Figure 2—An electret condenser microphone (not shown) is biased to 5 V via R4. The signal is then amplified by U2a. C2 and R6 (along with C3 and R10) form a high-pass filter. The output is fed to the second op-amp, which is configured as a comparator whose output is connected to PB4 of the 68HC705J1. The EEPROM has a two-wire I2C interface, which is connected to PB1 and PB0. The remaining pins of the processor are connected to LEDs and push buttons.