Looking for a receiver for 5 kHz

[Batucada]

I'm not sure if I've landed in the right forum with my request, but I'll try anyway.

I am looking for a receiver module or a receiver IC, whatever it may be, to receive a radio frequency at 5 kHz and then to be able to evaluate the transmitted signal.

The transmitter that emits this radio frequency is a chest strap transmitter that records the heart rate and transmits it to a receiver. The maximum range should be several meters.

My search on the net was unfortunately overshadowed by the results for a receiver with 77.5 kHz, which is close, but not really. I am therefore looking for a receiver for 5 kHz whose output signal can be forwarded to an Arduino or Raspberry Pi.

Thank you for all the answers.

[radiolistener]

you can easily receive such frequency on sound card. It's better to use one with 24-bit ADC and 384 kHz sample rate, some Chinese dongle like CX31993 can do it through mono mic-input. It has good sensitivity and dynamic range.

you can connect it to raspberry pi and use as usual sound card.

[joeqsmith]

First few hits from Google:

http://circuitszoo.altervista.org/heartbeat_monitor.html
https://maker.pro/forums/threads/wireless-heart-rate-monitor.6623/
https://forum.arduino.cc/t/heart-beat-coil-signals-problem/108327/4

[RoGeorge]

Such devices are usually made using the ISM band for radio.  5kHz seems wrong, I don't think 5kHz has such usage, or any usage at all.  The lowest I have tinkered with was a chest belt heartbeat monitor emitting on 110kHz, which is one of the free ISM frequencies:  https://hackaday.io/project/13142-sniff-the-wireless-data-of-a-sports-wrist-watch

How do you know yours is emitting at 5kHz, where from did you found the frequency?

[shabaz]

There are some chest transmitters that work on 5 kHz, but maybe it's worth just buying a more modern one (maybe also Bluetooth ones exist nowadays - I've not checked).
I have a 5 kHz one somewhere, and out of curiosity I once tried to receive the signal with it. I can't recall all the detail, I think the 'packet' is something like three small pips of 5 kHz, and you need to measure the distance between a couple of them. I'm sure you'll find the documentation online by googling. I don't have a circuit, I merely experimented until I could see something on the 'scope. I used a coil wound on a loo roll cardboard tube (Edit: actually now I think more, it was a piece of a poster tube, so probably approx 3 inch diameter), and some capacitor across it to act as the resonant circuit. Then, I simply used a few op-amp stages, some with filters. I think I also experimented with a switched capacitor filter but it wasn't necessary. That was all sufficient to see a clear signal from half a meter away or so (enough distance to reach a running machine etc), but I didn't decode with a microcontroller. Note that the frequency from the transmitter isn't precisely 5 kHz by the way (I think mine was off by two or three hundred Hz), so you might not want to filter too sharply.
Edit 2: If you want it to work over several meters I think you'd be better off using different type of chest strap, e.g. BLE if it exists, since then there are hundreds of off-the-shelf chips/microcontrollers, very decent range, and it's more sophisticated, won't interfere with other chest straps.

[Batucada]

Hello,

I don't know exactly. I just followed an advertisement that used the term 5 kHz in its presentation. In general, however, this type of sensor, which supplies the corresponding signal, seems to be classifiable with this term.

[Batucada]

Hello,

Thank you very much for your answer, it's really to my taste. I think I will go down this route, unless I can find a supplier of a small module.

[Stringwinder]

"5kHz seems wrong" - No it's NOT!

Polar uses 5 kHz transmission in some products (and I have quite a few).
Works under water and coded versions exist for "crowded" places.

Polar H1 sensor - GymLink (5 Khz)
Polar H2 sensor - GymLink & WIND (5kHz + 2,4 GHz)

Easy to receive 5 kHz but since ignorance thrives here I won't go into details.

[jpanhalt]

Here's a very old design for 10 kHz.  The link no longer works with my Google.  Russian subs used extremely low frequency RF, but not sure it went that low.

Ten years ago I had a passing interest is such low frequencies.

[Batucada]

Hello,

I am currently on the way to realizing such a receiver. I would be very happy to hear more from those who use this technique and can certainly tell me interesting details. I have picked up on joeqsmith's post.

[RoGeorge]

"5kHz seems wrong" - No it's NOT!

Polar uses 5 kHz transmission in some products (and I have quite a few).
Works under water and coded versions exist for "crowded" places.

Polar H1 sensor - GymLink (5 Khz)
Polar H2 sensor - GymLink & WIND (5kHz + 2,4 GHz)

Easy to receive 5 kHz but since ignorance thrives here I won't go into details.

That last sentence in your reply is not only arrogant, but also illogical.

You decided to not go into details about the VLF receiver, which is the subject of this topic, because somebody else (myself) have had doubts about the 5kHz frequency.  Then why did you even bother replying?  The 5kHz frequency was already confirmed 3-4 times before.  Are we all, here, where ignorance thrive, supposed to be intimidated and never ask what we don't know?

Anyway, rhetorical questions.  The idea is to reply about the subject of the topic, and not about the participants.  First rule here, on EEVblog, is to be nice to each other:  https://www.eevblog.com/forum/chat/forum-rules-please-read/

[RoGeorge]

My search on the net was unfortunately overshadowed by the results for a receiver with 77.5 kHz, which is close, but not really. I am therefore looking for a receiver for 5 kHz whose output signal can be forwarded to an Arduino or Raspberry Pi.

You may want to search for VLF receiver schematics. 

There are plenty of examples, from very simple to very complicated, but the are usually schematics that you need to build, not a ready made module like the 77.5kHz receiver for the DCF77 time beacon signal.

If you have an oscilloscope, try to receive the signal by simply hooking the GND aligator to the tip of the probe, like in this:


The loop of wire will behave like an antenna.  That will show the exact frequency and the modulation type.  Mine was emitting 110kHz pulses, on/off keying like this:


If you don't have an oscilloscope, for 5kHz you could use a sound card as well, as suggested before, then record and look at the waveform with Audacity, or with some other sound editor.  If the signal is not audible, use a longer wire and make a bigger loop with more turns.  If still not enough signal, then make a tuned antenna instead of just a loop of wires.

Once you can see the signal, you can decide if a simple tuned loop is enough, or if it needs amplification.  Any audio amplifier should do it. 

I would start with the above tests in the hope that the signal might be strong enough to feed the ADC of an Arduino (with only a tuned antenna and no additional amplifier).

[radiolistener]

The sound card is the most cheap and easy solution for VLF reception. You can use HDSDR software on Windows or some other sdr software on Linux or MacOS like SDR++. It allows to see spectrum and select proper filter and modulation.

Original SDR++ don't allow to use sound card with mono input as a source, but you can use my mod, it unlocks soundcards with mic input:
https://github.com/qrp73/SDRPP

See attachment how it works on Raspberry Pi 4 with sound card mic input.

Sound card with 16-bit ADC is good enough for this because there is pretty high environment noise floor on VLF band.
But if you want better sensitivity and dynamic range, you can use sound card with 24-bit ADC.

I can listen to submarine communications with using just PC onboard realtek soundcard with 16-bit ADC. They using different bands, VLF band is used for common communications, but this band requires the submarine to swimming at shallow depth for about 20 meters depth and using very long antenna. This is used for planned communication. For emergency commands like nuclear strike command they using ELF band which allows to receive it while submarine swimming in deep water, but it is very slow because it uses low frequency carrier about 72 or 82 Hz. It's hard to listen such band in the city due to high environment noise. But VLF communications can be listened with no issue, usually submarines very active at about 19 kHz.

The main issue with VLF and ELF band is that it requires very large antennas and very high power due to low antenna efficiency. In order to send commands to submarines from aircraft, military using very long antenna about 8600 meters (!) length. Such kind of antenna comes out of an airplane in the form of a very long steel rope. Just imagine aircraft with almost 9000 meters steel tail... It also provides for emergency steel rope cutting in case weather conditions lead to uncontrollability of the aircraft with the antenna extended

[ledtester]

...
I am therefore looking for a receiver for 5 kHz whose output signal can be forwarded to an Arduino or Raspberry Pi.

Coincidently, if you want to do all of the detection in the microcontroller this video was just released today:

Building a Digital Filter: How it works + Simulation + Example
https://youtu.be/Aq_SOvR1Sxs

The example it uses is a Biquad notch filter which can be implemented in just a half page of code. After viewing that video you'll know enough to use this design tool to generate the biquad coefficients:

https://arachnoid.com/BiQuadDesigner/

According to this post:

https://arduino.stackexchange.com/a/16354/13673

even the lowly Arduino UNO can perform ADC conversions at 70 ksps in free running mode so doing it all in the Arduino seems quite feasible.

Btw, if you use an UNO this writeup:

https://www.instructables.com/Girino-Fast-Arduino-Oscilloscope/

looks like a good resource for getting the most out of the Atmega328's ADC subsystem.