Unknown Schematic Symbols

Newbie here.

Hello, I found a schematic online and want to create a pcb design, however I don´t know what the circled symbols mean or how to find them on Kicad.

Do they have standard footprints? I was thinking about changing the chest1 symbol for a header pin.
Captura de tela 2023-01-04 184216

Could anyone tell me what their names are on Kicad?

Thanks a lot

not a perfect match, but OUTPUT is basically Connector:Conn_Coaxial which is a generic coaxial connector (BNC, RCA/Phono, SMA). No standard footprint, it depends what type of connector you want to use.

CHEST1 looks like Connector:Testpoint although its use in circuit seems more like it should be a connector that goes to a skin contact…

Many symbols in KiCad’s library are generic, i.e. they do not refer to a specific part and therefore do not have a specific footprint. You’ll have to choose which real part you want to use based on your application, and from there select or make an appropriate footprint.


What would this symbol be on Kicad? AudioJack3?

Do the position I put global labels on them matter? Because it has L and R pins. What is the other unlabelled pin on the JP2?

Captura de tela 2023-01-05 131739

Thanks a lot gkeeth, you solved my problem!

AudioJack3, yes.

The eagle symbol you show has (I think) poorly named pins, or at least they are only useful names for unbalanced, stereo audio applications.

  • The unlabeled pin is the sleeve connection, which is usually/always grounded in audio applications. KiCad calls this pin S (for sleeve).
  • The pin labeled L is the tip connection, labeled T in the KiCad symbol. T is the left audio channel when you’re using a TRS plug for unbalanced stereo audio.
  • The pin labeled R is the ring connection, also labeled R in KiCad. R is the right audio channel for unbalanced stereo audio.

For reference see Phone connector (audio) - Wikipedia

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Hi gkeeth,

So I read the article and found this:
Tip: Signal
Ring: Ground or no connection
Sleeve: Ground

In this schematic the current jack is for a PC output so it´s fine to have only one connection. But for instance I want to add another jack for ECG electrodes which are 3.

Could I connect CHEST1 to T, CHEST2 to R and LEG which is grounded to the sleeve? In case if I inverted the CHEST1 and CHEST2 connections would the graph be backwads?

I don’t know how your CHEST1 and CHEST2 signals are graphed, so I’m not sure what swapping them would do.

I don’t think I can help you much further - I’m in no way qualified to provide advice on an ECG circuit or medical electronics.

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Thanks for helping gkeeth, it´s an educational project. I´m building an open-source version.

A few years ago I built several BPM gizmos and an EKG(unfinished). Did them with different divices (LM741, LM324, Arduino_Nano… some PICchips…)

Lot’s of schematics out there but I found it takes more time messing around with them than to just understand what needs to be done, then to go about doing it.

Have done them with IR and $3 devices… Yes, they are accurate (confirmed with EKG in lab)

Two examples (LM324 with IR and Nano with $5 sensor (mounted in 3D housing)


ADEED: That rectangular black thing is a Cover/Finger placment 3D-printed part (it slips over those two IR LED’s (emitter and recvr). That thing with the White shaped heart is the $5 sensor used on the Nano)…

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Hi BlackCoffee,

Nice to see the projects you worked on.
Did you design any filters for your projects? What do you think is the best option a passive low-pass RC filter or a 3-pole active filter?

Would you mind sharing some of your files? Like pictures and schematics?
Do you have a github page?

Not posted but, below are some images…

Filtering: I did use several filters but, in the end, I did not use them, I just tweaked the Trimpots for voltage/sensitivity… Oh, the Arduino IDE has BPM library with tunable parameters (that’s what the Nano is using…)

Screen Shot 2023-01-05 at 12.56.26


Screen Shot 2020-08-06 at 12.16.22 PM

That finger torture device is called a “plethysmograph”

I had to do an internet search for that one.
I only found “Penile plethysmography”, but my wife does that by hand, no need for a 741 :wink:.

So BlackCoffee I made some changes on the circuit based on a forum´s advice and added two 60Hz filters to filter the noise out of the CHEST1 and CHEST2 electrodes before they´re fed into the OP AMP.

They adviced me to swap the LM741 for the TL072 because it´s a better chip.
The values of the filter components were calculated using a tool online: (Sample)3rd order Sallen-Key Low-pass Filter Design Tool - Result -

What do you think about the NETLABELS and connections?
What is your overall opinion on the circuit?

It is based on this link here: DIY ECG with 1 op-amp

Opamps are not square. They are triangular. I don’t even look at schematics like that.
It’s also got the horrible rectangles around schematic sections which obfuscate more then they clarify and on top of that it’s an senseless burden on schematic maintenance because they get redrawn and resized often.

I do like to have some text to clarify the meaning of schematic sections, but I always make them bigger so you can also see them when zoomed out and your attenion gets drawn to it. Just like chapter titles in a book.

Also, have a look at openeeg. It’s an open source project with both hardware and software and you can even buy kits.

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Well, sorry, but your new circuit ruined the essence of that little 1 opamp design, since you lost the body ground, and killed the high-impedance input. I have been down this path as I design eeg systems which are much like ecg circuits.

In the original circuit, the battery is divided in half to create a ground voltage. A floating battery in combination with that divider provides a pretty clean pos/neg supply for the opamp. That ground also gets connected to the leg (right leg usually) and is the body ground for the circuit – without that connection to the body you will get garbage signals. You took that lead off your new circuit.

The big problem is that your two chest leads now feed into low impedances of your filters, and the millivolt signals will get lost (well, chest-1 is the desired signal and chest-2 is for noise cancellation). The original design had the main chest lead feeding into the opamp pos input which is biased to the half-supply ground via a 10 meg which is a nice high impedance (the choice of a TL072 is also probably good, as it is a fet input opamp with a much higher input impedance than an ancient bipolar like the old 741).

The circuit is amplifying the Chest-1 lead by 1+R5/R3 which is a gain of about 57. You can tweak these to change gain if needed. Now R3 needs to return to the same dc reference as the positive input which is that same half-supply ground, but they are connecting it to chest-2. This is interesting for a couple of reasons: the chest will be at about the same dc level so the opamp should get biased properly, AND it will be picking up about the same amount of 60-cycle (or 50) powerline noise as the chest-1 lead, but the opamp is subtracting that out to a large extent. This is called a common-mode signal (in-phase noise on both inputs).

I like a good sallen-and-key filter as much as the next guy, but you need to re-visit the original circuit which probably works ok. And please don’t draw a ground symbol facing any direction but down :slight_smile:

If the opamp output just sits at the pos or neg rail, there is a dc offset that needs to be cancelled out. If it is oscillating at MHz freqs like you often see on a breadboard, add a small cap across R5. In fact, you should always have a small cap across a feedback resistor to keep the opamp from oscillating or at least to limit bandwidth – it provides a rolloff of high freqs starting at a -3dB point of 1/(2piRC); eg: a 100K with 100pF across it is a low-pass opamp circuit starting about 16KHz.

Try it in spice. Spice is great for opamp circuits – start with an ideal opamp to get the circuit topology figured out and then put real opamp models in to see how they affect the circuit. I have not yet tried the spice in kicad but have done a lot of ltspice, and many other versions going back to when it was loaded from a 9-track tape. I have learned a lot about circuits by simulating before prototyping even begins. Some spice opamp models even do a decent job with 1/f noise corners and other subtleties so you can simulate system noise floors as well as signals.

Caution: connecting things near your heart can be dangerous, and this basic circuit has no current limiting or protective considerations at all. To be completely safe you should not connect this to a human. If you choose to do so, ONLY use a battery to power this, and a battery to power the arduino/micro, and use a laptop on battery as well. No usb cable to a desktop – no connections to the ac mains anywhere…

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OpenEEG has some protection on it’s inputs. Series resistor and transistors that short the leads if a voltage gets above 600mV. You write you have some experience with this stuff. Do you consider that a good design? I once saw a datasheet of an IC that was especially designed for this, but I got a big shock when I saw it’s price.

In defense of OP’s preference for Square/Rectangle OP-Amps…

I agree that Triangular shape is (usually) the standard however, Analog Devices (the big company that produces devices and the popular LTspice software), about 1/2 of the LTspice OPAmps are Square/Rectangles… They also use Triagular shapes, too… a mix of them…

I know there are plenty of low quality libraries out there.
But that does not mean I’m goin to use them or look at them.

It may be justifiable if you have some special part such as the high side current sensors or integrated instrumentation amplifiers, but for “regular” opamps there is no valid excuse. Especially when you use a single rectangle for two opamps. I’m not spending a single second to figure out how the surrounding circuitry is connected.

I’m not going to invest that effort to compensate for a lazy person who does not use decent and standardized schematic symbols.

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Well I don’t know anything about openeeg except what I saw in the last few minutes of looking it up. The series current-limit resistors are on the low side, but the crowbar clamping is not a bad idea. There are different approaches, and this is using an instrumentation amp to knock down common mode signals (of which powerline is the dominant culprit), but a differential adc is another way to go.

Electrode impedances play a big part of successful signal extraction and the impedance from electrode through conductive paste and tissue will vary quite a bit. A good paper from 2001 analyzes differential and common-mode gains – a big take away is that unwanted common-mode gain is not driven so much by the absolute value of the electrode impedances, but by the imbalance:
2001-aaa-electrode-impedance–ferree-et-al.pdf (274.0 KB)
But this is getting way off-topic for a kicad forum.

For single opamps I do like to put them in one box with power and optional trim pins (or 5-pin without). For duals or quads I use separate devices. The positive terminal should always be at the bottom for opamps and the top for inamps, imho.
Schematics should be easy to understand at a glance, but are also a form of art.

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