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Forum: µC & Digital Electronics Designing a Digital Tuner for Electric Guitars by Using STM32F4


Author: Ada Lee (Company: Tesco) (jonnydep)
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I am currently involved in an academic project which the topic is to 
design a Digital Tuner for Electric Guitars. My knowledge of electronic 
is very basic, so here I am.

I need to pre-process the signal that goes out from the pickups of a 
common electric guitar, let's consider passive pickups so....we have an 
output voltage from 200mV up to 1V of peak, with also negative peaks (of 
the same magnitude).

I have to use the STM32F4 (Discovery 
Board:http://www.kynix.com/Parts/3597871/STM32F4-GAME.html), so I need a 
very simple circuit that drives the On-Board 12-bit ADC, that takes, as 
inputs, signals with voltage between 0 and 3.3V. So I have to design a 
pre-amp circuit that amplifies and clamps the guitar's output.

I want to underline that I have never realized an electronic circuit (a 
real one, not in the simulator) before. I am wondering about:
STM32F4 provides some voltage sources (5v or 3v). Shall I use these 
sources or is it preferable to go with an external voltage source (e.g., 
batteries)?
I have chosen an opamp that is not, let's say, "handy" because it needs 
2 voltage sources: the TL082. What op-amp do you suggest me to use? 
(maybe with only 1 voltage source)
I have tried to design a circuit by using LTSpice, but I don't know if 
it is correct at all. You can also notice that I have inserted a 
filtering stage (LP w/ cutoff freq. 2,2 KHz) and a "protection" Schottky 
diode before entering in the pin of the board.

Author: Lothar Miller (lkmiller) (Moderator)
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Ada L. wrote:
> STM32F4 provides some voltage sources (5v or 3v). Shall I use these
> sources
The board supplies the voltages, not the STM32! And yes: you can use 
them for your design, as it will draw only a few mA.

The TL082 is useless here due to 2 problems:
1. you do not have a negative supply.
2. it's input and output ranges are not adequate for 5V single supply.
Use a rail-to-rail amplifier like the TS912.

: Edited by Moderator
Author: matzetronics (Guest)
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With the large level from a guitar pickup your concern is problably more 
to
* convert a high impedance source like the guitar pickup to a low 
impedance input of the ADC
* clamp the zero level of the AF signal roughly to the middle between 
VREF and GND of the STM32.

Note that the STM32F4 Discovery will not supply 3.3V but a little less 
than that due to the diode built in after the 3V3 regulator.

I suggest a single stage amp like a BF545 FET or similar with a little 
above unity gain.
Clamping the ADC input is easy. Supply two resistors of the same value 
(e.g. 4k7 to 10k) in series from 3V to GND and connect the ADC to the 
center tap. Inject the AF via a capacitor of 0.47µF to 4.7µF to the same 
connection.

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