Electronic-Optical Counter

Feed te output of a foto transistor to a frequency counter.

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pim.

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 =3D RC) But this is all good fun and your students might learn something. Ask them how to make it faster?

George H.

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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?

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.

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?? 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.

Hi John,

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

Mohib. PS: I posted some additi> ---

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.

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

--- OK, I'll have something for you the early part of next week.

--- OK.

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

Thanks,

--- JF

If economy is your main point, the best option would be to simply buy a commercial tachometer for RC hobby, such as:

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:

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.

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.

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RoRo

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:

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

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!

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).

What you want is already on line. Manufactured by Heathkit back in the golden age of kits.

The schematic:

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

$30 to $40 (gets you a DVM too)

--- 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:

--- JF

For the counter, how about

You can add more digits if needed. Or

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