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A way to count motor rpm's

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Deltakilo925

New Member
I’m looking for a simple circuit to count the revolutions of a motor. The motor is 12vdc runs at 9 rpm and will be mounted up on a communication tower. I thought about attaching a rotary encoder on the shaft of the motor and from there to the counting circuit. The circuit needs to be able to count up and down with possibly three 7 segment displays to display numbers 000-999. I’m not to good at digital circuits, but I figured it can’t be all that difficult to do. I’ve looked at a few hobby circuit sites for ideas but seem to be coming just short of what I need.

Thanks Dave
 

BrownOut

Banned
Any counter circuit will do. There are thousands of them on the web, books, etc.

For the encoder, you only need to count full revs, right? Something as simple as a cam and sail switch ( I think that's the right term ) will do that. A rotary encoder seems like overkill.
 

ericgibbs

Well-Known Member
Most Helpful Member
I’m looking for a simple circuit to count the revolutions of a motor. The motor is 12vdc runs at 9 rpm and will be mounted up on a communication tower. I thought about attaching a rotary encoder on the shaft of the motor and from there to the counting circuit. The circuit needs to be able to count up and down with possibly three 7 segment displays to display numbers 000-999. I’m not to good at digital circuits, but I figured it can’t be all that difficult to do. I’ve looked at a few hobby circuit sites for ideas but seem to be coming just short of what I need.

Thanks Dave
hi,:)
Is the motor driving a gear box.? If yes, do you want to count motor revs or gear box output shaft revs.

As you require up/down counts a simple uSwitch will not give any sense of direction.
 

crutschow

Well-Known Member
Most Helpful Member
To generate pulses you could attach a small magnet to the shaft and use a reed relay or a hall-effect IC to detect the rotation.
 

Deltakilo925

New Member
Thanks Guys for the fast replys. Yes the motor is driving a gear box so the gear box is rotating at 9 rpm not the motor. The gear box is connected to a drum which has 14AWG antenna wire spooled around it. Reason I need someway of counting is to have a basic way of knowing wire postion or how much wire was released or takin in it dosen't have to be accurate just some relative number displayed on the 7 segment displays. For example wire is spooled out and when the display reads #150 the antenna becomes resonant say at 10.125mHz, then more wire is spooled out to #250 which retunes the antenna to a different freq say 10.250mHz so in order to get back to 10.125mHz all I would need to do is reverse to motor and count back to #150.

Dave
 

crutschow

Well-Known Member
Most Helpful Member
all I would need to do is reverse to motor and count back to #150.
Thus you will need a circuit to count up and down. The simplest would be to control an up/down counter direction by the signal that controls the motor direction. Thus the counter would count up when spooling out and down when spooling in.
 

BrownOut

Banned
As you require up/down counts a simple uSwitch will not give any sense of direction.
I glossed over that in the original post. So then, just use two switches oriented 90 degrees apart in rotation, and the circuit below. Inputs CW and CCW are the two switches. Then you only have to 'sense' if the direction changes while the shaft is positioned between the switches. That would be very easy to do.

NOTE: It took MANY tries to get the drawing right. You may have noticed a couple of my posts being deleated.
 

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Deltakilo925

New Member
Looks good guys I like the idea of using switches 90 degrees apart in fact I thought of this approach before but kept thinking of switch failure becoming a problem from constant cycling, but I guess failure of switches vs a rotary encoder would be about the same.


Dave
 

crutschow

Well-Known Member
Most Helpful Member
Electronic rotary encoders and hall effect devices would have a very long life. Next would be reed relays, and finally standard switches.

But if you are not reeling the antenna in and out frequently then a standard switch should probably have sufficient reliability (most have a operation life estimate you could use to determine how long it might last in typical use).
 

BrownOut

Banned
If you go with contact switches, use SPDT and de-bounce them.

And they should be "break before make" I'm not really sure if they all aren't.
 
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