Lab 1: Timing chip

The 555 timing chip is one of many which uses internal circuitry to produce high/low discrete alternating output signals at specific times. There are multiple ways of using the chip, but most involve creating a circuit around it which will exploit the voltage drops over resistors, and the time it takes for certain capacitors to charge.

The circuit we were interested was once which created an oscillating square-wave output. This is referred to in the data sheet as astable operation. We were able to adopt the circuit from the spec sheet without making very many changes to create a 1Hz proof-of-concept output signal, and then one of a much higher frequency to run the speaker. Below you can see the basic structure of the circuit, as referenced in the spec sheets and the specific values and calculations leading to the values we used for our final circuit (expansion on basic circuit diagram in previous blog):

(The image can be clicked to view in full resolution)

High and low times are a function of the two resistors, and a gain caused by the capacitor. The exact functions could again be found in the spec sheet, which we referred to to create an oscillating output at Hz.

One design consideration which played a major role was to use variable resistors as resitors A and B, instead of fixed ones. This meant that the capacitor could be used as a gain control to get the oscillation in the right range (1 Hz for testing on the oscilloscope and 10-20k times that for the speaker). Then, adjusting the two potentiometers would result in a change in the high time or the low time, and therefore the duty cycle and period. This combination of effects enabled us to create a sliding effect in the sound produced by the speakers.

As it pertains to the blog-question outlined in the lab hand-out, this variation of resistance translating to a variation in tone can easily be adapted into an human-machine interface for the musical instrument project. One simply has to decide on an interface. As lab 2 will no doubt show, there are a few ways to produce a variant resistance based on the forces applied (to the instrument) or the proximity one has to the instrument (theremin). Changes in these values will correspond to changes in pitch, which then can be exploited to play pretty pretty music.