Circuit board rendered by OSH Park.
I’ve learned a bunch about making circuit boards at work recently, so I decided to re-do the ol’ strobe guitar tuner project from 3 or 4 years ago. The original still works… sort of. I have to power it from a 3x AA battery holder that’s clipped to the power terminals with alligator clips. It doesn’t tune the low E string properly for some reason. The input’s inflexible, and it’s hard to reprogram.
The hardware has been updated in the following ways:
More like version 0.1, every part of it was bad in some way. We did robotic self-tuning monochords—single-string musical instruments—last spring in the mechatronics class. The students did pretty well.
Most of their robots were designed along similar lines: they had a servo with a pick to pluck the string, an electric guitar pickup from which the robot read the string’s frequency, and a stepper motor to spin a machine head to adjust the string’s tension (and thereby change the frequency). The design had some issues that the students didn’t have time to fix before they had to move on to their next projects. We found that the biggest problem was that the tension controls weren’t responsive. The machine heads are made more for precision than speed—with a guitar you have to tune a string precisely, but you shouldn’t have to do it very often so it doesn’t matter if it’s slow. One team adjusted the tension using a linear actuator that stretched the string by moving a bar back and forth, which was a novel idea but which also suffered from unresponsiveness since they necessarily used a lead screw as a linear actuator.
I figure a robot should be inherently precise compared to a human, so machine heads are the wrong tool for the job. We ought to be able to find a speedier way of adjusting the string frequency. Instead of adjusting the tension I wanted to try adjusting the string length like you do with your fingers in most real stringed instruments.
First try at a guitarbot.
The old guitar tuner I made works fine, but I’m thinking of some improvements:
- The first priority is to switch the power supply from a 16 mm coin cell to a 20 mm coin cell. 20 mm cells are way easier to find: the dollar store down the street carries 2032 cells (2032 means 20 mm diameter, 3.2 mm height), but 1632 cells are expensive and hard to find. Right now I’m clamping a 3x AA battery holder to the tuner’s + and – power pins, which is not comfortable.
- The rotary switch is too expensive. A 0.1″ two-row header with a jumper to select the tuner’s frequency will be cheaper, and won’t be an ugly blue box. This is still not as flexible as the 7-segment display the original project used.
- I’d like to use a low profile DIP switch to turn the power on and off instead of (or in addition to) mucking around with sleep mode.
- It would be cool to use a surface mount microcontroller and crystal. Getting an STK 600 routing card for 14-pin SOIC AVR chips might be worth it if I find money somewhere.
I considered replacing the microcontroller with a 555 timer, but I don’t think the 555 timer will generate sufficiently precise frequencies because of the tolerances in the resistors and capacitors.