black propeller and cause incorrect and unstable readings.

Sounds like you refined it pretty well. You wouldn't happen to have a
schematic laying around would you? :) I would start out like I described
and then let it spiral out of control from there letting my old oscilloscope
lead the way. ;-)
 
R

Robert Roland

Jan 1, 1970
0
You wouldn't happen to have a
schematic laying around would you? :)

I do, but if I tell you where to find it, I am worried I'll spoil
Dylan's project :)

Besides, the solution is very old-fashioned. A microcontroller would
reduce the chip count by 80%. If you have a uC with A/D, you might be
able to connect the phototransistor almost directly to the uC and do
all the filtering and calibration in software.
 
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