I am trying to make an infrared proximity measurement device.
I want it to be in the range of 10cm or 4" (maybe 15 cm?). The frequency I use is 10 KHz. Here is the circuit I used, except that I have used 1 nF capacitors and resistors that suits them for band-passing 10 KHz. I have used LM358A for the OP-AMP and I don't know the part ID of my IR diode.
To increase the sensitivity and remove the offset, I added a difference amplifier with a gain of 10 using the other OP-AMP inside the LM358A. I've used a potentiometer to set the voltage to be subtracted from the out of below circuit.
It works! With a reasonable linearity. However, the voltage levels change with the day light intensity.
Is there any way to make this device immune to the daylight using an LDR? I've tried to connect the LDR in parallel with the offset removing potentiometer, however, as obvious, that didn't give good, logical results. I do not have any IR filters and it is really expensive to get them from Farnell or such in Turkey.
Here is my schematic: