Light Sensors

Cameras@ LED@ Wearables@ Infra-Red I/O@ Fiber optics

Richard Ottoson says:

...a quick rule of thumb about opto's:

• Photo diodes have about a 1us response time,
• photo transistors have about a 10us response time, and
• photo darlingtons have about a 100us response time.
• CdS (Cadmium Sulfide) photo resistors have about a ___s response time.

These times assume a "reasonable" resistor value. There are amplified optocouplers that are in the 100ns range as well.

Application notes on photodiodes ( A Primer on Photodiode Technology http://www.centrovision.com/tech2.htm has some simple circuits for photodiodes, and tons of nit-picky detail on how they work. Offline as of 2009-06-13 -- has it moved elsewhere?  Perhaps to http://www.osioptoelectronics.com/application-notes.asp)

Brian Kraut says:

Go to your local disco and theatrical lighting supply store and ask for for a gell sample book. Voila, about 100 different color plastic filters in 1 X 3" size. Some of them even have spectral response graphs for each sheet.

James Newton Says: " For optical interruption sensors, don't forget your standard super low cost "mechanical" mouse! "

Phototransistors

Glossary:

Dark Current

A small value current that flows in a photodiode while reversed biased, due to thermal generation of carriers and surface leakage with no incident light (also called leakage current).
Range: a few pA to 10 m A

Photo Current

Flow of current generated by a photodiode in response to an incident optical power. The absorbed photons generate free charge carriers that are collected within the

Photo Voltaic Mode

Unbiased mode of operation of a photodiode, preferred in low frequency (up to 350 MHz) as well as ultra low light level applications.

Photo Conductive Mode

Reverse biased mode of operation of a photodiode, in which lower capacitance and faster rise time are obtained. Generates more noise (dark current).

Shot Noise

Noise related to the statistical fluctuation in both the photocurrent and the dark current. It is the dominant noise in the photoconductive mode.

Shunt Resistance

Defined as the current flow in the dark of a photodiode when reverse biased at 10mV. More current flow reduces the effect of noise.

Thermal Noise

All devices have a thermal (or Johnson) noise associated with them, due to thermal generation of carriers

Noise Equivalent Power

Optical power necessary to create a photocurrent equal to the noise, to obtain a signal-to-noise ratio of 1. It depends on the noise current and the responsivity.

Responsivity (or Sensitivity)

The ratio of output current (in Amperes) to input light power (in Watts). It is a constant in the dynamic range, at a given wavelength.

Degradation

A drift in output for a detector submitted to a constant input optical power for a long period of time.

See also:

• http://vorlon.case.edu/~flm/eecs245/Datasheets/Sharp%20photodevices.pdf Gives overview of basic photoiode application and then provides a number of transistor and opamp based application circuits which demonstrate practical aspects. Brief but more than usually usefuil.
  
• http://www.osioptoelectronics.com/application-notes.asp A collection of app notes, FAQ's, Glossary, Links, Resources, Calculators, Tables (see left side)
• "Line Tracking Robot Project" http://www.seas.upenn.edu/~ese111/robot/Robot.html by Sohrab Rabii and Siddharth Deliwala. Shows a complete line-tracking robot circuit, including photosensors, in excruciating detail.
• "Visible and Infrared Light Detectors" http://home.cogeco.ca/~rpaisley4/PhotoDetectors.html by Rob Paisley
• "Laboratory Fundamentals in Biological Engineering" http://ocw.mit.edu/NR/rdonlyres/Biological-Engineering/20-109Spring-2006/51EE27E4-D56A-4292-BAA3-BAAFE889223D/0/mod3_proto15.pdf
  Shows a simple photosensor circuit, and compares it to much more complicated (living) stuff.
  

  The purpose of the photodiode is to serve as a light sensor (like Cph8 and the phycobilins) and the LED as a readout (like lacZ). The 150Ù resistor is there to ensure that the voltage drop across the LED isn't too high (which can cause the LED to breakdown). The inverter provides a convenient way of connecting the photodiode to the LED. The other resistor RL is there to ensure that the signal between the photodiode and inverter are matched. When light shines on the photodiode, its resistance is decreased and current flows through it. Thus, point a in the circuit diagram is at high voltage (near +5V) and the input signal to the inverter is high. The low output signal (point b) from the inverter creates a voltage drop across the LED causing it to turn on and emit light. In the absence of light, the photodiode has high resistance such that little current flows. Point a in the diagram is at low voltage (near 0V) and the inverter produces high output at point b. The resulting lack of voltage drop across the LED means that the LED is turned off. Much of this exercise will focus on RL and ensuring that the signal between the photodiode and inverter are matched for proper circuit operation.
   
  V_IH / I_L < R_L < V_IL / I_D
   
  • V_IL: Highest voltage at point a that is a low input signal for the inverter.
  • V_IH: Lowest voltage at point a that is a high input signal for the inverter.
  • I_L: Current that flows through the photodiode when light is shone on it.
  • I_D: Current that flows through the photodiode when it is in the dark.

• "Simple optical switch" http://www.electronics-lab.com/projects/misc/017/ by Jonathan Filippi

  

  When the phototransistor is struck by IR light it conducts and the voltage between the 1Mohm resistor(arbitrary) and the phototrans drops from VCC to lower values. When the voltage drops lower than VCC/3 the 555 is triggered and goes high (from 0 TO VCC). The amount of light that strike the phototrans necessary to bring his collector to VCC/3 is determined by the resistor (Vdrop = Icollector * R , so , if Vdrop= 2*VCC/3, the resistance needed to set the threshold on current is R=2*VCC/(Icollector*3)). High sensitivity phototrans would need a smaller resistor, and weaker phototransistors higher value resistor, you can also use a trimmer to set the "on" threshold level with precision. The type of phototransistor isn't critical. The 555 has high current capability and can drive various devices, such as Bipolars, relays, bipolars+relays, mosfets, mosfets + totem pole , or give a logic output (see pic).

  In case you need to trigger something when the gate is blocked (for example a burglar alarm, or a multistage coilgun) you need to invert the output, which is accomplished using a small bipolar transistor wired in an inverting setup (see pic) or by swapping the positions of phototransistor with the resistor, so the voltage will drop under VCC/3 when blocked: The formula to determine the resistance to turn off at Icollector is R=VCC/(Icollector*3).