Hi everybody.

I want to design a reflection sensor with phototransistor and ir led. I have to use this system behind a glass (thickness is 10mm). Also this sensor must work in both dark and bright ambient. The sensor works good in bright ambient because of characteristic feature of phototransistor. In very bright ambient the output of sensor is 5V. When i shade, the voltage start to reduce to 1.5V.

But in dark ambient it not works. so i used an ir led. when it is dark, phototransistor detect the ir light which reflect from object. And also because of glass characteristic, some of ir led reflects from glass. So when it is dark the output voltage is 0.5V and if there is an object above the sensor, the output start to increse to 2V.

So in bright ambient the output 5V--->1.5V Reduce
in dark ambient the output 0.5v--->2V Increase

if i capture the voltage between 1.5V and 2V, i can use this system, i think. I need a circuit which gives me an output between 1.5V and 2V. Is it possible?

Thank you.

 

Have you done everything you can to put a barrier in place to block light taking a short path from source to sensor?

My best suggestion is use a source that's pulsed at a particular frequency, and a sensor that has a filter that passes only that frequency. You could also try to null out the output that's caused by the reflection off the glass, by subtracting some amount from the output with no target in place. Then continue to subtract that amount all the time, and any output that you see beyond that, must come from the target.