> Marcelo:
>
Eric is right. Inductively data and even power can be transmitted with
loosely coupled windings in "air transformers". The data transmission
can be bidirectional using reflected load techniques. Some smart
security tags use inductive coupling.

I did medical research at Stanford using this technique and developed a
number of transcutaneous instruments.

To make your own link just wind two "pancake" shaped coils. Face coils
flat to each other and centered. Start with 1:1 turns ratio. The number
of turns depends on the carrier frequency or data bit rate. Start with a
100 turns for 100kHz. You want the inductance to be high enough at the
effective carrier rate so the drive current is not too high. On the
receive side start with a high input impedance comparator if. If the
coupling is tight enough it is possible to drive CMOS logic directly.
Besure to use input protective clamping for very close coupling. The
secondary voltage varies greatly with coupling or distance between
coils.


> ----------
> From:         Eric W. Engler[SMTP:englere@SWCP.COM]
> Reply To:     pic microcontroller discussion list
> Sent:         Saturday, January 10, 1998 12:20 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: Inductive transmitter
>
> At 01:43 AM 1/10/98 -0200, you wrote:
> >    I need some help from the list...
> >
> >    I need transmitter data with:
> >        - low power (2mA max.)
> >        - moderated cost (<  $3)
> >        - distance: 5 cm (2 inch)  across the wood (can't use optical
> >devices)
> >
> >    I thought in a inductive transmitter, but...  any ideas how can I
> do
> >this ?
> >
> >
> >-regards
> >Marcelo Rib
> >
>
> You've just described the transmitter used in the Heart Rate monitors
> from the Polar company.  They are like magic - they use almost no
> power.  Their TX distance is over a foot. Thet come in a sealed
> enclosure that includes the battery.  The battery is good for a couple
> years!
>
> They don't release schematics, but someone may want to take one apart
> and check it out.
>
> Eric Engler
>