Comparator, but need custom reference.

[oasacorp]

I designed an IR sensor. The RX of the IR LED without any obstacle shows a voltage of about 2mV. Now when some object is present, it reads b/w 8mv and 50mv. I would like to design the circuit in such a way that when that happens an LED should glow. What are the solutions ? An obvious solution would be to use an opamp as comparator. But i dont know how to set V- (vref as 2mV with consistency). Please help me. Any other soultion is also appreciated.

 

[sijukrishnan]

I understand RX of IR LED is pulsating, isn't it?
If the output is DC as you described, the S/N ratio is poor. You can't expect a stable High or Low state at the output of the comparator if the signal is directly fed. To provide a healthy hysteris to the comparator, amplify the signal to a sufficiently strong signal compared to the noise floor. If you provide gain of 100, you can set 0.4V reference and design a realizable circuit.

 

[prakashvenugopal]

hi,

As suji said, You have to amplify your 50 mV signal with the OP-AMP and fed this to the comparator I/P to turn ON your LED.

 

[ea.arun]

a transimpedance amplifier with a hysterysis comparator should solve this issue i believe.....also make sure to add a filter with amplifier.....

 

[oasacorp]

Sorry for replying late, So what you mean to say is: I amplify this 50 mv enough to light an LED switch ? I tried that but the problem is I want it to be like a switch. When I do this it faintly glows @ a low output also Thank you for replying

 

[SWINI]

can you try applying a feedback circuit to that int he form of ratio of resistances. then the Vref = ratio(r) * Vout

 

[Pjdd]

We'll be able to help you better if you give more details about your design and perhaps suggest alternatives to your present scheme. A schematic showing averything involved would be best.

For example, unless you're operating the sensing system in a cold dark environment, there are many interefering sources of IR radiation. The usual approach is to drive the IR emitter into pulses and use a frequency-selective receiver.