Very cheap, very low bandwidth, very short range wireless

Wireless & IoT
1

Hi guys,

I need to add one-way short range wireless to a microcontroller project. My estimated max range is only 3 feet, but cost is a big issue. The units may be covered, so infrared communication is not possible.

I’ve been playing with [rfpics, but I think they are overkill for my project.

I want to know if it’s possible to do simple LF (like 125khz) communications with a microcontroller. Has anyone gotten something like this to work before? If so how? I imagine the transmitter would just turn on or off (on off keying). I don’t really know how to make a low frequency antenna though. The receiver may be more complicated. Could a diode peak detector and a built in comparator work?

Any idea on where to start with the hardware?

BTW, I don’t need any software help. I’ve dealt with Manchester encoding before. I’m just looking for hardware advice for extremely cheap, short range, low bandwidth wireless.

Thanks.](http://www.microchip.com/ParamChartSearch/chart.aspx?branchID=1201&mid=&lang=en&pageId=76)

2

What about certification and approval? Cobbling your own system together like that is going to be very expensive.

Leon

3

I’m not worried about that yet. I figured certification would be easy, as I’m only looking to transmit 3 feet. I thought I may have more certification problems with an RF PIC.

In any case, pre-certified units would be to big and bulky. I’m thinking of using a PIC 10F micro-controller for the transmitter and receiver. So size and unit cost are my primary concerns for now. I hope it’s doable since I need very low bandwidth and range.

Thanks.

4

You should check the relevant regulations to see what certification is needed, before you start designing the system. It might be very expensive.

Leon

5

Thanks for your warning, but right now I’d really just like to focus on the hardware. I’d like to do this for education purposes, if nothing else.

Can anyone help me with a starting point for the hardware? Is it even possible?

6

Look at near-field communications, there is a lot of research being done in that area. The field strength drops off by the cube of the distance, unlike far-field communication.

Leon

7

OK, I did a really simple experiment with breadboards. Basically, I feed a square wave (from a PIC) into an inductor and capacitor for my transmitter. It ran at 83 khz because of the components I had on hand.

My receiver is just one comparator. My idea is that it compares the present voltage of the antenna loop to its previous voltage, which is remembered by a capacitor.

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Anyway, it works reliably to about 3 inches, giving a very clean square wave output. I couldn’t ask for anything simpler or cheaper. Now I just need to improve the range (by about 12x :lol:). If the power drops off by the distance cubed, does that mean I need to be about 1,800 times more sensitive?

I don’t really know anything about RF though. Where should I start for improvement? I’m quite excited that I got this to work at all, but am I even on the right track?

Thanks again.](ImageShack - Best place for all of your image hosting and image sharing needs)

8

That definitely is near-field! You will need much more receiver sensitivity.

If you want to learn about radio get a copy of the ARRL Handbook for Radio Amateurs.

Leon