Illumination ring provides focused intensities
Light up an area for optical inspection using LEDs under PWM control.
William Grill, Honeywell Aerospace, Olathe, KS; Edited by Martin Rowe and Fran Granville -- EDN, July 9, 2009
If you use a camera-based inspection or soldering fixture, you need to see images in a small area. Often, side lighting produces shadows on an image that result in contrasting colors and poor quality. Thus, your monitor views may be difficult to clearly see or interpret. Centering a light ring on the image provides illumination on all sides of the object and may illuminate everything you need to see. In a camera application for controlling a light ring, this implementation not only controls the light, but also enables you to direct the light intensity by maintaining two levels of control. It also lets you maintain and rotate the second-tier levels about the illuminated object.
Based on a seven-LED set, you select three consecutive LEDs; the second-tier settings will define the three LEDs' intensities (Figure 1). The remaining displays are maintained at a base-tier-intensity setting. Using four pushbutton switches, the Microchip 16F505 rotates, distributes, and provides PWM (pulse-width-modulation) control of these two power tiers across the seven LEDs. Two of the buttons increase or decrease intensity, or they group or ungroup the tier-intensity settings; the other two buttons rotate the resulting second-tier display clockwise or counterclockwise.
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The implementation uses just a few parts, exploiting the controller to provide light level, state maintenance, and PWM control. The application debounces the buttons and indexes the intensity controls. An eighth LED indicates tier-grouped or -ungrouped mode. When you group the tiers together, their intensity-setting indexing is common, but their register limits remain independent. You can download Listing 1 which is the assembly code for the circuit.
The controller provides a PWM period of approximately 7.5 msec to all the LEDs. It also controls each LED's duty cycle, according to the registered levels the button sets, in defined and maintained register masks and intensity values. The controller provides six bits of intensity, corresponding to 64 levels of resolution, although 8-bit resolution is available. The operating voltage is 5V. You can reconfigure the controller, the display, and their limiting resistors to operate at voltages as low as approximately 3.1V. High-millicandle, white, 5-mm, T1¾ through-hole LEDs provide the light source. The controller provides about 8 mA of current to each of the LEDs. By constraining the total power, surface-mount or other LED configurations are possible.
You can lay out the four momentary-action pushbutton switches for operation by the left or the right hand. With one representing the pushbuttons' asserted position, the controller's coded sequences provide the button-control functions found in Table 1.
