Sensing optical tape
Revision as of 20:54, 26 December 2006
Reflectivity differences can be sensed using optoelectronic components. An Emitter and a Receiver are arranged so that more of the Emitter's light is seen by the Receiver when the target are is white, than when it is black.
Emitters provide a cone of illumination, and receivers have a cone of sensitivity, sometimes called a receiving angle. These components are available with narrow (e.g. 10 degrees) through wide (e.g. 130 degrees) full-width cones. You can aim the Receiver and the Emitter so that their cones overlap at the desired detection distance.
Infrared (IR) opto devices are popular, as are visible light (usually red.) Infrared has the advantage that Receivers are somewhat most sensitive in IR, and daylight and indoor illumination are low in IR thus avoiding that confounding factor. IR Receivers are often encapsulated in black or dark blue plastic that is transparent to IR, thus even more attenuating ambient visible light. 900nm is a typical IR wavelentgh.
Visible light opto devices have the advantage that you can better see what you are doing, and they are pretty. 660nm is a typical red wavelength.
Infrared emitters are sometimes called IREDs. Visible light emitters are just called LEDs.
Receivers may be phototransistors or photodiodes. Phototransistors have more gain: more current for a given amount of light. The have the disadvantage of being slow, typically 5uS rise time for phototransistors vs. 5nS for photodiodes. They also have more variability in gain, part-to-part, and more dependence of gain upon temperature.
Each LED or IRED has a maximum forward current If(max), typically 30mA. It also has a forward voltage drop (Vf) like any diode, which depends weakly upon forward current. 0.6v is a typical forward voltage for a silicon diode. A typical AlGaAs IRED may have a forward voltage of 1.9v at 20mA forward current.
Some LEDs and IREDs are much brighter than others. There are many figures of brightness. Some are intensity per unit area in the brightest spot of the cone of illumination, while others integrate the total light output over all angles. You have to be careful about this if you are comparing narrow-cone to wide-cone emitters.
To operate an LED or IRED you usually need a limiting resistor in series with it. Choose the limiting resistor so that when the supply voltage minus the emitter's forward voltage, is across the limiting resistor, it will pass the current you want, not to exceed If(max). For instance with a 5v supply, and anticipating a forward volatge across the IRED of 1.9v, we will have 3.1v across the limiting resistor. If the value of that resistor is 100 ohms, the limiting resistor (and the IRED) will have 31mA through them.
The longer lead is the positive one (top, in the diagram)