OK, I have what is probably a more complicated question than I imagine:

I'm a beginner when it comes to RF circuits. I'm trying to put together a system that operates wirelessly but only over short distances (less than 1-2 meters).

The system incorporates a Heart Rate Monitor. The HRM transmits wirelessly at 5 kHz and can only go maybe 2 meters. I have a circuit that receives the 5 kHz 'magnetic pulse' and amplifies and feeds that into an micro-controller for processing.

My problem is that I would like to have other inputs that operate in the same manner as the Heart Rate Monitor transmitter. That is, I need to know how to start developing a circuit that transmits the same 'magnetic pulse' at a slightly higher frequency (but still in the VLF band which 3-30kHz).

I've searched high and low for reference circuits but they are generally WAY more complicated than could possibly be necessary and generally are targeted to amateur radio or ISM or 2.4 GHz

The question: does anyone know the inner workings (or know of a schematic) of a Heart Rate monitor strap (the transmitter)? Or know generally how to make a circuit that behaves the same way? Or better still, an easily sourced IC that performs as a VLF transmitter?

Thank you very much for your time.

Also this site rocks.

 

At a guess, both from the limited range and the "magnetic pulse" wording, the transmission is through induction rather than radio.

Do you have the make and a model number for the heart rate monitor?

 

Try a coil of a dozen turns on a couple of metres diameter inside the ceiling or under a carpet ( a 'speaker' without cone nor magnet) fed by the 3KHz to 30KHz signal and a 'T loop' style receiver.
Miguel

 

Yes it is most likely by inductance. I was referring to RF as all things using the sub-microwave end of the electromagnetic spectrum. Perhaps that was misleading in the context of electronic circuits.

The HRM transmitter is the Polar T31

 

Try a coil of a dozen turns on a couple of metres diameter inside the ceiling or under a carpet ( a 'speaker' without cone nor magnet) fed by the 3KHz to 30KHz signal and a 'T loop' style receiver.
Miguel

Yes, I understand the wavelength of this low a frequency is HUGE and thus the antenna for radio transmission. But bear in mind that the transmitter in question is no more than 2in x 1in and the receiver fits on a watch.

I guess I'm not talking about radio comunication. If it would help, here is an example circuit that that I used as a reference that receives the signal in question:

http://rick.mollprojects.com/hrm/hrm00pub.pdf

Notice the 5kHz resonator on the top left.

Would the transmitter be similar to that setup?

 

If there are other simoultaneous signals on the same frequency, the receivers will pick both and get a confused decoding.

Unsure of the receiver shown; seems to be for an AM modulated 5KHz carrier.

I was referring to no carrier, transmitting your 'ultrasonic' signal directly by feeding it to a T loop. If you need amplification, I do not know.
http://www.mreilly.com/WebPages/loop/how.html
The system is normally used for hearing impaired.

Miguel

 

It is absolutely an AM modulated 5kHz signal. It's almost a digital transmission (in that I'm only interested in two states: is the amplitude above a threshold or not). I have built the receiver, know how it works, and it does operate correctly.

In an effort to provide as much information as I could, I think I muddied the issue.

The goal is to find a simple circuit that emits that 5kHz signal like the sealed sport heart rate monitor strap does. Or an explanation of the ideas involved in a transmitter like this so that I can adapt that into a circuit that I can tune to transmit in other frequencies.

ICs will only work if I can select the frequency.

Thank you, for your responses.

 

Then; build a plain low power 'transmitter' at say 9KHz carrier, or 23.5KHz; or any frequency apart from existing ones.
Modulate the transmitter collector with 1 or 0 (high or low) to send the 'digital' output of your future device in bursts. That could be done with a digital level driven mosfet.
There is dozens of AM transmitters on the web; but doubt any under 30KHz.

It would then be an (ultrasonic) burst audio oscillator = AM modulation. Oscillator circuits are also plenty on the web. I wish I knew how your heart rate monitor transmits to suggest better.

The new receiver will have to resonate at the new carrier; or use a NE567 IC. A NE566 could work as 'transmitter' ¿?

Miguel