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I have a major problem. A couple days ago, I ran into an issue with my circuit. The problem is that I can only get noise from my ECG, not actual readings. I'm using opamp LM358, not opamp OP37 like shown in the schematic and breadboard below. Here are my pics.

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NOTE: In the schematic, I took out the diagram of the Bluetooth because it's wrong. I don't know what I'm doing here that's wrong. I'm not going to post Arduino code here because I know that the Arduino is processing everything fine (software problem ruled out) I'm totally fine with the noise (that's easy to fix), but the fact that I can't get any actual readings from my ECG is what I've been to fix. I'm a total noob at hardware design and I don't know much about circuits. This is just supposed to be a simple design, so that's why I didn't place any capacitors or resistors in my breadboard. I would appreciate it if I could get some feedback on this. Thank you!

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    \$\begingroup\$ The LM358 IS NOT a drop-in replacement for an OP 37!!! Show us a correct schematic of the circuit you are actually using (with pin numbers on the OP-Amp). You should have bypass capacitors across the supply and ground pins of the op-amp, no matter how "simple" a circuit you are building. \$\endgroup\$
    – Peter Bennett
    Jan 1, 2015 at 22:46
  • \$\begingroup\$ This is your third almost ecg circuit with an inaccurate circuit diagram that you're asking for help with. \$\endgroup\$
    – Scott Seidman
    Jan 1, 2015 at 23:56
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    \$\begingroup\$ Not to mention, an ECG is a quite sensitive device, even though I have only a vague idea of how to build one, I'm pretty sure the LM358, which is about the worst op-amp money can buy (only still available because it is such an old design) is not part of the solution. \$\endgroup\$
    – Phil Frost
    Jan 2, 2015 at 3:08
  • \$\begingroup\$ Please, please, PLEASE tell me you're powering the arduino a laptop running off it's battery ONLY. If the arduino is connected to ANYTHING run off wall power, you're one power supply failure away from killing yourself. ECG/EKG/EEG design can be very dangerous because the electrode pads basically defeat the skin barrier, making it VERY easy to electrocute yourself to death if something goes wrong. \$\endgroup\$
    – Connor Wolf
    Apr 13, 2017 at 22:11
  • \$\begingroup\$ @Connor Wolf, I think you're suggesting that when power supply fails, then wall AC power can find it's way to the ECG electrodes. Is this really realistic? I would think that even if a wall wart can fail in such a way to put 110 or 220 VAC on the DC line, the op amp and/or arduino would burn out in an open rather than a short. And burn out quickly. \$\endgroup\$
    – bfris
    Jul 1, 2021 at 16:03

1 Answer 1

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I'm almost afraid to tackle this one, but here goes.

Stop. Just stop. Forget EKGs. Get a cheap DMM, find a web page that deals with really simple op amp circuits, and start building really basic stuff, like a voltage follower, then a non-inverting amp with a gain of 2, and work up from there. The LM358 data sheet has some very basic circuits that you can try. At this point there is simply no way help you "fix" what you've done. Your schematic makes no sense at all, and the Fritzing does not correspond to the schematic.

In the schematic, there is no pin connected appropriately. Not one. First, you show an OP37, then you say you don't actually have an OP37, you have an LM358. Why did you do this? You have +5 going to the V- input, and D9 going to the V+. The LM358 doesn't have trim inputs, and on the OP37 that's not how to connect them. And the output doesn't go anywhere. Finally, an op amp is not run "open-loop" - that is, without feedback from the output to at least one of the inputs. So just running your signals directly into the inputs is not going to work. Like I say, there is not a single pin connected appropriately.

The Fritzing shows, for instance, +5 connected to pins 4,5,7 and back to DIN. There is no ground connection to the breadboard, so there is no ground connection to the op amp.

I'm sorry, but when you're this far off, there's no easy place to start.

Cease with the EKG for now. Concentrate on making a few simple circuits that work, and only then move on to something more complicated.

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