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differential amp help

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krismoly

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Hi Folks,

This is my first post.

I require an amplifier that has two inputs and one output. The output of the amplifier has to increase as the difference of the two inputs increases. If the two inputs are at the same input then ideally the output should be zero.

Size and component count are quite important.

The inputs are varying levels of DC (0-5V)

Can anyone help.

Any help or suggestions are greatly appreciated.

Regards Kris
 
Hi Nigel,

Thanks for your speedy reply.

what configuration should I use for the op amp?
basically the amp needs to amplify the difference between two Ldr's.

you said that my post is somewhat vague? what other information do u require?

regards
Kris
 
krismoly said:
Hi Nigel,

Thanks for your speedy reply.

what configuration should I use for the op amp?
basically the amp needs to amplify the difference between two Ldr's.

you said that my post is somewhat vague? what other information do u require?

For a start, what you're trying to do (you've mostly answered that now), and what gain you need. If you google for 'opamp tutorial' you should be able to find plenty of opamp differential details.
 
hi guys,

i have investigated the op amp route but it is not quite what i'm looking for.

I will try to expand further, and i will also try to explain why the op amp differential amp is not suitable my for purpose.

I have two potential dividers consisting of an ldr (connected to + power rail (5v)) and a 10K resistor. each of these potential dividers is connected to and input. (with me so far?)

the output should be something along the lines of this

input 1 = input 2 then output should be ov

the input voltages will be anywere from 0v to 5v

the output should also be 0v to 5v depend on the difference of the two potential dividers.

The differential op amp is not suitable because I am not using a split rail supply.

I hope you all can understand what I mean?

any help or suggestions greatfully appreciated

regards
Kris
 
light tracker.jpg

as requested here is the full circuit diagram. protection diodes have been omitted.

hope this helps

regards kris
 
Opamps don't need a split supply. Their inputs must be within their power supply voltages, or within the positive supply and ground if there isn't a negative supply.

Most opamps have inputs that don't work if they have a voltage within 1.5V below the positive supply and within 1.5V above ground. Most opamp outputs cannot go higher than 1.2V less than the positive supply and 1.2V higher than ground.

Rail-to-rail opamps are available that have inputs and outputs that work as high and as low as the positive supply voltage and ground, but you can scale your input voltages to work with an ordinary opamp.
 
Why do you need a differential op-amp at all?

You are using a PIC with analogue inputs, let the PIC calculate the difference between the two voltages and drive the BFY50 accordingly.
Or am I missing something?

Also there is a mistake in your circuit as drawn, pin 12 of the PIC is connected to +5v and the second LDR is shorted out.

Also, the two relays could be replaced by one double pole relay, or driven from the same transistor. (Unless you intend to short out the motor to give dynamic braking).

JimB
 
Thanks Jimb,

Sorry for the mistake on the circuit diagram, it was a glitch with the software. It isn't actually connected to +5 (pin 12).

At present the pic calculates witch ldr is brighter or darker and turns on appropriate relay. (I need to control the direction of the motor.)

The amp is intended to control the speed of the motor, without this control the motor will overshoot the target, and will result in constant seeking.


I need to leave some spare pins on the pic for future developement.

The pic I am using doesnt have PWM, I am new to PICS so I am not able to do a bit banged pwm.

Hope this has clarified your query:confused:

Regards

Kris
 
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