Need help understanding this circuit.

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In summary, this is a finger plethysmograph that measures the pulse rate. The frequencies look about right for that purpose.
  • #1
integrant2
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lab9circuit1.png


This is finger plethysmograph i found on Cornell site. I can kind of understand how the circuit works, but i am confused on the circuitry around the op amps, especially the 10K potentiometer, i have no idea what that does.
My guess is 10uf cap and 20K ohm is a high pass filter and a low pass filter (the 100K ohm and .47uf cap) is fed back to the inverting input for amplification. Please correct me if i am wrong.
Any help is greatly appreciated.:smile:
 
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  • #2
Welcome to PF integrant2.

Did you notice that the 10 kΩ pot is labeled gain?
 
  • #3
dlgoff said:
Welcome to PF integrant2.

Did you notice that the 10 kΩ pot is labeled gain?
Yes i did. Does that mean that pot controls the gain on the op amp in the middle or both?
 
  • #4
integrant2 said:
Yes i did. Does that mean that pot controls the gain on the op amp in the middle or both?
Specifically it controls what fraction of the signal from the output of the first op-amp makes it to the input of the second op-amp circuit. It provides a variable attenuation which ultimately of course controls the gain of the overall circuit.
 
  • #5
Thanks a lot for the reply, it helps me a lot.
But how about my guess on the filters? I am quite sure the 10uf and 20KOhm is a high pass filter, but how about the 100KOhm and 0.47uf in parallel? Is that a low pass filter?
 
  • #6
The 10uf and 20KOhm looks like it would be a very low frequency (0.1 Hz) filter and the 100KOhm and 0.47uf would pass around 21 Hz.
 
  • #7
integrant2 said:
Thanks a lot for the reply, it helps me a lot.
But how about my guess on the filters? I am quite sure the 10uf and 20KOhm is a high pass filter, but how about the 100KOhm and 0.47uf in parallel? Is that a low pass filter?

It's a crude way to do it, but the combination of a low pass filter and a high pass filter provides a simple band pass filter. In this case the high pass filter cuts in about 0.8Hz and the low pass cuts off at about 3.4 Hz. The fact that two of these stages are cascaded however reduces the pass band somewhat.

BTW Integrant2. Is this for measuring pulse rate? The frequencies look about right for that purpose.
 
Last edited:
  • #8
uart said:
It's a crude way to do it, but the combination of a low pass filter and a high pass filter provides a simple band pass filter. In this case the high pass filter cuts in about 0.8Hz and the low pass cuts off at about 3.4 Hz. The fact that two of these stages are cascaded however reduces the pass band somewhat.

BTW Integrant2. Is this for measuring pulse rate? The frequencies look about right for that purpose.

Yea, it measures the pulse rate by shining IR into the finger tip, then the phototransistor on the other side detects the signal and transmit it to the op amps for filtering and amplification.


But anyway, thank you so much for the help, you guys are really helpful :)
 

1. What is the purpose of this circuit?

The purpose of this circuit is to control the flow of electricity and perform specific functions, such as amplifying or filtering signals, in order to achieve a desired outcome.

2. How do I read a circuit diagram?

A circuit diagram is a simplified visual representation of an electrical circuit. The symbols used in the diagram represent different components, such as resistors, capacitors, and transistors. The lines connecting the symbols show how the components are connected and the flow of electricity through the circuit. To read a circuit diagram, familiarize yourself with the symbols and their meanings, and follow the lines to understand the connections between components.

3. What are the basic components of a circuit?

The basic components of a circuit include a power source, such as a battery or power supply, conductors, which allow electricity to flow, resistors, which limit the flow of electricity, capacitors, which store electrical energy, and transistors, which can amplify or switch electrical signals.

4. What is the difference between a series and parallel circuit?

In a series circuit, the components are connected one after another, so the same amount of current flows through each component. In a parallel circuit, the components are connected in branches, so the current splits and flows through each component separately. This means that the total current in a parallel circuit is equal to the sum of the currents in each branch, while in a series circuit, the total current is the same at all points.

5. How do I troubleshoot a circuit that is not functioning properly?

First, check all connections to make sure they are secure and correct. Then, use a multimeter to test the voltage and current at different points in the circuit to identify any potential issues. You can also systematically remove components to isolate the faulty one. Additionally, checking for damaged or burned out components can help pinpoint the problem. It is important to use caution and follow proper safety procedures when troubleshooting circuits.

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