A Belmont Report

3. CONSTRUCT A FILTER

⟵ 2. BUILD AN AMPLIFIER

The LM741, the chip with the polished circle on its upper-left hand side, is a generic operational amplifier, “op-amp” for short. The particular wiring used here allows one to construct a “filter” that gets rid of parts of the signal that are not wanted. Specifically, we are creating a “bandpass filter” that will get rid of low frequencies and high frequencies.

Step 6. Place an op-amp on the breadboard

  1. Place the op-amp across the same trench as the instrumentation amplifier, oriented such that the op-amp’s top side aligns with the top of the board
  2. Wire R1 and C1 in series from the output wire of the instrumentation amplifier to Pin 2 (–IN) of the op-amp 

[Note: electrolytic capacitors have a specific direction they are designed to work in. Connect the big leg of C1 to R1 and its little leg to Pin 2]

Step 7. Wire the op-amp for power

  1. Wire the positive rail to Pin 7 (+Vs)
  2. Wire the negative rail to Pin 4 (–Vs)
  3. Wire the reference rail to Pin 3 (+IN)

Step 8. Finish the filter

  1. Use a parallel combination of R2 and C2 to connect Pin 2 (–IN)  to Pin 6 (OUTPUT)
  2. Create a loose output wire and connect it to Pin 6; this will be our “system output”

Check in 

  • The “lower corner frequency”, C.F.L is the region below which we attenuate low frequencies and the “higher corner frequency”, C.F.H is the region above which we attenuate  higher frequencies. These are calculated by 

C.F.L = 1/(2*pi*R1*C1)

C.F.H = 1/(2*pi*R2*C2)

Using your values of R1 and C1, what is your lower corner frequency? 

Using your values of R2 and C2, what is your higher corner frequency?

4. DETECT A HEARTBEAT ⟶