I'm building a chronometer for high speed projectile speed measurement. Currently i'm in the process of designing the projectile detection gate. It needs to be able to detect obstructions in light for a duration of just under 1 uS.
The design is supposed to work as follows: - The reverse biased photodiode wil be lit by a stable light source (battery powered led) - If the path of light is blocked from the lightsource the voltage on MP1 wil rapidly fall. - The voltage on MP2 is slightly negative biased to ensure it is below MP1 under normal conditions - Since MP2 has a RC circuit it's voltage wil change slower than MP1 - If the voltage on MP1 gets lower rapidly the gate will output 1.
So far so good, it seems to work with simple tests of breaking the path of light.
The problem i currently run into is noise, for some reason the signal at MP1 (measurement point 1) is very noisy, it seems irregular (for as far as my equipment can tell) and it measures over 20mV peak to peak, which renders the sensor unusable.
The following is measured with an ADC of an arduino (atmega328p) @ 200Hz +/- 1mv per unit.
The gradual fluctuations are caused by changes in light (probably by my hand) and are intended. However the irregularity in the blue line is causing the problem. As can be seen the noise is higher than 20mv and also higher than the offset voltage and object detection limits. This causes the output to constantly bounce up and down.
- Is this amount of noise normal for an Photodiode (SFH213)? and thus should i just deal with it?
- Is there anything obvious that i'm missing that could cause this noise?
- Could i filter the noise in a way that whould still enable me to see dips in the light of slightly under 1 uS?