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Opto Electronics/Sensors Help!

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fireworks1

New Member
Hello guys.
I'm brand new to this forum and quite new to electronics itself.
I have quite basic, fundamental knowledge but am acquiring some help
from genuine electronics people..

I live in Australia and as apart of our secondary school subjects, we have a
subject called systems engineering. In this class, we are asked to build a
system that consists of both an electronic and mechanical sub-systems.

I have opted to build a digital tachometer and to get a reading, I have decided to build a signal generator that is made up of a 12v DC Motor, that rotates a blade through a sensor and is able to generate a reading.

So far I have attempted building an opto-isolator circuit consisting of an
IR LED and a phototransistor. The problem exists in how I would be able to
turn this into a pulsating output signal that would be able to replicate or simulate a running motor and generate a reading on the tachometer. Would I need some sort of PICAXE Chip that would do this? An encoder? A driver? I'm not sure what to do.

Any help would be greatly appreciated :)
 

fireworks1

New Member
What would people suggest if I was to use and opto-interrupter?
Is there any schematics available for this circuit?
 
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ericgibbs

Well-Known Member
Most Helpful Member
I have opted to build a digital tachometer and to get a reading, I have decided to build a signal generator that is made up of a 12v DC Motor, that rotates a blade through a sensor and is able to generate a reading.

So far I have attempted building an opto-isolator circuit consisting of an
IR LED and a phototransistor. The problem exists in how I would be able to
turn this into a pulsating output signal that would be able to replicate or simulate a running motor and generate a reading on the tachometer. Would I need some sort of PICAXE Chip that would do this? An encoder? A driver? I'm not sure what to do.

Any help would be greatly appreciated :)
hi,:)
Normally the blade blocking the IR beam as the motor rotates would give a pulsed signal.

If you want to simulate a pulse the you could use a 555 timer ic as an astable.

This could be controlled to give pulses at a variable rate, for simulation.

http://www.eleinmec.com/article.asp?3
 
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fireworks1

New Member
I think that the reason I don't understand it all is that
I don't quite understand the term "output signal".
I mean does the fan blocking the path create a change in
voltage as a result in a change in resistance or photoelectric current?

And how does this ΔV lead to an output signal that is able
create a reading on the tacho? Does it need to be passed through
a programmed PIC that creates the High/Low signal?
 

ericgibbs

Well-Known Member
Most Helpful Member
I think that the reason I don't understand it all is that
I don't quite understand the term "output signal".
I mean does the fan blocking the path create a change in
voltage as a result in a change in resistance or photoelectric current?

And how does this ΔV lead to an output signal that is able
create a reading on the tacho? Does it need to be passed through
a programmed PIC that creates the High/Low signal?
hi,
Consider you have an infra red [IR] emitter which 'shines' the infra red beam to a IR detector.
Say the emitter and detector are spaced 100mm apart so that the IR beam is 'seen' by the detector.

Say you block the beam so that the detector cannot see it, this change in the detector circuit voltage is the 'output signal'
Consider now you shine the beam thru the path of the fan blades, so that every time a blade blocks the path you get an output signal.
If the fan has 4 blades you will 4 output pulses for every revolution on the fan motor.

The circuits for the detector amplify the 'weak' signal from the IR detector to a voltage level suitable for a PIC or other logic.

OK.?:)
 
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fireworks1

New Member
How will I calibrate it so it is only 2 pulse per revolution as the blade is only 2 blades?
And so the PIC amplifies the small photocurrent the photo-transistor receives?
Also would it be better to use an IR LED or just a basic LED?
Sorry about all the questions..
 
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ericgibbs

Well-Known Member
Most Helpful Member
How will I calibrate it so it is only 2 pulse per revolution as the blade is only 2 blades?
Well the pulse from the IR detector would occur twice per revolution, so the rpm would be the pulses divide by 2.

And so the PIC amplifies the small photocurrent the photo-transistor receives?
No, the PIC dosnt amplify the signal, often an external amp is used.
You could use the analog input on the PIC to see the signal change.

Also would it be better to use an IR LED or just a basic LED?
A basic RED led is not suitable, you need a matched pair of IR emitter/detector, you dont want the ambient light being detected by the system.

Sorry about all the questions..
The questions are OK, but I guess you havnt done this sort of thing before.:rolleyes:
 
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fireworks1

New Member
Yep, as cliche as it may sound, I definitely jumped into the deep end.
Actually, I think its more the fact that I actually have to design this
circuit, where as every other student just has a "kit" that they just
put all the components together.

Ahh well, I'll be at uni next year, studying electrical engineering,
looking back and thinking how ridiculous some of the things I am saying
on this forum may actually seem for you "experienced" electronics
engineers and hobbyists.

Ill keep you updated with my findings and collaborate on what things
need to be done to enable proper function.
 

fireworks1

New Member
So I have finished rigorous testing and have come to a conclusion that I
believe may be feasible.

When the tachometer is connected to the sig.gen of the power supply
the tacho shows an RPM reading. But when it is connected to my Sig.Gen
output signal, the reading shows for a quick time and then disappears.
So my prediction is that the output signal of the signal generator is not
smooth enough and gets distorted so it does not show a reading.

What do people think?
If so, how do you suggest that I mend it?
 

Attachments

ericgibbs

Well-Known Member
Most Helpful Member
So I have finished rigorous testing and have come to a conclusion that I
believe may be feasible.

When the tachometer is connected to the sig.gen of the power supply
the tacho shows an RPM reading. But when it is connected to my Sig.Gen
output signal, the reading shows for a quick time and then disappears.
So my prediction is that the output signal of the signal generator is not
smooth enough and gets distorted so it does not show a reading.

What do people think?
If so, how do you suggest that I mend it?
hi,
In the second part of your post, are you saying that you are driving the tacho display with a different signal generator and then it fails after a short time.

If Yes, are you connecting the SG to the tacho via a series capacitor.??
 
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fireworks1

New Member
hi,
In the second part of your post, are you saying that you are driving the tacho display with a different signal generator and then it fails after a short time.

If Yes, are you connecting the SG to the tacho via a series capacitor.??
That is correct. I have made a signal generator out of a 12VDC motor
that has a fan blade connected to it. The blade obstructs the IR Beam
of the IR Opto-Interrupter I have constructed.

I do also have a capacitor in series to attempt to help smooth the wave.
Maybe increase the value of the capacitor?
 

ericgibbs

Well-Known Member
Most Helpful Member
That is correct. I have made a signal generator out of a 12VDC motor
that has a fan blade connected to it. The blade obstructs the IR Beam
of the IR Opto-Interrupter I have constructed.

I do also have a capacitor in series to attempt to help smooth the wave.
Maybe increase the value of the capacitor?
hi,
So its not really a SG, but an interrupted IR beam circuit.:)

On your scope when using the IR detector, what signal to the tacho display, do you measure.?

I need to know the amplitude of the signal and is there any 'dc' offset.

If there is a 'dc' offset this may be cause of the failure after a few seconds.

The idea of using a coupling capacitor would be to remove this 'dc' level.

Post a picture of the scope display, but tell me the scope settings, mV/cm, ac or dc etc... OK.:)

Do you have a diagram of you final IR detector circuit that you could post.?
 

fireworks1

New Member
hi,
So its not really a SG, but an interrupted IR beam circuit.:)

On your scope when using the IR detector, what signal to the tacho display, do you measure.?

I need to know the amplitude of the signal and is there any 'dc' offset.

If there is a 'dc' offset this may be cause of the failure after a few seconds.

The idea of using a coupling capacitor would be to remove this 'dc' level.

Post a picture of the scope display, but tell me the scope settings, mV/cm, ac or dc etc... OK.:)

Do you have a diagram of you final IR detector circuit that you could post.?
Sorry dear sir for the misconception.
The photo attached of just the oscilloscope screen is the output signal.
I will try get some schematics up shortly.
 

ericgibbs

Well-Known Member
Most Helpful Member
Sorry dear sir for the misconception.
The photo attached of just the oscilloscope screen is the output signal.
I will try get some schematics up shortly.
hi,
Nothing to be sorry for, I just wanted to be sure I didnt mislead you.:)

I have seen your scope image, but I cannot see the scope settings, so the waveform dosnt mean anything.
 

gaspode42

Member
I think that unless the OP changed the scope between photographs if looks as if the scope is configured for 2V/div on AC.
 

ericgibbs

Well-Known Member
Most Helpful Member
hi FW1,
If the scope is set for 'ac' input, as gaspode suspects, change the setting to 'dc' input, lets see the full signal.

Do you have any info on the tacho display input,?,,, what input pulse levels are specified.?
 

fireworks1

New Member
By increasing the speed of the motor, I am not getting the desired result.

Can only seem to get the amplitude of the output wave to change
and I'm having trouble getting the frequency to change..

Am buying a photo interrupter tonight and will try again.
 
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