probing the CD4093 including Vcc should offer some hints for us. the first gate is the NAND function, the second does an invert and the 3rd basically increases the current drive. datasheet:
https://www.ti.com/general/docs/sup...=26&gotoUrl=http://www.ti.com/lit/gpn/cd4093b
NAND is NOT AND.
AND IS (Both inuts high, the output is high)
Inputs, AND, NAND
00 0 1
01 0 1
10 0 1
11 1 0
NAND has the output inverted, so 1, 1, 1, 0
Would need the actual voltages at the pins.
When the inputs are the same. you get inversion.
The voltage across the LED portion of the sensor should give an idea about aging. The LED intensity will drop with age. Compare the reflective surfaces with a known good sensor.
If you have the sensor pointed at the surface, you can look at
At Vcc = 5V, you have to be below 0.9V for a valid low.
The board looks easy enough to power by itself and use an isolated DC source for IN What is Vcc?
So, I see aging LEDs which will reduce the light output, Reflective surface issues and not adjusted to maximize signal.
When there is enough light, the voltage between the b-e junction could be as low as 0.4 V. Generally 0.6 to 0.7 V is assumed if you don;t know. In this case, it's important.