I am working with a photodetecting circuit built as follow:
When I drive an LED ON and OFF, I get the following waveforms:
Blue indicates that the LED is turned on when a voltage is high and turned off when low.
Yellow shows brightness reading after RC high pass filter.
The lower envelope of the yellow line looks like a voltage discharging pattern of an RC circuit.
The problem is, however, the upper envelope seems like a saturated signal.
I can it more clearly by reducing the capacitance to 10 uF:
By reducing capacitance from 47 uF to 10 uF, a discharging pattern in the upper envelope after a short saturated duration can be seen.
But I am curious that
- Why only the upper envelope is saturated? Why not for the lower envelope?
- Why the upper envelope is saturated at nearly 200-300 mV, not around
VCC+, 5 V?
For your information, both the upper and the lower envelopes are not saturated if I remove the RC HPF and see voltage readings: