Practical Coilgun Design How Phototransistors Operate So you
want to detect the projectile's postion. A useful device is a
phototransistor. How does it work? Phototransistor Frequency
Response All silicon photosensors (phototransistors, etc.)
respond to the entire visible radiation range as well as to
infrared. In fact, all diodes, transistors, Darlingtons, triacs,
etc. have the same basic radiation frequency response. This
response peaks in the infrared range. This is why manufacturers
offer infrared-emitting diodes. Their goal is to maximize the
signal-to-noise ratio, by using an emitter with the best match
to the phototransistor response. However, note the response is
very broad and virtually any light source will work. Basically,
a phototransistor can be any bipolar transistor with a
transparent case. There are some variations provide advantages.
For example, a focusing lens can be built into the case for
directional sensitivity. Coatings can be applied to block some
higher or lower wavelengths. The trans