February 21, 2020

Building A Drum Machine: Part II - Tuning the Snare

After a rough time getting any sort of snare sound last week, I moved to step two; tuning. Slight circuit variations can dramatically alter a circuits transfer characteristics. Trading resistors for potentiometers allows you to adjust a resistance value on the fly. After achieving a desired effect, I used a multimeter to measure the pots resistance so that it can be replicated in my design.
Here’s a few changes I made:

1. Decrease the decay potentiometer from 1M ohm to 100K ohm. I found that the range of decay time was from no sound at all to about 10 times too long. T=RC , reducing resistance, R, will reduce the time constant T.
2. To further reduce decay, I also swapped out the 10uF capacitors for smaller ones. It should be noted that I used a 3 mm LED for the indicator light which inherently draws less current than a standard 5 mm. If you use a 5mm, you may find that the decay is too short with the R and C values I have choosin.
3. Remove the input 4.7k Ohm resistor. This little guy screwed up my input circuitry in that it created a voltage divider. Instead of a 12V trigger input, I had somewhere in the range of 9 volts which greatly changed the circuit output.

Once everything was just right, I re-drew my breadboard circuit as an EagleCAD schematic. It is important to include any design or construction instructions and notes. Notes will help immensely when you’re ready to produce the final product or if the circuit ever needs debugged. In the past, I have drawn circuits on engineering paper, but it is difficult to make changes. EagleCAD makes it easy to go from a symbolic circuit to a protoboard layout. My current circuit layout was designed with the intent to be soldered onto a matrix-style protoboard, but also has the ability to be moved to an actual PCB board. In the future, I hope to be able to sell a few Eurorack modules. Having a my designs on printed circuit board allows me to quickly make duplicate items.