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Accelerometer Interfacing

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jimbo1954

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I'm trying to build an indicator to show acceleration either to the left or right, using a bargraph of LEDs.

The Bargraph is fairly simple, using a LM3914 bargraph driver. This device wants to see an analog voltage input on one of its pins, and, as the voltage on this pin rises, so it switches on more of the LEDs.

A solid state accelerometer along the lines of an ADXL202 delivers, amongst other things, an absolute voltage for left-right and front-back acceleration. Looking at left right acceleration, if there is a 1g left acceleration on it, it will deliver 0v. A 1g right acceleration on it will deliver 8v. Zero g acceleration delivers mid-range voltage, i.e. 4v

What I need is a circuit that can take the linear, composite 0-8V accelerometer output, and deliver two voltages, as follows:
* one which runs 0-5v (say) ON ONE OUTPUT WIRE when the accelerometer output ranges between 4v and 0v (so accelerometer output =0v/circuit output=5V, accelerometer output =4v/circuit output=0V);

*the other that runs 0-5v (as before) ON A DIFFERENT OUTPUT WIRE when the accelerometer output ranges between 4v and 8v (so accelerometer output =4v/circuit output=0V, accelerometer output =8v/circuit output=5V);

This will allow me to drive two LM3914 bargraph drivers, one which will respond in a linear manner to 4-0V, the other which will also respond in a linear manner, but this time to 4-8V.

I'm at a loss....I guess I will need to bias the circuit with a Vcc/2 battery , I maight need to use comparitors, whatever. Any body got *any* ideas???

Cheers

Jim
 

Roff

Well-Known Member
I posted a schematic on the other forum.
This should work. There may be a simpler way to do it.
V1, the -4V battery, is actually a voltage reference, and it could have another value, so long as V1/R3=400uA. For op amps, try TL074 or something similar.
U1 inverts the input and level-shifts it to be symmetrical around zero volts. Full scale (FS) left at the U1 output is +4V, FS right is -4V.
U2 reinverts the signal. FS left at the U2 output is -4V, FS right is +4V.
U3 and U4 are half wave rectifiers that clip off the negative portions of their respective iputs.
U5 and U6 are voltage followers, to give you a low output impedance that will drive a light load without error being introduced by the load.
You probably don't need U5 and U6 if you are driving LM3914s.
 

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jimbo1954

New Member
That is *exactly* what I needed! Thanks Very Much!

One small last question (this guy is *never* satisfied!), the diagram shows the Op Amps as 1N4148...and Google as I might, I can only find diodes referenced as such...

My supplier has several variations of op-amps, but none with that number (it may be because I am in the UK and have been looking at products available at www.maplins.co.uk).

Can you give me any further help on the nature of the Op Amps...are they common or garden variety, that I can swap with any similar product? or do they have carefully selected characteristics?

Thanks

Jim
 

Nigel Goodwin

Super Moderator
Most Helpful Member
jimbo1954 said:
That is *exactly* what I needed! Thanks Very Much!

One small last question (this guy is *never* satisfied!), the diagram shows the Op Amps as 1N4148...and Google as I might, I can only find diodes referenced as such...
That's because the reference is for the two diodes to the right of the opamps! :D
 

jimbo1954

New Member
You make a *very* fair point....D'oh!

Can't see the wood for the trees some days...it's sad, I can even see the OpAmp referred to as TL074 in the answer now....I probably shouldn't be out on my own:)


Thanks

Jim
 
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