Step 6: SIGNAL CONDITIONING - Smoothing + Amplification
In this last phase of circuit assembly, we will be using an active low-pass filter to filter out the humps of our signal to produce a smooth signal for our microcontroller.
You will need the last TL072 chip (chip D), the two 80.8 kOhm resistors, the 100 kOhm trimmer, the 1 kOhm resistor and the 1.0 uF ceramic disc capacitor.
First, plug in chip D and connect +9V to pin 8, -9V to pin 4, and GND to pins 3 & 5. (image #1).
Now, grab one of the 80.6 kOhm resistors and connect one end to chip C’s pin 7. Connect the other end to chip D’s pin 6. Next grab the other 80.6 kOhm resistor use it to connect chip D’s pin 6 and 7. Do the same thing for the 1.0 uF capacitor. (image #2)
That’s the end of the filter circuit. However, since this is an active filter, there is a side effect of inverting the signal. We will need to invert the signal one more time (and have the ability to amplify it more if desired) using another inverting amplifier circuit with a trimmer configured as a variable resistor.
Use a jumper wire, connected to chip D’s pin 7, and the 1 kOhm resistor to bridge the board’s center gap. Use another jumper wire and connect the 1 kOhm resistor to chip D’s pin 2. Next, place the trimmer one row over with the pins laid out and a jumper wire connecting two of the pins as pictured. Finally, place the last two jumper wires as indicated. (image #3)
By using a screw driver and turning the trimmer, you will be able to adjust the gain of your signal to account for different signal strengths from different muscle groups. Start out with it set pretty low and go up from there (~20 kOhms).