# Accelerometer sensing

Discussion in 'General Electronics Chat' started by bentomo, Sep 17, 2013.

1. ### bentomo Thread Starter New Member

May 6, 2012
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Hello, I'm trying to build a voltage sensor to trigger a speaker when my accelerometer reaches a certain angle. I'm still only in my basic electric circuits class and we're just getting into Kirchoffs laws and the like. I do have a bit of digital background with logic gates though.

My accelerometer outputs a 0 to 3.3v depending on it's angle. I just need an adjustable circuit, preferably using a resistor for the adjustment, that triggers a speaker when it's within a certain voltage range.

Say 1.6v to 1.8v I need the speaker to sound.

I would really appreciate the help guys! This forum has been very helpful in the past and I haven't been disappointed yet.

Thanks!

Also any notes on filtering caps a decoupling caps to clean up the signal would be helpful.

2. ### BillB3857 AAC Fanatic!

Feb 28, 2009
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So if I understand your question correctly, you need a way to COMPARE a voltage from the accelerometer to a variable voltage that you could set by means of, maybe, a potentiometer. (CAPS=HINT)

3. ### Brownout Well-Known Member

Jan 10, 2012
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Accelerometers typically output a signal that is proportional to acceleration, not angle.

4. ### sirch2 Senior Member

Jan 21, 2013
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... and gravity appears to be acceleration, so if the axis of the accelerometer is pointing to the centre of the earth +/- max reading, pointing tangential to the earth min reading.

5. ### Brownout Well-Known Member

Jan 10, 2012
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Since when is gravity an acceleration?

6. ### BillB3857 AAC Fanatic!

Feb 28, 2009
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32'/sec/sec at equator. If I have a rock in my hand and drop it, does it not accelerate toward the ground?

7. ### Brownout Well-Known Member

Jan 10, 2012
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Are all forces then acceleration? Should we re-write Newton's F=ma to a=ma???

Ok, I guess an inertial acceleromoter might work that way, since the measurement element would respond to a gravitional force. Never really thought about it...

Though it would measure 0 - 1g. Not necessarily full scale.

8. ### bentomo Thread Starter New Member

May 6, 2012
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Alright so I'm looking for something that compares voltages, I'll start my research there! Thanks guys, figuring it out is more fun than just being given a circuit.

Oh and I completely missed the sticky about decoupling and filtering caps, I'll just take a good read of that one.

9. ### Brownout Well-Known Member

Jan 10, 2012
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It's called.... drumroll.... a comparator. Connect the output of the accelerometer to one intput and set the threshold with a variable resistor or resistor divider at the other input. Don't try to drive too much current. Use a power output stage or power transistor if you need more than a few milliamps.

10. ### wayneh Expert

Sep 9, 2010
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Since the apple hit Newton in the head.

11. ### Brownout Well-Known Member

Jan 10, 2012
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I thought the apple accelerated due to the force of gravity. Silly me.

12. ### bentomo Thread Starter New Member

May 6, 2012
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You're thinking of the force of gravity, which requires a mass; such as you standing on a scale to weigh yourself, gravity by itself is acceleration.

Anyway, I was thinking of using a voltage divider as my ways of setting the comparators as to what to compare to. However is there another accurate way of setting my high and low voltage? Resistor voltage dividers seem to change based on the amount of current that changes in the circuit.

13. ### wayneh Expert

Sep 9, 2010
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It's properly referred to as the "force due to gravity". The force (weight) is the result of a mass experiencing acceleration.

14. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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The whole point of General Relativity (as opposed to Special) is that an object observed with respect to an accelerating reference frame is identical to the same object observed relative a non-accelerating reference frame within a gravitational field.

15. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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Nitpick: Force != Weight

Edit: Foot in mouth. Weight is force, as you said, measured in Newtons, which is [kg][m]/[s^2], and is dependent upon the magnitude of the gravitational field.

Last edited: Sep 17, 2013
16. ### Brownout Well-Known Member

Jan 10, 2012
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The weight is the same wether the object is accelerating or not. The force of gravity pulls the same if the object is accelerating, still or moving at a constant velocity. Gravity and acceleration are not the same.

17. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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As I corrected myself above, weight is dependent upon acceleration, and is not a constant. Mass is. This is why you can be "weightless" in space, but not mass-less.

Eric007 likes this.
18. ### Brownout Well-Known Member

Jan 10, 2012
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Weight does not depend on acceleration. In space, the weight is the same regardless of the acceleration. Weight is a force that arises from gravity acting on a mass. Force is described in Newtons while acceleration is described in Δvelocity/Δtime. The two are related by Newton's equation G=mA. Near the earth surface, gravity is a constant, and thus weight is a constant for constant mass.

EDIT: Sorry, the equation is W=mg, where g is the acceleration due to the force of gravity = 9.8m/s^2.

Last edited: Sep 17, 2013
19. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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As my old physics professor used to say: "Want quick weight loss? Go to the moon!"

Aug 5, 2011
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