I have just built the Microphone Sound Input Module: http://www.freetronics.com.au/products/ ... rXOnfkrLcs
I did my own PCB layout according to the schematic V1.1 (2013-05-24). The module works but I see some unexpected outputs on my scope.
I'm mostly interested in the Sound Pressure Level Output (to measure peak sound level) so I hooked up my scope to the output and started making noises. The module is driven from 4xAA batteries outputting 5.1V.
1. The first thing I noticed was that the Sound Pressure Level Output is not a flat line on the scope, it varies with the frequency of noise (me singing tones). This will make it harder to measure the sound pressure level with a microcontroller.
2. I also noticed that no matter how loud a noise I was making, the Sound Pressure Level Output would never reach more that around 4V even though the input voltage is 5.1V. So I took a look at the Amplified Audio Output and saw that the positive going wave would clip before the negtive going wave. I verified the VCC center ref to be 2.55V which is exactly 50% of VCC. This does not seem right to me. Is this because the TLC272 is not rail-to-rail output capable? If this is the case, why would we need 50% VCC ref? According to the datasheet the "High-level output voltage" is typ 3.8V with 5V VCC, so the 50% VCC should probably be more like 38% VCC to take the limited TLC272 output voltage into consideration.
3. Sound Pressure Level Output during my air compressor running under my work table.
4. Sound Pressure Level Output with noise from my air horn.
5. Amplified Audio Output with noise from my air horn.
6. Amplified Audio Output with noise from my air horn zoomed in so the clipping can be seen. How can the gain be adjusted so the module is less sensitive to sound pressure level and a higher volume sound can be measured?
7. Sometimes I experience some self-resonance or something, like others have reported. The LED stays on and I can measure this high frequency signal (around 31.2kHz) on the Amplified Audio Output.
I hope you can help me a bit here, using microphones is very new to me so I just picked the circuit that seemed to be the best, but it still has some problems to be solved before I can use it.