The board was designed for the SEm VHDL FSM lab.
It has a beeper which can emit a Morse code signal and a microphone for receiving it.
|V1.1||HEB-microphone Schematic PDF||10 fully mounted|
The beeper is driven directly by a digital I/O line.
The microphone's output is amplified and connected to an ADC. The amplified signal is also triggered in order to deliver a simple digital input to the FPGA.
The microphone's output is quite small. It is first fed to a fixed amplifier through a passive RC highpass. After this, it is brought to an amplifier with a variable gain, and finally to a trigger.
The buzzer/microphone couple have 2 resonant frequencies: one around 3 kHz and the other around 4 kHz.
The trigger is a Schmitt trigger. When there is no signal, the trigger output can be as well low or high.
The following picture shows the signal levels
- at the buzzer
- at the output of the first amplifier (testpoint near buzzer)
- at the output of the variable amplifier (Ampli testpoint)
- at the output of the trigger (Trig testpoint)
To come to these signals:
- set the gain of the amplifier with the potentiometer close to the buzzer: screwing reduces the gain
- set the trigger range with the other potentiometer: first unscrew until the output is too noisy then screw until it looks fine.