# Building a Cheap Tachometer

L

#### luki

Jan 1, 1970
0
Hi everybody,
I need to build a tachometer circuit to measure the speed of a motor.
Any suggestion on the technique,circuit, problem that might exists,
estimated cost of building such device,etc ? Any pointer would be
highly appreciated

Regards,

Lukman

A

#### Anthony Fremont

Jan 1, 1970
0
luki said:
Hi everybody,
I need to build a tachometer circuit to measure the speed of a motor.
Any suggestion on the technique,circuit, problem that might exists,
estimated cost of building such device,etc ? Any pointer would be
highly appreciated

We need more informaton. What kind/size/speed is the motor? What kind of
power does it run on? At this point, I can only offer the advice that Don
Lancaster would probably give you......use a PIC (and don't try to get rich
by writing books). ;-)

M

#### martin griffith

Jan 1, 1970
0
Hi everybody,
I need to build a tachometer circuit to measure the speed of a motor.
Any suggestion on the technique,circuit, problem that might exists,
estimated cost of building such device,etc ? Any pointer would be
highly appreciated

Regards,

Lukman
not really enough info, but you could start here
http://www.national.com/pf/LM/LM2907.html

martin

L

#### luki

Jan 1, 1970
0
Hi,
A small DC motor maybe, i don't know (i don't decide it yet) but would
not be very big sized motor. Is there any circuit that would generally
be able to measure motor speed?
If not, what really caused that? Is there any special attention i must
deal regarding each type/size/speed?
At this point, I can only offer the advice that Don
Lancaster would probably give you......use a PIC (and don't try to get rich
by writing books). ;-)

This is the background\purpose of this tachometer :
1. I was developing a motor controller, so i need a speed sensor to
control my motor
2. The motor would be a small DC motor, to be easy to control
Any suggestion about the motor itself?
not really enough info, but you could start here
http://www.national.com/pf/LM/LM2907.html
A cheap frequency to voltage converter would be nice. That means i
must measure the frequency change according to Doppler? What kind of
signal that must be transmitted?
What if I can't have LM2907, quite hard to find things like that in my
country. Is there any alternative. Or could i made frequency to
voltage my own?

I really appreciate your posts. Looking forward to hear more.

Lukman

L

#### linnix

Jan 1, 1970
0
Hi,
A small DC motor maybe, i don't know (i don't decide it yet) but would
not be very big sized motor. Is there any circuit that would generally
be able to measure motor speed?

How small? All CPU fans (three wires) have pulse sensors. Can you
If not, what really caused that? Is there any special attention i must
deal regarding each type/size/speed?

This is the background\purpose of this tachometer :
1. I was developing a motor controller, so i need a speed sensor to
control my motor

If you need to control it anyway, why not a micro to count the pulse
sensor.

D

#### D from BC

Jan 1, 1970
0
Hi everybody,
I need to build a tachometer circuit to measure the speed of a motor.
Any suggestion on the technique,circuit, problem that might exists,
estimated cost of building such device,etc ? Any pointer would be
highly appreciated

Regards,

Lukman

Some Wild Ideas:
Pimp up an optical mouse?
Hook up the motor to a potentiometer. Measure delta ohms/sec.
Put a magnet on the shaft and use a hall sensor connected to a timer.
Connect the motor to a dynamo. Measure Vout to measure rev/sec.
Drill a hole through the shaft. ...
Beam a laser through the hole..Measure laser light interruption rate.
Put a mirror on the shaft..
Beam a laser at the mirror .Measure laser light interruption rate.
Go to scrap yard...Get old tach from an old car for $5.00. Go to automotive store..get new tach. D from BC A #### Anthony Fremont Jan 1, 1970 0 luki said: Hi, A small DC motor maybe, i don't know (i don't decide it yet) but would not be very big sized motor. Is there any circuit that would generally be able to measure motor speed? There are many, but you need to specify how. For example, RPM can be determined optically, mechanically, electrically even acoustically. Allot depends upon your accuracy requirements, the speed of the motor and how often you need updates. You could count pulses (magnetic, optical, etc.) over a defined time interval, or measure the time between pulses. What works best depends upon your application. If not, what really caused that? Is there any special attention i must deal regarding each type/size/speed? Just the obvious things like hanging a magnet on the shaft of a pager motor probably won't work well etc... ;-) This is the background\purpose of this tachometer : 1. I was developing a motor controller, so i need a speed sensor to control my motor 2. The motor would be a small DC motor, to be easy to control Any suggestion about the motor itself? Is this for something like a robot where measuring the wheel speed/rotations might be easier/better? Can you be more specific about your end goals? A cheap frequency to voltage converter would be nice. That means i must measure the frequency change according to Doppler? What kind of signal that must be transmitted? What if I can't have LM2907, quite hard to find things like that in my country. Is there any alternative. Or could i made frequency to voltage my own? I really appreciate your posts. Looking forward to hear more. You probably shouldn't mix responses from different people into one reply like this. S #### scada Jan 1, 1970 0 luki said: Hi everybody, I need to build a tachometer circuit to measure the speed of a motor. Any suggestion on the technique,circuit, problem that might exists, estimated cost of building such device,etc ? Any pointer would be highly appreciated Regards, Lukman You could hack an old CD player, many of the motors have tach pulse outputs. L #### luki Jan 1, 1970 0 Anthony said: There are many, but you need to specify how. For example, RPM can be determined optically, mechanically, electrically even acoustically. Allot depends upon your accuracy requirements, the speed of the motor and how often you need updates. You could count pulses (magnetic, optical, etc.) over a defined time interval, or measure the time between pulses. What works best depends upon your application. Is this for something like a robot where measuring the wheel speed/rotations might be easier/better? Can you be more specific about your end goals? I am developing a motor controller, to have the optimum response of a motor. Like a PID controller, i will use the motor speed as an indicator of it's response and compare it to a desired response. A descent accuracy would be enough, what i need is that the sampling rate is high enough to catch enough information. L #### luki Jan 1, 1970 0 D said: Some Wild Ideas: Pimp up an optical mouse? Hook up the motor to a potentiometer. Measure delta ohms/sec. Put a magnet on the shaft and use a hall sensor connected to a timer. Connect the motor to a dynamo. Measure Vout to measure rev/sec. Drill a hole through the shaft. ... Beam a laser through the hole..Measure laser light interruption rate. Put a mirror on the shaft.. Beam a laser at the mirror .Measure laser light interruption rate. Go to scrap yard...Get old tach from an old car for$5.00.
Go to automotive store..get new tach.
D from BC
Hi D, from your wild idea i think i would like to try using one of
your idea. Optical mouse, and dynamo seems interesting. I will search
for infomation on that subject, or if you have some can you give me a
pointer?

L

#### luki

Jan 1, 1970
0
linnix said:
How small? All CPU fans (three wires) have pulse sensors. Can you
This would be interesting. I'll try to dig more information on that.
Thanks
If you need to control it anyway, why not a micro to count the pulse
sensor.
I think the measurement should be done independently. The computation
in the micro is quite heavy.

L

#### luki

Jan 1, 1970
0
You could hack an old CD player, many of the motors have tach pulse outputs.
I think i have an old CD player in my house. I will look for it, i
will be a good and cheap alternative.

L

#### luki

Jan 1, 1970
0
I like the wild ideas, i never even think of that (mouse, CPU fan, CD
player, etc). Great replies. I really appreciate the all of the
response and attention. Thanks a lot, it really helps me. Hope to be
able to finish my project with this.

Regards,

Lukman

J

#### jasen

Jan 1, 1970
0
Hi,
A small DC motor maybe, i don't know (i don't decide it yet) but would
not be very big sized motor.

decide first. techniquies that are applicable to brushless DC motors aren't
applicable to other types.
Is there any circuit that would generally be able to measure motor speed?

it's basically a frequency counter calibrated in per minute (or per
minnute/100) instead of per second (if for display)
This is the background\purpose of this tachometer :
1. I was developing a motor controller, so i need a speed sensor to
control my motor
2. The motor would be a small DC motor, to be easy to control
Any suggestion about the motor itself?

that depends what you want to do with it, the speed sensors on VCR and
floppy drive motors generate several pulses per rotation to allow closer
control of the speed
A cheap frequency to voltage converter would be nice.

if you don't need great precision that's one way to go.
What if I can't have LM2907, quite hard to find things like that in my
country. Is there any alternative. Or could i made frequency to
voltage my own?

getting repeatable accurate results from a design using discrete parts
reqwuires more skill (more than I have), how much precision do you need?

J

#### jasen

Jan 1, 1970
0
How small? All CPU fans (three wires) have pulse sensors. Can you

the two wire fans have them too, they are just missing the third wire
(the sensor is needed to switch the coils in the motor)

Bye.
Jasen

J

#### jasen

Jan 1, 1970
0
I am developing a motor controller, to have the optimum response of a
motor. Like a PID controller, i will use the motor speed as an
indicator of it's response and compare it to a desired response. A
descent accuracy would be enough, what i need is that the sampling
rate is high enough to catch enough information.

Put some numbers on those goals, make them measurable: how much is "enough".

Bye.
Jasen

S

#### Spehro Pefhany

Jan 1, 1970
0
I am developing a motor controller, to have the optimum response of a
motor. Like a PID controller, i will use the motor speed as an
indicator of it's response and compare it to a desired response. A
descent accuracy would be enough, what i need is that the sampling
rate is high enough to catch enough information.

A resolver or a quadrature encoder with a large number of pulses per
revolution will allow you the possibilty of getting very close to
optimum response, but probably not "cheap".

Best regards,
Spehro Pefhany

M

#### MooseFET

Jan 1, 1970
0
I am developing a motor controller, to have the optimum response of a
motor. Like a PID controller, i will use the motor speed as an
indicator of it's response and compare it to a desired response. A
descent accuracy would be enough, what i need is that the sampling
rate is high enough to catch enough information.

You may also find that getting 1/RPM is an easier thing to measure.
If you only need to run over a narrowish range of speeds, this may be
a better feedback in your servo. Unlike some of the other measures,
you don't have to wait for several rotations to get it.

Once the motor is turning, all you need to do is count how many clock
cycles go by as the shaft makes one turn. The trick to not waiting
too long when the motor is stopped is to only wait long enough to
create the full error signal if the motor is slower than desired.

You can also make an analog ramp that you capture when the shaft
completes the rotation. A DAC controls the slope of the ramp. The
ADC grabs the voltage when the shaft makes a full turn.

M

#### MooseFET

Jan 1, 1970
0
A resolver or a quadrature encoder with a large number of pulses per
revolution will allow you the possibilty of getting very close to
optimum response, but probably not "cheap".

Look up a "Digital pot" if you want a lower cost encoder.

If the motor is turning a gear, you could place a hall device or other
thing so that it picks
up the teeth going by.

J

#### joseph2k

Jan 1, 1970
0
Anthony said:
We need more informaton. What kind/size/speed is the motor? What kind of
power does it run on? At this point, I can only offer the advice that Don
Lancaster would probably give you......use a PIC (and don't try to get
rich
by writing books). ;-)

Your attitude could be better, just direct the slob to s.e.b where it
belongs. Better yet, take the whole "conversation" there.