Implementation

For implementation we decided to start small with only two bandpass filters. This is easy because we are using LM358 opamp chips which each have two op-amps per chip. Each filter requires 3 resistors as well as a gain potentiometer. For this reason, we got MCP4261 chips. These are variable potentiometers with two potentiometers per IC. This lets us create a full filter with only two resistor chips and half of an op-amp chip. We also decided to add the unmodified input to the summing amplifier. This lets us set a gain across the spectrum in the case that the user has general hearing loss across all frequencies.

In the center, there are five ICs. Four of them are the MCP4261. Under the crossed yellow wires is the LM358 chip. These five ICs make up the two bandpass filters we have so far. Beneath them on the same column is another MCP4261, this chip is used to set the gain on the pass-through data. On the bottom left, there is a summing amplifer that takes the three voltages from the filters and pass-through and creates a final output.

On the left is an mbed. We use this to program the MCP4261s over SPI.