lock in amplifier for IR LED and Photo darlington

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4tuty

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i need a simple lock in amplifier for my circuit i used 555 timer with 1khz to irl81a and photodarlington op560c

doo i need any non inverting amplifier for my photodarlington output?
 

A lock-in amplifier or synchronous detector is a simple AC-coupled opamp with a key operated from your refetence source (555). There are many good versions available, check google. Analog Devices offers one IC designed as the sync detector but is expensive. I used a LM324 quad opamp with CD4016 or CD4066 quad key in my circuits.

After your darlington some AC gain amplifier may be needed: what is the 1 kHz signal range at its output?
 
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Did you check if a synchronous rectifier is actually required for your measurement application? A simple AC measurement may do as well?

If you are attempting any quantitative optical measurement, a photodiode should be used instead of a phototransistor, because the transistor is very nonlinear and has temperature-dependent sensitivity.
 

I have updated my circuits and waiting for your valuable comments
 

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  • New Doc 2.pdf
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Your schematic looks OK. What is the signal range at photodiode output?

- - - Updated - - -

For start you can also use an audio millivolltmeter connected to photodiode or opamp output.
 

Hi,

a more detailed schematic of your photodiode circit would be good.

do you use an ADC and microcontroller to process the data?

Klaus
 

i am using +15 and -15 V to bias opamp shows good results. But i have to make a device to work on 5volt


i have attached so for i did and spec sheet of IR and Photo detector.

need more guidance regarding circuit whether i am going in a correct way:???:
 

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  • 1st schematic.pdf
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  • IRL 81A, Lead (Pb) Free Product - RoHS Compliant.pdf
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  • op560a (1).pdf
    336.1 KB · Views: 137

You can't use AD630 or LM741 with 5V single supply. Also the amplifier circuits must be biased differently for single supply operation.
 
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    4tuty

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Hi,

don´t use the old 741 opamp. There are way better opamps.

Use lower ohmic resistors for both OPAMPs. Like 1k / 10k (lower noise, more speed)


**
To make it work on +5V only supply, at first step you could add a 10k to your photodarlington in parallel. This lifts up the 100R voltage to 50mV.
This is enough for a Rail to Rail I/O OPAMP. It should generate no disadvantage.

If the DC makes problems you need to supress it.

Klaus
 
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FvM said:
Instead of the AD630, you can use a "sign-multiplier" made by an OP and an analog switch. https://www.edaboard.com/threads/272103/

What's your motivation for using asymmetrical (10%) duty cycle?
Click to expand...

i want to detect small change in refractive index of water when 0.1mg of sugar is added
duty cycle i am not quite sure about it,, shall i use 50%. let me try signmultiplier will it good in small change in detection with accurate and less noise compared to lockin?


@Klaus

thanks for the reply sir. i will try your method. could you suggest me a good opamp:???:
 

The "signa multiplier" does essentially the same as AD630. There should be no difference in noise behaviour.

It's not bad to pulse the LED with 10 % duty cycle, but using the 10% duty cycle signal as reference for the synchronous demodulator will hardly give optimal signal-to-noise ratio. A straightforward scheme filters the 100 Hz fundamental before the synchronous demodulator and uses a 50 % duty cycle reference.
 
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Hi,

Good Opamp:

There are so many around. And almost every is better than 741.
Look for RR I/O, specified with single 5V supply, GBW > 500kHz.

Klaus
 
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Last edited:

IR and Photodarlington circuit doubt

concept:
I use IRL81[LED] as source of 1KHZ 50% duty cycle PWM and OP560A[photodarlington] as Photo detector .
Connecting source and detector via Fiberoptic with small Portion decladded where different samples to be introduced.
When samples introduces in the decladed fiber there is a Changes in intensity. This change in intensity changes the Photo Darlington(PD) voltage
now i need to find the difference between the reference signal 1khz 50% duty cycle PWM and PD.

prob:
Need to detect change in amplitude as well as phase.

Working model: i used 1khz 50% duty cycle(don't get confused as shown in Image 100hz)



 

Re: IR and Photodarlington circuit doubt

Hi,

Look for "lockin amplifier".

For phase detect I recommend to generate a second 50% duty cycle signal but withexactely 90° phase shift.
(Easy with a PLD with twice the input frequency).
Use this signal as reference for a second lockin amplifier.

With the output signals of both lockin amplifiers you can calculate amplitude as well as phase shift.

Klaus
 
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Re: IR and Photodarlington circuit doubt


exactely 90° phase shift. (Easy with a PLD with twice the input frequency).

could you help me with the circuit for 90 phase shift of 1khz 50% duty cycle pulse please:-(

if this is the circuit i need capacitor and resistor value ir help me how to calculate the values
I searched EDA boad but cannot find idea
 
Last edited:

Re: IR and Photodarlington circuit doubt

Hi,

Just recognised you nee four times the desired output frequency.

In a PLD with 4 kHz input clock, you just use a two stage binary synchronous counter and a DFF.
Use the second output (Q1) of the counter as one clock signal.
Feed the same signal through the DFF and use it's output as second 90° shifted clock.

Because both outputs are clocked from the same clock source I expect the deviation from 90° to be far below 1ns.

Klaus
 
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I used AD630 [bias changed to +9v and -9v] for lockin application and the Yellow color is Photodarlington Output[CH1], Blue color waveform[CH2] is my Lockin output. The circuit which i have used is also attached.








The image 20151012_211607.jpg is before sample and 20151012_211655.jpg is after sample.

Now i am trying to measure the amplitude change. I want the output waveform to be a DC output so that i can give input to Arduino A0 to measure the data in serial terminal.

Guide me whether i need to go for Low pass filter/ Bandpass filter/ or i need to use any instrumentation amplifier to get precision readings

Advance Thanks to EDAboard
 

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