This week in lab I created an oscillator based on a 555 Timer chip. It produced a relatively wide range of frequencies in a rough squarewave pattern:


EXPERIMENTS:

1. When you press the switch, the signal that is sent to the output is connected to the output and you can hear the signal. When the button is not pressed, there is no connection between the oscillator and output to headphones so pressing the switch will connect the two portions of the circuit and you can hear the signal.
2. When you press the switch, it connects the output of the circuit to a line to ground so all of the signal bypasses the headphone out and goes straight to ground so when you press the button, you hear nothing.
3. This circuit works similarly to the switches in 2 and 3, when the potentiometer is turned all the way to one side, for example if it was turned to low resistance, it would pass all the signal to the side that is connected to that pin on the pot. If it is turned to high resistance, it will be sending the signal instead to the other side.

QUESTIONS:

1. Some common mistakes you can make when building a circuit from a schematic include mistaking wires crossing for junctions in the circuit, mistaking the order of components, and connecting the incorrect pin or connecting a component incorrectly (backwards instead of forwards, etc). These mistakes are easy to make and can cause your circuit not to work or can fry your components. When creating a circuit, careful attention must be paid to every detail. The values of components, direction, and order must be taken into account when mocking up a circuit on a bread board or otherwise.
2. If you wanted to use the volume of some audio source to control the frequency of this oscillator, you would connect the output of the audio source and connect it to pin 5 on the 555. This pin is the Control Voltage pin and will take the voltage received from the audio source and use it to control the frequency instead of the voltage that was controlled by the voltage divider (potentiometer).
3. Instead of panning your single signal between left and right in the headphones, it is possible to crossfade between two different signals. Using a similar circuit to the one used for panning, you simply connect the headphones to a potentiometer and the two sources and turning the pot will provide different resistance to each source and allow each to go through to the headphones at any position of the pot.

FINAL PROJECT:

1. My final project is an onboard proximity-controlled guitar effects collection. With a set of four photoresistors inset into the pick guard, the user will be able to control different aspects of different effects by changing the distance his/her fingers are to the light sensors.
2. I will need quite a few parts:
1. 4 photoresistors
2. replacement pickguard
3. high-pass filter
4. low-pass filter
5. multi-position switches
6. distortion circuit
7. envelope follower circuit
3. Parts that I need to buy:
1. photoresistors
2. replacement pickguard
3. multi-pos switches
4. any extra components that I may need duplicates to create each effect

Here is a preliminary schematic of my final project. In future drafts, I will add an envelope filter and all variable resistors will be switched to photoresistors:

Experiments:

Reverse Diode:
     This circuit bevels off the top of the sine wave. When you change the volume, it simply deepens the curve on the bottom. When frequency is brought up, the back side of the wave cuts off more steeply.
Forward Diode:
    This circuit bevels off the bottom of the sine wave. Just as in the reverse diode circuit, when you change the volume, it simply deepens the curve on the bottom and when frequency is brought up, the back side of the wave cuts off more steeply.
Forward Diode to Ground:
   This circuit cuts off the top of the wave sharply and widens it significantly. As you raise the volume, the effect becomes more and more present. As you change frequency, there is little to no change in the shape of the waveform.
Reverse Diode to Ground:
  This circuit is the opposite of the forward diode to ground, cutting of the bottom of the wave and widening it. Similarly, as you raise the volume, the effect becomes more and more present and as you change frequency, there is little to no change in the shape of the waveform.


Questions:

A peak follower creates a decay by charging up and slowly draining a capacitor. It can be used to control any number of voltage controlled modules (VCOs, VCFs, etc). By simply adjusting the volume of the inputted signal, the shape of the curve can be made steeper or smoother depending on whether the volume was moved up or down. Using a peak follower, you could control the frequency of a VCO to make a tremolo effect or manipulate the cutoff frequency of a VCF to make an envelope.

Below is a diagram of a circuit that would take the low frequencies of a drum machine, applies a smooth, almost square filter to the signal and then uses that signal to control the frequency of a voltage controlled oscillator.
The initial resistor and then capacitor to ground form the filter to send through frequencies around 10-20Hz (see my previous post for more information about these types of filters), and then the following combination of reverse diode and reverse-biased diode to ground creates the filter.

Final Project:

     My final project is looking very plausible. I have been researching the potential for photoresistors in applications like mine. While the optimal device would be a proximity sensor, these parts cost upwards of $30 per unit so they are entirely out of contention. I have found that photoresistors are in a lot of things, from camera light sensors, to outdoor lights and night lights, these small, inexpensive devices are in many consumer electronics. I have many of these device at home not in use so I can take these apart and retrieve the LDRs (light-dependent resistors) from them. Unfortunately, I have hit one obstacle. Since the LDRs are essentially just variable resistors, I will have to do all the calculations to make sure that they are in the correct range for my application. For example, if I wanted to use my device as an extension of an existing effect pedal, I would have to be sure that my LDRs match the existing resistors in the device so that the circuit within the effect would not be put off balance. Because of this, there is a chance that I will not be able to find an LDR that works for my application and I will have to add on extra circuitry to make up for this. Also, because of this, it will be very hard to make the device compatible with a wide range of preexisting effects. Each effect has a different circuit and different value potentiometers. Because of these complications I am leaning towards an on board, or at least personally made set of filters that I will design to work well with the LDRs that are available.

Experiments:

The circuits pictured above are examples of high-pass and low-pass filters with adjustable cutoffs. Due to the reactance of a capacitor (Xc = 1/2pifC), when the frequency in a circuit is high, the reactance is low and inversely when frequency is low, reactance is reactance is high. Reactance is how much the current is impeded. When you have a capacitor going to ground and high frequencies are sent through the circuit, the capacitor is not stopping the signal from being sent out to ground so at high frequencies the output is very low because most of it is going to ground. When low frequencies are sent through the circuit, the reactance in the capacitor is high so it prevents the signal going to ground and therefore more can be sent to the output. Similarly, in a high pass filter, when the capacitor is in series in the signal chain, at low frequencies it simply prevents the signal from going through and at high frequencies provides little impedance to the flow of the current out to the output. The variable resistor simply acts as a voltage divider so you can change the voltage across the circuit and therefore change the frequency at which the circuit cuts off (the cutoff frequency).

Questions:
1. When you have your RC circuit set as a highpass filter, there is a point at which the corner frequency goes above he range of human hearing. To improve this, you can adjust the value of the potentiometer that you're using so that at it's peak, it does not allow the corner frequency to rise above 20,000Hz. 

2. When you have an RC circuit set as a lowpass filter, there is a point where the corner frequency goes below 20Hz and there is no change in sound beyond that point. To fix this, you can first change your potentiometer so that it is unable to reduce the corner frequency to less than 20Hz.

3. In order to make a filter with an even shaper cuttoff, you can put multiple filters in series. This way, you will essentially filter the filter or cutoff the cutoff. At the cutoff frequency you will sharpen that curve even further. 

Final Project:
The project I have been researching is entirely analog. As it is essentially only a controller (an extension of the potentiometers on a stompbox), all the parts are entirely analog. If I were to include filters in the design, they would also be analog (distortion, envelopes, etc.). A typical proximity sensor is $50 plus so I will have to stick with the cheaper light sensors. If I can find a way to use them in reverse so that a high amount of light turns the pot off and as the level of light sensed goes down, the value of resistance goes down. Here is a preliminary sketch of my design: