This page highlights the electronics theory behind the photointerruptor principal. The information on this page applies to those found in paintball markers as well as virtually every other emitter-detector photointerruptor out there (door sensors, automotive systems, etc).
Electronics knowledge is recommended for this article, but not necessarily required. I spare the heavy details wherever necessary. If you have questions then I'd be happy to answer.
Components:
All anti-chop eyes consist of one emitter/detector pair. The emitter is a light-emitting diode (LED for short) which emits light toward the detector. The detector is a small photo-transistor which reacts with light entering it, and sends an outbound signal. An assembled emitter and detector is often known as a photointerruptor or photointerrupting sensor.
Seen above is one possible package type for a pair of eye elements. Other packages can be seen further down the page.
Emitters are nothing more than LEDs, which are a type of diode (one way conductor) which produces light. The exact wavelength (color) of the emitted light depends on the speed of the electron displacement and the permeability of the semiconducting filament. Most markers out there use an infrared eye type, whereas others use a visible emitter. Infrared light emitted by the diode exists within the spectrum somewhere between 800-950 nm, which is invisible to the unaided human eye. Visible light is often seen as red colored LEDs since red is the longest wavelength of visible light. However, some eye systems can use other colors for visible eyes, depending on the circuit and components.
The detector element in the system is a type of phototransistor, which is a device that absorbs electromagnetic radiation and uses it to send a signal depending on how much light is received (this is done by amplifying the transistor's incoming voltage). Electrons are emitted through the transistor anode where they interact with electromagnetic waves outside of the lens, and then are later reabsorbed through the transistor cathode.
Some phototransistors use a visible-light filter, which is a coating that will prevent ambient light from entering the lens and interfering with the sensor. Detectors with VLF coating will appear dark or opaque; if the lens is transparent then it doesn't have a VLF filter. The VLF coating doesn't prevent other electromagnetic waves from entering (gamma rays, X-rays, microwaves, radio waves, etc.). Because of this, the detecting phototransistor usually has to be covered to prevent outside energy from interfering with the circuit. But the specifics of those waves interacting will vary and I don't want to discuss it here.
Eye Function Types:
There are two types of photointerruptors, a reflective photointerruptor and a transmissive photointerruptor.
Reflective eyes function by emitting light into space which is then bounced back when an object moves close enough to the sensor. The emitter and detector are placed right next to one-another, to help aid this reflective process. Check the pictures below for some examples.
Reflective eyes were used heavily by SP markers. Some other markers and equipment use reflective eyes, including E/X-Mags, Halo hoppers, E-Blade E1 kits, Autococker Raceframes, and possibly others. Halos in particular use two separate eyes (emitter and detector) which are normally used for beam-break purposes. The eyes are positioned so the emitter and detector are angled together.
Transmissive eyes are also known as beam-break eyes and function using a similar method. Beam-break eyes are positioned opposite each other inside the marker's chamber. When an object enters the chamber, the beam from the emitter to detector is broken, which denotes the ball being loaded.
Beam-break eye systems always use two separate eyes (emitter and detector) however they may be physically linked together to help make the assembly process easier. Some markers have separate, removable eyes which can be taken out individually (examples would be Intimidators, DM4/DM5, Egos, and others). Markers such as Ions, DM3s, and Revolution hoppers use an integrated eye set which will remove the emitter/detector as one component. See the below pictures for some examples.
Beam-break eye systems are more common in paintball and can be seen in most markers and parts.
Here are some sample pictures of different reflective eye types used in various markers.
Here are some sample pictures of different beam-break eye types used in various markers.
Both eye systems have their own advantages and disadvantages. In terms of raw performance, typically beam-break eyes can view a larger section of the spectrum in reference to paint loading. Reflective eyes have to be specially programmed to recognize a paintball loading, and if the programming suffers then the performance may be limited. The advantage to reflective eyes is that they can more accurately view the position and/or speed of the moving object, and can also prove easier to use due to the eyes being physically integrated/linked. Neither eye type is universally superior to the other, however most developers choose to use a beam-break system for the general reliability. In most cases I agree with this trend, although some situations necessitate the use for reflective eyes out of simplicity, or due to the arrangement of surrounding parts.
Circuit Design:
There are two circuit methods that are used with eye systems out there. In either case, the circuit design is laid out such that the detector will receive light from the emitter, and produce a varying response depending on the amount of light that is received. Either of these eye types can be used for reflective or beam-break eyes.
· Type-1 circuit design: This seems to be more prominent outside of paintball. This circuit uses the detector's output side as the eye signal. This means when light passes into the detector, the output signal increases; when light is cut off, the output signal decreases since the detector is no longer receiving light. The detector signal uses a pull-down resistor to pull the signal down; current is pulled away from the detector when energy passes through that portion of the circuit (this resistor is typically a 4.7-Kohm).
Beam-break - eye signal is high when no ball loaded; eye signal is low when ball loaded.
Reflective - eye signal is high when ball loaded; eye signal is low when no ball loaded.
· Type-2 circuit design: This system is the opposite of the above. The output side of the detector is literally grounded out, and the signal comes from the positive voltage (+V) side of the detector instead. Under this principal, when no light interacts with the detector, the signal will be high (since the detector isn't being grounded). When light passes into the detector, it becomes grounded which pulls the outgoing eye signal down with it. Effectively, this turns the detector into a miniature electronic gate. A pull-up resistor is used in this eye type, however it must be placed before the detector in the circuit (typically 20-Kohm is used, depending on the detector type).
There isn't an advantage to either system. Examples of markers that use a type-1 circuits include Impulse, Shocker/Nerve, Matrix, Viking/Excalibur, Halo boards, and others. Examples of markers that use a type-2 would include Intimidator, Ion, and others.