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Myopia of CNY70 remedied
Radiation pattern of CNY70 in front of the 'silver six' mirror on gas meters, separated by an acryllic window.
The CNY70 is myopic. It emits light in a diffused way, not in a bundle. It is designed for a working distance of 2..10 mm. Therefore, if you have the detector at a distance from the object that has to be detected and some window is in between, most light reflected will be from the window. Sometimes it helps to use a lens for focusing the beam onto the object.
A simple focussing mechanism is made from two sizes of electricity pipe. The larger diameter appears to fit snug on the smaller diameter. Two short pieces and a longer piece of pipe glued together with superglue make a simple platform on which the CNY70 can be fitted. The lens is glued onto a card, the card has a notch for the CNY70 and simply slid onto the CNY70, which has two grooves.
Radiation pattern of a CNY70 with a lens, in front of a water meter, where myopia is remedied by use of a tiny, short-distance focusing lens.
This solution works for a water meter, where the reflective surface is rather large.
The acryllic window is left of the bright white bump, and straight underneath the lens.
If a simple lens is not enough, because scattering light still influences the phototransistor more than the silver mirror does, an opaque sheet (1) can be slid onto the CNY70, that keeps direct or scattered infrared light of the LED away from the sensor. To top it off, a small lens (2) is glued onto the end of the separating board, projecting the IR light onto the phototransistor. The result is a reliable detection of the silver mirror.
The IR diode of the CNY is fed from +5V in series with a 220 ohm resistor. From +5V, a 100Kohm resistor is connected to the CNY, which on the other end is tied to GND. With this setup, the junction between the CNY and the 100K ohm resistor swings from 1.2V to 3V, which can easily be discriminated by a Soft-I/O alarm soft-device.
Successful projects of others can be found at this forum and here; they have both images and circuits.