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Help in boosting sensor output voltage

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aerogramma

New Member
Greetings to all,
I've a project where a Sharp distance sensor controls a range of other devices.

Those devices expect a voltage of 0-5v however the Sharp sensor by default only outputs between 0-3v.


My understanding is that I should use an op-amp stage to boost the output signal to reach 0-5 values but how to and the maths involved baffle me.

(The sensor is powered by a 5v supply and the rest of the project is on 9v.)

Anyone here can point me in the right direction?

thanks in advance :)

aero
 

marcbarker

New Member
No idea what you mean by "other devices". I thought "maybe it's me?" so I tried googling for a Datasheet for 'other devices', but millions of results came up. Maybe you need to be a bit more specific about your 'other devices'? :)

I'll guess the "5 V input range" devices you mentioned would work fine with the Sherp sensor's 3 V output, without any extra circuit needed.
 
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aerogramma

New Member
ahahah.. you're right i should have been more specific

one of the infamous devices is an oscillator, so i really need the 0-5v range to take full advantage of the Sharp sensor controlling it

the sharp sensor belongs to this family

GP2Y0A700K

:)

aero
 

ericgibbs

Well-Known Member
Most Helpful Member
Those devices expect a voltage of 0-5v however the Sharp sensor by default only outputs between 0-3v.
aero

hi,
This circuit will do what you ask.:)

I would make R2 a 5Kpot.
 

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ericgibbs

Well-Known Member
Most Helpful Member
Can R4, R5 be omitted?

(change R2 value needed)

hi,
They could be omitted, but the circuit would require R2 to be less than 1K, as the gain of the NI is G= [1+R2/R3].

Its a preference of mine to fit a simple resistor divider so that the INV input gain is equal to or greater than unity.

If the OP wants to change it, I dont mind.:)

EDIT:
Some of the 'earlier' OPA's used to be unstable with gains less than zero.
 
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crutschow

Well-Known Member
Most Helpful Member
Its a preference of mine to fit a simple resistor divider so that the INV input gain is equal to or greater than unity.
eric,
Don't understand the reason for this preference. You mention the INV gain but there is no INV gain. It's a NI amp with a gain of 1/2 input attenuator. This means the op amp has to operate at a higher gain to achieve a given I/O gain which increases the effect of input offset and reduces the bandwidth. Also the input attenuator reduces the input impedance.

So I don't see any upside to adding the input attenuator which requires two additional components.
 

ericgibbs

Well-Known Member
Most Helpful Member
eric,
Don't understand the reason for this preference. You mention the INV gain but there is no INV gain. It's a NI amp with a gain of 1/2 input attenuator. This means the op amp has to operate at a higher gain to achieve a given I/O gain which increases the effect of input offset and reduces the bandwidth. Also the input attenuator reduces the input impedance.

So I don't see any upside to adding the input attenuator which requires two additional components.

hi Carl,
As I mentioned, some of the early OPA's do go unstable, when the Rf/Ri is less than unity. I have no idea what the OP is going to use an amp.??

With regard to the BW its reduced from 914KHz to 875KHz, which I think in this application will have no effect.

With ref to the input impedance as you say it will be reduced to ~100K, which again I dont consider to be a problem.

Considering the possible misuse of a circuit while an OP experimenting a limiting resistor isnt a bad idea.

If the OP wants to drop the divider and trim the R2 pot to around 670R I dont mind.

Although I may sound I little negative, I do appreciate your comments on any of my posts.

Regards
 
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crutschow

Well-Known Member
Most Helpful Member
Eric,

OK. I missed what you were saying. I see now that if the NI gain is less then two, then Rf/Ri would be less than unity. To avoid this the NI gain must be >2 thus requiring an input attenuator for NI gains between 1 and 2.

Basically this is only a problem for any op amp that is not unity gain stable in the NI configuration.
 
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