Here circuits to read analog signal such as audio signal with Arduino are illustrated. Arduino analog input pins requires voltage between 0V to +5V biased at 2.5V DC. However, analog signals are often unbiased and swings negatively and positively around 0V. Also analog signal such as audio signal from electret microphone or analog sensors are low in amplitude and hence they need audio amplifier or appropriate signal amplifier. That is why biasing and signal amplifier circuit are required to acquire signal by Arduino or other microcontrollers.
One circuit that can be used with Arduino to bias and amplify analog signal before reading by analog pin.
The above circuit is also called electret microphone pre-amplifier. In the above circuit, the analog signal or audio signal is connected to vin port. The capacitor C1 is used to couple the ac signal into the analog signal amplifier. The amplifier is build using inexpensive and readily available LM358N operational amplifier. The op-amp amplifier is a single supply non-inverting amplifier where the R4 and potentiometer RV1 are the setting resistors, R1 and R2 are the biasing resistors(10KOhm to 100KOhm) that sets the output dc voltage at 2.5V. This method of biasing is called split resistor biasing method.
Another method of biasing is called voltage divider biasing. The following shows voltage divider biased non-inverting op-amp to acquire analog signal amplify it and bias at 2.5V output so that the analog signal can be read with Arduino or microcontroller.
The above circuit used non-inverting op-amp with split resistor biasing and voltage divider biasing to bias the op-amp output at 2.5V required by Arduino for reading full scale analog signal.
We can also use inverting op-amp configuration with different biasing method to design circuit that acquires analog signal, amplifies it and bias at 2.5V.
Shown below is a analog signal acquiring circuit, that uses LM358 inverting op-amp with voltage divider biasing.
Yet another way of connecting analog signal to an pre-amplifier and then to Arduino analog pin is shown below.
The biasing method used above is called voltage follower biasing. We can use inverting op-amp biasing calculator and non-inverting op-amp biasing calculator.
So here circuit to conditioning analog signal before it can be read with Arduino were shown. This are useful for acquiring analog signal.
Also read how to bias non-inverting op-amp amplifiers.
