- Induction loop sensors are fairly easy to design, just take an RL
oscillator such as a Colpitts and use the coil as your pickup. When
chunk of metal comes near it the frequency of the oscillator changes.
You can use a phase-locked loop to get your control signal. Just use
standard circuits (available from a variety of sources) and cobble
together something. A single transistor is all you need for the
oscillator and a single PLL IC can be used.
I've built a few induction sensors before but have always found them
a problem to adjust, they're always sort of cranky.
Why not use an IRLED/phototransistor combination to sense the train?
You can mount it between the tracks (reflective) or use a beam break.
Connect the phototransistor to the trigger of a 7555 (CMOS Timer) to
help debounce it. Proper masking of the source and sensor will
improve reliability greatly. One thing you could do also is to mount
bar codes on the bottom of the train and the phototransistor could
read these as the train moves past. All kinds of possibilities.
--- In firstname.lastname@example.org, "Jon" <brockj@h...> wrote:
> Does anyone have a circuit design for an induction-loop sensor for
> detection of trains in a model railroad?