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 transistor itself may provide higher gain, or higher frequency, or lower capacitance, etc.
The diagram above illustrates the frequency response of silicon phototransistor junctions, along with the spectral output of an infrared LED.
A Phototransistor Experiment
As an experiment, remove the metal top on a fairly large power transistor. You should use a hacksaw or Dremel tool. Cut carefully around the transistor until you can lift the metal top off the transistor.
Don't destroy what's inside the case! This is the silicon chip and it is what you need for this experiment.
Connect your current-reading meter to two of the transistor terminals. Set the meter to a low current value, say, a few milliamps. Then shine a strong flashlight directly into the exposed chip. Or, better yet, place the chip and meter hookup in direct hot sunshine.
You should see a current reading on the meter. If not, change your meter leads to two other terminals on the transistor. In some cases, you may need to use a small flame as a light source, such as a kitchen match or candle.
The reason for suggesting strong sunlight or a small flame is that when light of the proper wavelength hits a semiconductor material such as a PN or NP junction, it increases the concentration of charge carriers. Bright sunlight has both visible and infrared frequencies over a wide spectrum, and a burning object (such as a match) also radiates visible and infrared frequencies.
How It Works
The actual operation of a phototransistor depends on the biasing arrangement and light frequency. For instance, if a PN junction is forward biased, the increased current through the junctions due to incident light will be relatively insignificant. On the other hand, if the same junction is reverse biased, the increase in current flow will be considerable and is a function of the light intensity. Therefore, reverse bias is the normal mode of operation.
Now, if the PN junction is the collector-base diode of a bipolar transistor, the light-induced current effectively replaces the base current. The physical base lead of the transistor can be left as an open terminal, or it can be used to bias up to a steady state level. It is the nature of transistors that a change in base current can cause a significant change (increase) in collor current. Thus, light stimulation causes a change in base current, which in turn causes a bigger increase in collector current and, considering the current gain (hfe), a rather large increase at that.
Source: "Practical Solid State Circuits", by Robert C. Genn, Jr., Prentice-Hall, ©1983, ISBN 0-13-450643-X.
Last Update 2013-03-06
©1998-2017 Barry Hansen