Electronic-Optical Counter

M

Mohib

Guest
Hi,

My students are in need of an Optical-Electronic counter to count the
revolutions/minute (rpm) of a propeller driven by an electric motor.
The maximum expected rpm is around 20,000 rpm with most of the results
being in the 5,000 rpm range.

My initial thoughts are to be able to have a light source and a photo-
detector, with the light being interrupted by the rotating propeller.
The pulses from the photo-detector can be counted and compared to a
timer to give revolutions per minute.

Please let me know the most economical way of combining (the students
can solder etc.) parts to produce a DIY product.

Thanks,
Dr. Mohib. N. Durrani
Montgomery College, MD
 
Op 3-12-2010 18:50, Mohib schreef:
Hi,

My students are in need of an Optical-Electronic counter to count the
revolutions/minute (rpm) of a propeller driven by an electric motor.
The maximum expected rpm is around 20,000 rpm with most of the results
being in the 5,000 rpm range.

My initial thoughts are to be able to have a light source and a photo-
detector, with the light being interrupted by the rotating propeller.
The pulses from the photo-detector can be counted and compared to a
timer to give revolutions per minute.

Please let me know the most economical way of combining (the students
can solder etc.) parts to produce a DIY product.
Click to expand...
Feed te output of a foto transistor to a frequency counter.

--
pim.
 
On Dec 3, 12:50 pm, Mohib <dr.mo...@gmail.com> wrote:
Hi,

My students are in need of an Optical-Electronic counter to count the
revolutions/minute (rpm) of a propeller driven by an electric motor.
The maximum expected rpm is around 20,000 rpm with most of the results
being in the 5,000 rpm range.

My initial thoughts are to be able to have a light source and a photo-
detector, with the light being interrupted by the rotating propeller.
The pulses from the photo-detector can be counted and compared to a
timer to give revolutions per minute.

Please let me know the most economical way of combining (the students
can solder etc.) parts to produce a DIY product.

Thanks,
Dr. Mohib. N. Durrani
Montgomery College, MD
Click to expand...
Do you have access to an oscilloscope? Your idea is fine. Get a
photo diode, or even a solar cell may be fast enough. Connect a
resistor across the photo diode or cell and look at the voltage signal
on the 'scope. (The detector produces a current proportional to the
light, the resistor turns the current into a voltage.) You'll have to
play around a bit with the resistor value. Perhaps start with 10 k
ohms or 100k ohms. You won't get more than ~0.5 Volts out of the
detector. You'll want a nice bright source for the light. A red
diode laser pointer might be nice... easy to align it. Once you can
see pulses on the 'scope you can try sticking it into a counter.

At some point the capacitance of the photo diode or solar cell is
going to limit the response time. (tau = RC) But this is all good
fun and your students might learn something. Ask them how to make it
faster?

George H.
 
On Fri, 3 Dec 2010 09:50:54 -0800 (PST), Mohib <dr.mohib@gmail.com>
wrote:

Hi,

My students are in need of an Optical-Electronic counter to count the
revolutions/minute (rpm) of a propeller driven by an electric motor.
The maximum expected rpm is around 20,000 rpm with most of the results
being in the 5,000 rpm range.

My initial thoughts are to be able to have a light source and a photo-
detector, with the light being interrupted by the rotating propeller.
The pulses from the photo-detector can be counted and compared to a
timer to give revolutions per minute.

Please let me know the most economical way of combining (the students
can solder etc.) parts to produce a DIY product.
Click to expand...
---
The most economical way would be to build a counter using CMOS digital
logic; either the CD4000 or 74HC families, or both.

The simplest approach would be to use the output pulses from the
propeller interruptor to clock an up-counting counter chain, while
using a pulse of a fixed width to gate the counter.

Doing that, and using decimal counters, would allow the counter
outputs to drive seven-segment displays and have the displays read out
directly in RPM.

Would you like a schematic?


---
JF
 
On 2010-12-03, Mohib <dr.mohib@gmail.com> wrote:
Hi,

My students are in need of an Optical-Electronic counter to count the
revolutions/minute (rpm) of a propeller driven by an electric motor.
The maximum expected rpm is around 20,000 rpm with most of the results
being in the 5,000 rpm range.

My initial thoughts are to be able to have a light source and a photo-
detector, with the light being interrupted by the rotating propeller.
The pulses from the photo-detector can be counted and compared to a
timer to give revolutions per minute.

Please let me know the most economical way of combining (the students
can solder etc.) parts to produce a DIY product.
Click to expand...
hook a photodiode to the microphone inpput of a PC and use free
frequency counter software.

if ypur propellor has 6 blades multiplying the frequency (in Hz) by 10 will
yield RPM.

--
⚂⚃ 100% natural
 
Hello again,

Thanks to all for the replies and valuable info.
I should probably give a few additional details.

We want to only use economical electronic parts that can be easily
obtained from an electronics store (online, RadioShack etc.). We will
appreciate references to reliable sources.

We do have a continuously varying voltage/current power supply (one of
the Mastech models).
We do not want to have additional accessories (no oscilloscope, PC
etc.)

We also want a continuous (and varying) display of the rpm of the
rotating shaft (accuracy of ~ 0.1%). The students will note down the
changing rpm (as displayed) with changes in supplied voltage in their
lab report.

Please also note that some of the propellers have 2 blades and some
have 3, 4 or even 5 blades :)

Mohib.

On Dec 3, 1:50 pm, Mohib <dr.mo...@gmail.com> wrote:
Hi,

My students are in need of an Optical-Electronic counter to count the
revolutions/minute (rpm) of a propeller driven by an electric motor.
The maximum expected rpm is around 20,000 rpm with most of the results
being in the 5,000 rpm range.

My initial thoughts are to be able to have a light source and a photo-
detector, with the light being interrupted by the rotating propeller.
The pulses from the photo-detector can be counted and compared to a
timer to give revolutions per minute.

Please let me know the most economical way of combining (the students
can solder etc.) parts to produce a DIY product.

Thanks,
Dr. Mohib. N. Durrani
Montgomery College, MD
Click to expand...
 
Hi John,

Thanks for the info.
Yes, a schematic will definetely be very useful :)

Mohib.
PS: I posted some additional details of the project, relplying to my
initial post.

On Dec 3, 4:55 pm, John Fields <jfie...@austininstruments.com> wrote:
---
The most economical way would be to build a counter using CMOS digital
logic; either the CD4000 or 74HC families, or both.

The simplest approach would be to use the output pulses from the
propeller interruptor to clock an up-counting counter chain, while
using a pulse of a fixed width to gate the counter.

Doing that, and using decimal counters, would allow the counter
outputs to drive seven-segment displays and have the displays read out
directly in RPM.

Would you like a schematic?

---
JF

On Fri, 3 Dec 2010 09:50:54 -0800 (PST), Mohib <dr.mo...@gmail.com
wrote:

Hi,

My students are in need of an Optical-Electronic counter to count the
revolutions/minute (rpm) of a propeller driven by an electric motor.
The maximum expected rpm is around 20,000 rpm with most of the results
being in the 5,000 rpm range.

My initial thoughts are to be able to have a light source and a photo-
detector, with the light being interrupted by the rotating propeller.
The pulses from the photo-detector can be counted and compared to a
timer to give revolutions per minute.

Please let me know the most economical way of combining (the students
can solder etc.) parts to produce a DIY product.
Click to expand...
 
On Dec 3, 9:52 pm, Mohib <dr.mo...@gmail.com> wrote:
Hello again,

Thanks to all for the replies and valuable info.
I should probably give a few additional details.

We want to only use economical electronic parts that can be easily
obtained from an electronics store (online, RadioShack etc.). We will
appreciate references to reliable sources.

We do have a continuously varying voltage/current power supply (one of
the Mastech models).
We do not want to have additional accessories (no oscilloscope, PC
etc.)
Click to expand...
Oh, not even a counter? That's a lot harder. How much electronics
have you done?
Do you have digital multimeters?
They're dirt cheap. Hey some even have counters!

George H.


We also want a continuous (and varying) display of the rpm of the
rotating shaft (accuracy of ~ 0.1%). The students will note down the
changing rpm (as displayed) with changes in supplied voltage in their
lab report.

Please also note that some of the propellers have 2 blades and some
have 3, 4 or even 5 blades :)

Mohib.

On Dec 3, 1:50 pm, Mohib <dr.mo...@gmail.com> wrote:



Hi,

My students are in need of an Optical-Electronic counter to count the
revolutions/minute (rpm) of a propeller driven by an electric motor.
The maximum expected rpm is around 20,000 rpm with most of the results
being in the 5,000 rpm range.

My initial thoughts are to be able to have a light source and a photo-
detector, with the light being interrupted by the rotating propeller.
The pulses from the photo-detector can be counted and compared to a
timer to give revolutions per minute.

Please let me know the most economical way of combining (the students
can solder etc.) parts to produce a DIY product.

Thanks,
Dr. Mohib. N. Durrani
Montgomery College, MD- Hide quoted text -

- Show quoted text -
Click to expand...
 
"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:idc2au$4j4$1@reversiblemaps.ath.cx...
On 2010-12-03, Mohib <dr.mohib@gmail.com> wrote:
Hi,

Please let me know the most economical way of combining (the students
can solder etc.) parts to produce a DIY product.

hook a photodiode to the microphone inpput of a PC and use free
frequency counter software.

if ypur propellor has 6 blades multiplying the frequency (in Hz) by 10
will
yield RPM.

--
?? 100% natural
Click to expand...
Nice idea. So many people have hand held electro-impulse or optical counters
and RPM meters that I'd forgotten that PC sound cards have pretty good
linearity.
Here are some more ideas
1. If the OP has a PC with a spare 3-pin fan connector on the MB he might
try the fan sensor pin, I don't know how it counts or if it has a simple
interface.
2. I made hand-held two transistor rpm meter with analog output from a 1960s
Popular Electronics. It's a charge pump circuit and uses an old radio
antenna to pick up the EM pulsed by the ignition wires. Still works today.
3. The OP can look up VU meter or audio frequency meter. I hope the OP knows
Hertz times 60 equals RPM
 
On Fri, 3 Dec 2010 19:01:18 -0800 (PST), Mohib <dr.mohib@gmail.com>
wrote:

Hi John,

Thanks for the info.
Yes, a schematic will definetely be very useful :)
Click to expand...
---
OK, I'll have something for you the early part of next week.

Mohib.
PS: I posted some additional details of the project, relplying to my
initial post.
Click to expand...
---
OK.

BTW, the convention here is to bottom post and to in-line post when
required for clarity.

Thanks,

---
JF
 
On Fri, 3 Dec 2010 18:52:38 -0800 (PST), Mohib <dr.mohib@gmail.com>
wrote:

We want to only use economical electronic parts that can be easily
obtained from an electronics store (online, RadioShack etc.). We will
appreciate references to reliable sources.
Click to expand...
If economy is your main point, the best option would be to simply buy
a commercial tachometer for RC hobby, such as:
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXPT31&P=ML

Please also note that some of the propellers have 2 blades and some
have 3, 4 or even 5 blades :)
Click to expand...
Most commercial units will only do 2 or 3 blades, but a little bit of
arithmetic is all you need for scaling the measurement.

Note that these are intended for outdoor use, so they may not work
well with electrical lighting.

There are also cheap tachometers available for more general use. They
usually require some sort of reflective tape be applied to the
rotating part. Here's a cheap one:
http://www.dealextreme.com/details.dx/sku.41842

It may be possible to attach the tape to something stationary behind
the prop, and let the prop interrupt the laser beam. Again, you would
have to scale the result depending on number of blades.

Please let me know the most economical way of combining (the students
can solder etc.) parts to produce a DIY product.
Click to expand...
If the point of the exercise is to learn about electronics, I'd go
with a small microcontroller (PIC or AVR, perhaps) with a built-in
comparator or op-amp, a HD44780 LCD, a photo transistor, and a few
passive components. Learning about microcontrollers is very useful,
and opens up a world of possibilities for future projects.
--
RoRo
 
On Fri, 3 Dec 2010 09:50:54 -0800 (PST), Mohib
<dr.mohib@gmail.com> wrote:

Hi,

My students are in need of an Optical-Electronic counter to count the
revolutions/minute (rpm) of a propeller driven by an electric motor.
The maximum expected rpm is around 20,000 rpm with most of the results
being in the 5,000 rpm range.

My initial thoughts are to be able to have a light source and a photo-
detector, with the light being interrupted by the rotating propeller.
The pulses from the photo-detector can be counted and compared to a
timer to give revolutions per minute.

Please let me know the most economical way of combining (the students
can solder etc.) parts to produce a DIY product.
Click to expand...
Others have given you good advice on the photo-detector.
For the counter, you can use my Daqarta software with an
ordinary Windows sound card. The Frequency Counter option
has an RPM mode that allow you to set the number of events
per revolution (fan blades, gear teeth, engine cylinders,
etc) and read out directly in RPM at high resolution.

The problem with ordinary counters is that you get terrible
resolution unless you are willing to wait a *looong* time
between readings. For example, at one reading per second
you can only resolve to the nearest Hz. Daqarta uses the
"reciprocal period" approach (plus a few other tricks) to
get high resolution with fast updates (default is 10/sec).

See:
<http://www.daqarta.com/dw_freq.htm>

Daqarta has a 30-session/30-day free trial that is probably
long enough to do the entire project. Let me know if you
need more time.

If you have any questions, please let me know.

Best regards,


Bob Masta

DAQARTA v5.10
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
Frequency Counter, FREE Signal Generator
Pitch Track, Pitch-to-MIDI
DaqMusic - FREE MUSIC, Forever!
(Some assembly required)
Science (and fun!) with your sound card!
 
Mohib wrote:

My students are in need of an Optical-Electronic counter to count the
revolutions/minute (rpm) of a propeller driven by an electric motor.
The maximum expected rpm is around 20,000 rpm with most of the results
being in the 5,000 rpm range.
... the most economical way of combining (the students
can solder etc.) parts to produce a DIY product.
Click to expand...
Your students only need a strobe light; a VCO (like CD4046) and
a monostable (NE555) that can drive an LED, with either a
calibration on the knob, or an auxiliary counter (the frequency
input of many multimeters will fill this need).
 
On Fri, 3 Dec 2010 18:52:38 -0800 (PST), Mohib <dr.mohib@gmail.com>
wrote:

Thanks to all for the replies and valuable info.
I should probably give a few additional details.

We want to only use economical electronic parts that can be easily
obtained from an electronics store (online, RadioShack etc.). We will
appreciate references to reliable sources.
Click to expand...
What you want is already on line. Manufactured by Heathkit back in
the golden age of kits.

The schematic:
http://www.vintage-radio.info/heathkit/

Download this GIF:

# GD-69 - Thumb Tach - Thanks to W1DBN USA

Free but you have to verify an anti-robot code to download.

Uses ambient light (so outdoor, flashlight or incandescent - no
fluorescent lights, but an ordinary magnetic ballast fluorescent may
be a good way to calibrate it).

Or just use the National LM2907 chip designed to do just that after
you amplify the signal with the front end of the heath schematic

http://www.national.com/ds/LM/LM2907.pdf
 
"whit3rd" <whit3rd@gmail.com> wrote in message
news:ba86a491-f91e-427b-9d55-337ee862d1a7@z19g2000yqb.googlegroups.com...
Mohib wrote:

My students are in need of an Optical-Electronic counter to count the
revolutions/minute (rpm) of a propeller driven by an electric motor.
The maximum expected rpm is around 20,000 rpm with most of the results
being in the 5,000 rpm range.
... the most economical way of combining (the students
can solder etc.) parts to produce a DIY product.

Your students only need a strobe light; a VCO (like CD4046) and
a monostable (NE555) that can drive an LED, with either a
calibration on the knob, or an auxiliary counter (the frequency
input of many multimeters will fill this need).
Click to expand...
$30 to $40 (gets you a DVM too)
http://www.harborfreight.com/catalogsearch/result?category=&q=tachometer
 
On Fri, 3 Dec 2010 19:01:18 -0800 (PST), Mohib <dr.mohib@gmail.com>
wrote:

Hi John,

Thanks for the info.
Yes, a schematic will definetely be very useful :)

Mohib.
PS: I posted some additional details of the project, relplying to my
initial post.
Click to expand...
---
Sorry, but after having looked into it a little more carefully, the
effort needed to design the device is much more than I thought it
would be at first glance, and is more than I'm willing to expend.

Since, from your description, it seems all your students really need
is a non-contact tachometer, for $40 this would be hard to beat:

http://www.harborfreight.com/digital-photo-sensor-tachometer-66632.html

---
JF
 
On Fri, 3 Dec 2010 09:50:54 -0800 (PST) Mohib <dr.mohib@gmail.com> wrote
in Message id:
<309c5c6c-2cf5-4571-acfc-b4fa06dfb32f@k38g2000vbc.googlegroups.com>:

Hi,

My students are in need of an Optical-Electronic counter to count the
revolutions/minute (rpm) of a propeller driven by an electric motor.
The maximum expected rpm is around 20,000 rpm with most of the results
being in the 5,000 rpm range.

My initial thoughts are to be able to have a light source and a photo-
detector, with the light being interrupted by the rotating propeller.
The pulses from the photo-detector can be counted and compared to a
timer to give revolutions per minute.

Please let me know the most economical way of combining (the students
can solder etc.) parts to produce a DIY product.
Click to expand...
For the counter, how about
http://www3.telus.net/chemelec/Projects/Counter-UD/Counter-UD.htm
You can add more digits if needed.
Or http://www.edaboard.com/thread78990.html

You'll need to come up with the detector side of the circuit.
 
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