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Rotary sensor with logic output, kind of...

Garberg

Oct 21, 2015
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I need something that can activate and hold a relay as long as an axle is rotating (don't care about speed and direction)
I thought about an optofork and a raster disc (from a mouse wheel). This would create a pulse train which can not be used to activate the relay, but can I put a "reservoir" capacitor over the pulse train to hold the relay?

Please help.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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You have to care about speed! What if the axle is rotating one revolution per day? Do you want the relay to activate and stay activated once each day? How will you determine when the axle is not rotating? If you have something that generates pulses when the axle is rotating, you can use those pulses to trigger a re-triggerable one-shot multivibrator. The one shot will trigger "on" when the first pulse arrives, and then will be re-triggered to remain "on" when subsequent pulses arrive if those pulses arrive before the one-shot "times out". You need to determine how long an interval must occur between pulses to define a non-rotating state, after which another pulse is required to activate the relay. The circuit for this function is called a "missing pulse detector" and is easily implemented with a 555 timer or other integrated circuits. Google it.
 

Garberg

Oct 21, 2015
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I don't care about speed because it's fairly constant. It will either be rotating at maybe 100rpm or not at all. I want to keep it as simple as possible.
 

Minder

Apr 24, 2015
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Another name is a Watchdog Timer for the 555 cct.
Some methods of detecting rotation is a hall sensor, such as the Honeywell SS400 series and requires a small magnet on the rotator or a retro-reflective sensor and a strip of reflective tape on the axle.
M.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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I need something that can activate and hold a relay as long as an axle is rotating (don't care about speed and direction)
I thought about an optofork and a raster disc (from a mouse wheel). This would create a pulse train which can not be used to activate the relay, but can I put a "reservoir" capacitor over the pulse train to hold the relay?

Please help.
You can couple the pulse train to a peak detector through a diode connected to a "reservoir" capacitor with a "bleeder" resistor in parallel to discharge the capacitor when the pulses stop. When pulses are present, the "reservoir" capacitor charges up and you can use this as a signal to hold a relay actuated.
 

AnalogKid

Jun 10, 2015
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There are only about a dozen ways to detect that an axle or shaft is rotating, and without knowing more about your physical arrangement, all recommendations are merely guesses. For example, is gluing or taping a magnet to the shaft ok, or would that throw something out of balance? Must the sensor be contactless, or can a cam activate a microswitch? Is the location too dusty for an optical sensor? Etc. In any case, the sensor output will be a 100 Hz pulse train that can be frequency detected as, say, greater than or less than 50 Hz to make a go/no go signal. One common way to do this is with a retriggerable monostable, sometimes called a missing pulse detector. A lot depends on your skill set and the power available for the sensor circuit, but this should be a relatively simple project once we have enough detail.

ak
 

Minder

Apr 24, 2015
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One method I used using magnets using a SS400 style latching sensor to prevent unbalance was placing a miniature magnet on opposing sides of the rotor, one S pole the other N facing out.
One latched the other unlatched the hall sensor.
M.
 

dorke

Jun 20, 2015
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Garberg,
Another important missing fact is ,what is causing the axle to rotate .

From your "...mouse wheel", I guess it is a small thing.
From: "It will either be rotating at maybe 100rpm or not at all"
It may be a small D.C motor.

If this is indeed the case,the easiest way would be:
monitor the voltage that the motor's current creates on a a small series resistor placed
near GND supply-lead of the motor (see the simplified idea in pic).
Basically you need to know the minimum current of the working motor(can be measure if you don't have the motor's spec.)
note : the resistor can be changed to an optocoupler as well.

The great advantage: you do it all with no "mechanical stuff attachments"

You will get:
1.0V if the motor and axle are not rotating or
2.V proportional to i the current of the working motor and R if it is rotating.
.
use it to do what ever you want like
drive a tr. to activate a relay etc..
sensor.JPG .
 
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