Enrico,

 Why don't you build a small transmitter operating at say 200Khz or so and
 capacitively couple the modulated signal to the rails.   You can then pick
 it off from anywhere on the rails.   You're already transmitting power
 through the rails.  You could have your modulated signal riding on the
 rails too.  No pun intended.

                                             Regards,

                                               Jim


> I am planning to build a speedometer for the model railroad. It should
> measure the actual speed of a train via a sensor on the axle of a wagon
> (via a white spot on the black axle and an IR fork lite barrier). The
> impulses should be transmitted via ?? to a local base station, max.
> distance around 30m (90 ft). The power for the barrier and the ??
> transmitter will be delivered from the rails via a LT1073 (Micropower
> DC/DC converter) working as a regulator for 5V and a 1F Gold Cap.
> 
> In the local station should be the ?? receiver, a PIC for counting and
> calculating the "real world speed" (conversion from actual speed in
> scale 1:160 to speed in 1:1) and presentation on a LCD.
> 
> The PIC part is clear - counting the received pulses for a specified
> time, calculating the speed and multiply with the scale factor for real
> world speed and show the calculation on the LCD. An indication if the
> link is OK should even be included and shown on the LCD.
> 
> I am thinking about a good method to transmit the pulses to the local
> station - either via RF or IR. The advantage of RF is that there has to
> be no line of sight between transmitter and receiver, the disadvantage
> that a RF transmitter is a bulky element that is maybe not fitting in
> the space of a 1:160 wagon. In addition I have no idea if there are
> small, simple and cheap integrated building blocks at the market. IR
> looks much easier but there has to be a line of sight between
> transmitter and receiver, in minimum the reflections from walls or
> ceiling has to be strong enough to decode the pulses safely.
> 
> So I have a few questions - what do you think is the better method for
> transmitting the pulses, is there integrated RF equipment on the market
> which is simple to use, cheap and small enough, does somebody have
> experience with transmitting simple pules via around 30m max...
> 
> Last but not least - the maximum number of speedometers is very small
> (one for me, one or two for collegues in our model railroad club).
> 
> I really would appreciate your thoughts and help.
> 
> Regards
> Enrico
> 
> 
> 
> 
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