A prototype of a laser projector which can display dot matrix style text onto a screen a few feet away has been built. To further enhance the projectors functionality, it can be interfaced to a mobile phone (currently in progress), so that the projector may be controlled from a remote location.
The projector itself consists of a rotating polygon mirror assembly and 5 lasers focused onto it. As the mirror assembly rotates, it creates a sweeping pattern and by precisely timing of the on/off times of the individual lasers, in conjunction with the speed of the motor, useful information can be displayed onto a distant screen.
Currently, it accepts data via a serial; port from a PC and has a menu interface via the serial port on a PC, where the user can change various parameters of the display.
The use of a cell-phone as the input source for the projector will further enhance its usefulness by way of making it more flexible and user friendly, not to mention more portable. Using a mobile phone makes the projector accessible from practically any location that has cellular phone coverage and is especially of importance if it is used in an urban environment where running cables or providing a dedicated wireless link will be prohibitively expensive.
The projector will consist of a triangular mirror assembly as shown (4 lasers shown for convenience).The human eye can be tricked into believing that an image is actually static if a minimum frame rate of at least 24 frames per second is maintained
How the image forms:
Fig2 Formation of the alphabet A
The motor’s speed is controlled using PWM (pulse width modulation).The program calculates the time for each revolution, determines whether it is rotating too fast or too slow and increases or decreases the duty cycle of the supply to the motor accordingly
This projector currently displays a maximum of 10 characters, each character occupying a space of 8x5 “pixels” on a grid as shown in fig 3 (The actual prototype built displays each character as a 5X7 grid)
The characters to be displayed are input through the serial port provided in the microcontroller in ASCII format and stored in the memory of the microcontroller temporarily. A look up table is then be used to convert the data into a laser projectable format (example shown in figure for alphabet “A”). Ultimately, the entire system will be interfaced to a GSM phone so that the message to be displayed can be sent to the projector via SMS.
(This part is still under implementation)
Fig3-Interrupt setup and character formationThis gives the projector the advantage of being able to display a message sent from a remote location, without the operator having to be physically present near the unit. It also takes advantage of the fact that most urban environments have an up and running wireless network for cellular phones and therefore, no need arises to have a dedicated wireless link to the unit
Fig4- Proposed final system
Results achieved so far:
Fig5-Display in a dark room showing a static “HELLO” at a distance of about 1’ is very stable and legible. (Please note that these photographs have been taken with a 4 second exposure, making the text appear brighter than what it would seem to the eye during normal operation)
Fig-64 Front view showing the mirror assembly and lasers
Fig-7 View showing the microcontroller and other circuitry
SOME THINGS I"D LIKE TO IMPROVE:
- Brightness of display leaves much to be desired.
- Not visible in daylight as lasers are too feeble.
- Device must use a more efficient and compact drive system if it must be miniaturized.
- Mechanical system of mirror and laser array is rather complex and delicate
- Aligning the lasers was found to be a very difficult task which involved a complex laser stand and very delicate adjusting.
Scope for improvements:
- Brightness issue can be resolved by using green lasers (the human eye is 10 times more sensitive to green light than red) and using higher power lasers (in terms of wattage).
- Mechanical system can be greatly simplified by using only 1 laser and a polygonal mirror with sides tilted incrementally to provide a vertical as well as horizontal sweep.
- Brightness can be improved by increasing the time the lasers are on per second (laser duty cycle).Currently, the lasers are on less than 8% of the time. This can be achieved by increasing the number of sides in the mirror that reflect the laser but requires greater mechanical precision.