for an

by: Matt Remelts

6-21:

I changed my project description from a turntable to an automatic guitar tuner. I realize that it is a bit late to change, but I haven’t had much time to think about it as my six week history class took an inordinate amount of time. I decided that making a turntable would require a lot of mechanical work that didn’t have much to do with the controls portion of the project.

The tuner should take a frequency input from the "A" string of a classical guitar and drive a motor which will tighten or loosen the string based on the relative difference between the input frequency and a reference frequency.

6-28:

I began designing the mechanical setup (gear reduction, axle support, means of string attachment, where/how to pick up the frequency).

I also purchased bearings, gears (spur and pinion), and axle piping.

7-5:

I called many places for advice on how to build the circuit. Finally, I got some from an electronics engineering consulting company. I went to the Newark distributor in Grand Rapids to pick up a catalog. I found that it would be expensive to do it the suggested way.

7-7:

I talked with Prof. Jack about it and he suggested a different route.

After searching everywhere for a contact microphone without any success, I used one from school.

7-12:

I worked on the mechanical setup again. I ran into a problem building the reduction gear and had my dad build a conversion plug to allow for diametrical differences in the gears I was using. I also found that the motor I was using (from a radio controlled car) was way too fast for my purposes, so I got a 12V motor with gear reduction from school.

7-13 to 7-19:

I am getting very confused about the electrical aspect of this project. I have been told about four different ways to do this and haven’t had the chance to get very far. I found a circuit in a circuit design book that looks similar to what I need, but there are still many unanswered questions.

7-21 to 7-26:

I got some clear direction from Dr. Jack as to the electrical circuitry. I built the input pick-up with the mic, an op-amp, and a schmitt trigger. The reference frequency was built with a crystal clock oscillator divided down to 444 Hz. The PLL chip that I was using to compare the signal is not doing what I need it to do. I tried to get a voltage swing using a NOR gate. I could not get the output of the NOR gate to give me a convertible voltage swing, so I am going to try using a frequency to voltage converter (LM2907N). The input frequency will be changed to a voltage. This voltage will be compared with a reference voltage to drive the motor. This eliminates the need for the whole clock oscillator/divider circuit.

I also finished the mechanical portion of the project. All I need to do is get the right signals from the circuit to drive the motor. I will, however, need to fix the mic pick-up so that I get a strong enough signal.

7-28:

The frequency to voltage converter worked! With the function generator on the training board, I can vary the output voltage from 0 to about 9 volts with 4.6 volts representing an input frequency of 440 Hz. I used a comparator to throw this voltage positive if the input is less than 4.6 volts and negative if it is more. The output from this comparator then goes to the motor drive circuit. I tested it and the motor tightens the string when the input frequency is less than 440 Hz. and loosens the string if it is more.

7-29:

Today, I was able to get a good signal from the mic by attaching it to the end support for the string. This allows the diaphragm inside the mic to vibrate in the direction that the string forces the support as the string vibrates.

I also hooked up a couple LED’s - one for a vibration frequency of less than 440 Hz. and one for a frequency over 440 Hz.