 Using differentiator for high speed optical sensors
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TheMerovingian
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Tue Aug 31 2010, 10:32AM
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Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
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Despite having an interesing sensibility (tunable using the load resistor) the response time (in particular from light to dark transiton) is slow introducing errors due to intrinsic response time differences from the two sensors. Don't know if this type of design is used (probably yes). Well, the idea is to use a differentiator (or high pass active filter) to sense the projectile passage, also rising a bit of immunity to external lighing (if the sensors are not saturated). The circuit would consist in the two phototransistor sensors (with load resistor) and an active integrator (using an opamp with reasonable bandwitch). The time constant is chosen to attenuate the 50/60Hz hum of lighting to avoide false readings. The output impedance during light to dark transition will limit the gain at high speeds to a chosen value proportional to the ratio of Rf and load resistor (C will become negligible and the phase will approach 180° just like an inverting amplifier). At higher speeds the gain will be limited by the slew rate /bandwitch.
It works like an edge detector substantially.
Preliminary experiments show interesing results using a phototransistor-IR led (3mm) pair with 3.3K load resistor and the differentiator (1uF 100K), all connected to a scope (one channel for phototransistor, one for output)
After solving some strange drift problems for subsequent shots (the capacitor had not the time to discharge if the shots are too close in time) the system worked very well, and was able to detect the passage of a 24 gauge magnet wire(partial coverage) with a 5 cm sensor separation in daylight (not direct sun).
Two sensors will give two pulses that can be used to turn on and off a SR flip flop allowing a PIC microcontroller to measure the duration of the pulse itself.
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