High-Level Design

Rationale and Logic Structure

Certainly the most difficult portion of the project to implement was detection of the beats.  Rather than relay on an ADC and complicated algorithms to calculate the beats within the MCU (using software), we opted for an analog approach.  The analog approach simply takes a mono line-level music signal; low pass filters it, and does beat detection.  The output of the beat detection is passed to the MCU and is assumed to be the beats of the music (for more explanation, see Beat Detection below).  

After beat detection, a randomizer was created using software.  timer0 was polled every time a beat was detected, and the current timer value was divided out by 11 and the remainder (0 to 10) was used to determine the position for the next color (since ten colors are possible using our color wheel).  This scheme offered an easy way to randomize the colors.  

CPU

We decided to use an ATMEL 90s8515 processor instead of the 8051C or the Mega 163 series.  The 90s8515 offered better speed (8 MHz) than the Mega (4 MHz) and more processing power than the 8051C.  Also, this was the chip that we felt most comfortable programming with.  It also turned out that the extra instruction space was not a factor in choosing our MCU because our final line count was around 250 lines!  Furthermore, since all the interfacing was done using analog circuitry, no ADC’s were required.  Hence, the 90s8515 was the most logical choice.  

Stepper Motor (Hardware)

We built the control interface circuits to run the 10-position color wheel, which is controlled by a 200-step unipolar stepper motor.   

Stepper Motor (Software)

The microchip also had to be coded to properly turn the stepper motors.  Initially, upon power-up, the stepper motor (when facing the front of the light) has to be rotated fully clockwise until the stop-pins hit.  This is absolutely required upon each power up because the microchip needs to know the initial color (black).  To go to the next color (white) the stepper motor must transition counter-clockwise through 20 stops.  Each successive color is 20 stops further than the previous.  However, one cannot exceed 200 stops (ten colors) or the reference point is lost.  Hence, knowing the initial position at startup, and keeping track of the current position, and the next position, the microcontroller can easily instruct the motor to turn the required number of stops to the next color (number of stops to turn = current_position-next_position, where the sign (±) indicates the direction to turn).  

Buffers

All components that ran off of 12 volts (the stepper motor, muffin fan, and control lines for the lamp relay) were buffered from the main processor using TIP31-C’s.  This offered several advantages.  The TIP31-C can sink up to 1 Amp of current, which is more than enough to run each of the components.  Additionally, the TIP31-C will provide a fairly good buffer to protect the ports on the microchip from the EMF spikes created by inductive loads (like the motors).  

Cooling System

Another component that we felt needed upgrading was the muffin fan.  The muffin fan was directly wired; hence it was always on when AC power was supplied.  We felt that this was unnecessary, and would make the light cooler (pun intended) if we used an IC to detect the temperature and have the fan turn on once the internal temperature exceeded a certain point.  Not only this saves power consumption on the long run, but it also flexes our analog interfacing skills.  Instead of using an ADC to convert the voltages put out by the temperature sensor, we simply set a comparator to trigger at a voltage point that coincided to a temperature of 100°F (roughly).  

Beat Detection

The original circuit used a capacitive microphone to detect the beats of the music emanating from the speakers.  However, the microphones were incredibly sensitive and had no gain control.  Hence, in moderately loud environments, the lights would simply lock up and turn off.  Rather than detect a beat using logic and software, we took an analog circuit approach.  The main reasoning behind this is that these club lights are generally used in conjunction with dance music.  Dance music usually has a very distinct low frequency, high amplitude beat.  Hence, one could easily detect a beat by low-pass filtering the left (mono) line voltage off of a CD player and passing the low-passed signal into a peak detector.  Hence, every time a peak is detected, it is directly correlated with a beat.  This is the signal that is then detected by the MCU.  Every millisecond, the MCU would poll the port and determine if it was high.  If the port was in fact high (signaling a beat), the MCU would send the appropriate signals to change the color.

Random Color Generation

The original light seemed to be stuck in certain patterns, always choosing the same colors.  In our software, we decided that we would implement the colors randomly.  To generate a random number we would poll timer 0 and then take modulo 11 of the resulting number.  This would output a number between 0 and 10 (0-9 colors, 10 strobe).  Hence, all of the colors in the 10-color wheel are now being used.  

Power Supplies

Finally, extensive power supplies were built to run the all the motors at the proper voltages.   120 VAC power was stepped down to 12 VAC using a transformer, and this voltage was then passed through a full-wave bridge rectifier.  The unregulated DC voltage off of the FWBR was passed into several voltage regulators.  Additionally, all the microchips’ ports were buffered from the motors using TIP-31C transistors.  In the end, the entire electronic guts of the light were completely replaced.  The original transformers, case, light bulb, color wheel, motor, and fans were used and interfaced to the microchip.