Pulse Fix

[ MCP6001 ] [ TL071 ] [ APDS-9008 ]

[ TL071 and single supplies ]


We burnt a pulse sensor some time back. That is, we plugged 5V into the output of an opamp, which subsequently stopped working…And as we should be ambitious engineers who can repurpose school supplies to achieve our own goals, I want to see if I can switch that burnt opamp out with opamps I do have.

So, I am working on replacing a single-supply [ MCP6001 ] with a dual supply [ TL071 ]. Why this particular switch? Because my personal collection is overflowing with TL071s that I used for Dr Brewer’s labs! How would I make the switch? I’ll need to make sure that I can get the same filtering and gain (which could require switching some passive components out), and I’ll have to make the TL071 work on a single 5V supply (if possible.) I could work with a dual supply, but it gets trickier to make a demo for BBB / Arduino, as there isn’t a clear -5V supply to work with!

By the way, here’s a neat chart for reading off component values:

[ source ]

Before I can just switch out the opamps on an open source pulse sensor, I am going through Joel Murphy’s design:


[ source ]

Which, interestingly, used to be:


[ source ]

I have redrawn the ‘amped’ circuit to help me make sense of it:


Aha! A negative-feedback configuration! With the +ve terminal hooked up to mid-supply (sort of.) Not sure why they have a capacitor hooked up where I have a ‘?’. I have seen it on Omer’s slides for getting rid of supply noise (especially as the voltage divider uses big resistor values: 100kOhms in Joel’s design.)

Not sure what the resistor and capacitor T-network are doing upfront. Joel’s [ description ] says that the first resistor is a load for the Avago photosensor. And the R-Cs together are forming some high pass filter.

Its interesting to compare this solution to Omer’s slides on photodetectors (using a transimpedence amplifier):

[ source ]

Anyway, food break, and we’ll figure the supplies and passive filter out.

[ single-supply op-amp design ]


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