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Optocoupler schematic

Long time lurker, first time asker. I've been working through a biosensor schematic from a BME class to build an EKG (I'm a physician, aware of risks of these devices etc) (typical voltages being detected in the range of <100mV). After passing through a series of amplifiers and filters, it connects to an Arduino via a optocoupler for electrical safety.

In the original schematic I'm working with, there are 3 generic red LEDs in series with a 220 Ohm resistor with the input side of the optocoupler (CNY17-3) producing about 10mA of bias current through the optocoupler. With this set up, the output waves look crisp with good amplitude. I tried to ditch the LEDs and just up the resistor value to maintain 10mA (in this case, supply voltage of 9V battery w/ resistors with values between 600-1k ohms), but the output waveforms I'm getting have much lower amplitudes compared to with LEDs.

CNY has a CTR of 100% at 10mA, forward voltage of 1.2 at that current as well (max is 60mA).https://docs.broadcom.com/docs/AV02-0772EN datasheet

Any thoughts on if this is a special effect of LEDs (if so, any way to use a different component to replicate this) or whether I'm choosing inappropriate resistor values?

Thanks!

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  • \$\begingroup\$ I would venture to guess that the LEDs are there as a cheap voltage regulator. to supply a more or less fixed drop between the signal and the resistor. In this day and age it is a profoundly cheezy way to implement such a circuit; do you know how old it is? \$\endgroup\$ – TimWescott Apr 2 at 21:23
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The arrangement limits the current through the isolator LED by adding diode drops, thus lowering the voltage that the 220 ohm resistor sees. Without them, you'd likely saturate your phototransistor, or damage the LED. I see no reason why adding a bigger resistor wouldn't do the same thing, though the fixed diode drops to get rid of some voltage probably take out some of the guesswork.

Personally, I tend to use those types of isolators as digital devices. For analog isolation, I'd point you to something more appropriate, like an HCNR200. Yes, you need some support circuitry, but you've already got a full biopotential amp on the left side, so you probably have the resources you'll need in place.

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