PRESENTING THE BE2-Vivid:
My final project is an onboard effects unit for the electric guitar. It is a built-in device that uses light to control the amount and quality of the effects. Included in the unit is a distortion effect and a tremolo effect. You can control the amount of distortion and amount of tremolo by passing your hand and fingers in front of the sensors which are embedded in the guitar’s pickguard below the strings. This placement allows the user to use his/her strumming hand to manipulate the effects while still plucking/strumming the strings. The effects are built into the electronics of the guitar so the normal output jack can still be used. In the future, with larger funds, the electronics will all be housed in the body cavity under the pickguard.
The flow of signal from the out of the pickups and guitar volume and tone pots is split and used as the input signal for each effect. In the distortion, the signal is run through an operational amplifier with a feedback loop set with diodes to clip the signal and provide a soft clipping to the signal peaks and a set of diodes post op-amp to ground to provide a harder clip on the peak of the signal waves. I placed my photoresistor in a voltage divider formation between the input signal and the output signal of the effect so that when the value of the photoresistor changes, it lets more of the clean signal through when there is a lot of light (low resistance is present), and lets more of the distorted signal through when there is little light (high resistance is present). This same blending technique is used on the tremolo unit to control the amount of tremolo signal in the output. The tremolo unit takes the same output signal from the original guitar electronics and passes it through a transistor whose base control voltage is controlled by a 555 timer. The fluctuating voltage shunts the signal to ground at the same speed as the timer and therefore creates the tremolo effect.
One of the biggest problems I faced in my project was using a photoresistor instead of a potentiometer. One of the issues with these components is they only work in one direction. There is no way to switch the part to provide high resistance when there is a large amount of light and low resistance when there is little light. In my project, I wanted the off or low amount position to be when there is a large amount of light hitting the sensor so that when you are not manipulating the sensors the effects are off and they only come on when you put your hand over them. In order to overcome this, I discovered the voltage divider method with a constant resistor soldered to one of the photoresistor’s leads. This is not perfect, however, as the direction of resistance is still not always easy to keep track of and the taper of the makeshift “potentiometer” is very steep, not entirely suitable for all audio applications.
The second issue of I faced in my project had to do with the tremolo unit. The LM555 timer introduces some inherent signal modulation to the power signal when it is plugged in to the battery power source. When the output signal is hooked up with the same power as the LM555, the modulation gets into the output signal and creates a click whenever the timer chip goes on and off. The only foreseeable solution to this issue is to completely isolate the timer with its own power source. The presence of the LED and photoresistor to modulate the signal in the original schematics I looked at is one step in the right direction, as there is no physical connection between the timer and audio portions of the circuits, but the power is still connected. I could use two batteries, but for my application this is a little excessive as my goal is to fit the entirely of the unit under the pickguard.
Below is a video of the device in action!
Here is the full schematic of my device:
And finally, a recording of the device doing its thing!
Work on my project is winding down. I currently have a fully functioning light-dependent distortion unit and a tremolo unit spliced together to the output of my guitar. I am still having issues with the tremolo. The circuit I am using utilizes an LM555 timer and when it is introduced to my circuit it creates a small click every time it turns on and off. Read through an oscilloscope, this modulation actually occurs through the power source as well, so to eliminate it, I will need to completely isolate the 555's power, which, for my application, is not entirely plausible. I also cannot use the photoresistor to vary the speed of the tremolo as there is no way to reverse the direction in which the resistance fluctuates due to light exposure. One idea I have is to use an oscillator and a JFET. This way, I could use a photoresistor to vary the frequency of the oscillator and feed the output into the base of the JFET so that the wave restricts the current flow through the JFET. When I splice the JFET ends into my circuit, it should sound relatively like a tremolo.
I tried to use a summing amplifier to combine the signals from my different effects, however the quality of op amps that I have, the LM358s do not provide a very clean signal at all. In fact, when I hooked it up, the amplifier provided more distortion than my distortion unit. Simply splicing the outputs together does fine currently so that is what I will stick with.
My biggest development from now till the end is putting my project onto my guitar. I will drill holes in the pickguard to mount my photocells and potentiometers. My eventual plan is to house the entire system within the electronics cavity of the guitar, however, I will need to route out the cavity more to accept more electronics and I also need to create a PCB for the system so that it will be solid instead of just on a breadboard. For now, I will just run the photoresistor leads out from under the pickguard and into a small box that I will mount on the body of the guitar which will house my breadboard.
Below are video demonstrations of