ChepkwonyChatterjee

LED Flicker system design

Background

Human vision science has been generally defined based on the presence of four types of photopigments in the retina: rods and three types of cones. Rods are responsible for peripheral vision under scotopic and mesopic lighting conditions. Rods are generally not associated with color vision. The cones on the other hand are present in the foveal region and are responsible for color vision under photopic conditions. The L,M,S cones are sensitive to different wavelength ranges of light.

There has been a recent discovery (This paper talks about it) of another type of non-rod non-cone photopigment in the human retina, and this is melanopsin present in the specialized ganglion cells of the retina. Melanopsin in the retina has been studied to the extent that its primary function has been determined to be signaling changes in ambient light levels to the brain throughout the day for unconscious visual reflexes, such as pupillary constriction, and regulating a number of daily behavioral and physiological rhythms, collectively called circadian rhythms. It has been suspected that Melanopsin may play some role in the human visual system as well. The role of melanopsin in color vision and temporally varying light intensity patterns is an emerging research area.

To understand the role of Melanopsin in the human visual system, researchers need a setup for conducting psychophysical experiments. The goal of our project was to build such a device which makes it easy to carry out such experiments.

High Level Device Specifications

1. Should be capable of producing 4 or more primary colors.

   To test the theory that four color sensitive photopigments (L,M,S cones and melanopsin) contribute to color vision

2. The flicker rate of each primary color should be programmable.

   To test the response of varying temporal frequencies of light flicker.

3. The light intensity levels should be adjustable

   To allow for variation of lighting conditions to stimulate the photopigments to different levels.

4. Should be an easy to use compact portable design.

   The ideal design would be that of a black box with commands for input and a optical fiber which brings the light output. The hope is to integrate this device into an fMRI testing environment.

Design

Components

1. LED Array of primary lights

   For our final design (to be implemented in future): 7-in-1 round assembly LED Array

2. Polymer Optics 7 LED Cell Cluster Concentrator lens arrangement

   Shown in this page Lens Optics

3. Arduino Mega 2560 microcontroller Arduino website
4. A simple heat sink like Heat sink picture
5. A perforated circuit board (for the final design) and a breadboard (for making a prototype)
6. Circuit components
   1. N channel MOSFETs like the 50N06L Datasheet or similar.
   2. npn BJTs like the 2N3904 Datasheet or similar
   3. 2 Ohm 1-2 Watt power resistors (for final circuit), 30-35 Ohm resistors (for prototype)
   4. Large value resistances 10k-1M Ohm
   5. 5V regulated Power supply

Circuit

Driver circuit for the LEDs

To get a system that produces a stable light output, the LED must be driven by a constant current. An approach to get a constant current is using the circuit to the right. The way this circuit works is that the R_feedback resistor and BJT combination sense the amount of current flowing though the LED and feed this signal back to the MOSFET gate controlling its voltage and hence the current flowing through it.

The key parameter that must be selected with caution is the R_feedback resistance since that controls the maximum amount of current flowing through the circuit. This page mentions that an empirical estimate to the maximum current flowing through the circuit is 0.5/R_feedback. We performed some circuit simulations in SPICE to get a sense of the appropriate parameter values.

Seven copies of this circuit should be made to drive each LED, with the seven PWM output signals coming out from different channels of the Arduino microcontroller.

Results

Simulations

We performed simulations of the circuit in HSPICE, to understand the appropriate values of resistances required for the design and also to check the temporal characteristics in the ideal case.

• Simulation waveforms
• 
  Current and voltage waveforms R_feedback = 2 Ohm
• 
  Effect of temperature sweep on current

References

Software

Appendix II - Work partition

Hardware assembly - Tirthankar Chatterjee
Firmware edits - Isaac Chepkwony
Simulations and Testing - joint