Re: DCC occupancy detections false positives

In article <483085f1$0$11208$822641b3@xxxxxxxxxxxxxxxxxxxxxxxx>,
David Nebenzahl <nobody@xxxxxxxxxxxxxxx> wrote:
Which says that any higher-frequency harmonics are going to be so weak
as to have negliglbie effect. Which means that the capacitive effect of
two conductors in the form of HO track can be disregarded.

Well, I would more or less agree with this analysis _but_ the detector
is inductively coupled, which means it will be much more sensitive to
higher than to lower frequencies.

My own block occupancy detectors work off the voltage drop from two
diodes in series, which is used to feed an optocoupler[1]. I find
it's very reliable: no spurious detections even though it will detect
a firmly-pressed-down dry finger (across N-scale track) or an inactive
function decoder drawing a fraction of a milliamp.

I've got lots of big runs of wire in parallel and quite a bit of EMI
(I had to shield a different part of my circuit with paper-wrapped
tinfoil) and I see no problems.

[1] circuit, best I can do in ASCII art:

from booster output
|
| 5V
+--------+-----------+ ------+---------
| | | |
| | | +-+
| | +-+ | |
----- +---+ | | | |
/ \ \ / | | +-+
+---+ ----- +-+ | detector output
| | | +--+--------------
| | | | / to microcontroller
| | | __ |/
----- +---+ +---+ ,. /| |\
/ \ \ / \ / ' ~~ | V
+---+ ----- ----- |
| | | |
| | | |
| | | | GND
+--------+-----------+ ---+--------------
|
|
to track

This has a minimum number of parts (and solder joints) per block: one
bridge rectifier, 1/8th of an octal optocoupler, 1 ordinary resistor,
1/nth of a SIL resistor array for the pullup, and of course one
microcontroller pin. I can fit 21 of these on one modest PCB along
with the microcontroller, circuitry to control a dozen point motors,
etc.

I built the booster myself and its output voltage is higher than the
12V nominal track voltage to compensate for the 0.7ish per diode
forward drop.

--
Ian Jackson personal email: <ijackson@xxxxxxxxxxxxxxxxxxxxxx>
These opinions are my own. http://www.chiark.greenend.org.uk/~ijackson/
PGP2 key 1024R/0x23f5addb, fingerprint 5906F687 BD03ACAD 0D8E602E FCF37657
.

Relevant Pages

  • Re: 556 Trigger Voltage Question
    ... >measure the two wires I soldered on the detector circuit board. ... stated what supply voltage you're using for the timer circuit. ...
    (sci.electronics.design)
  • Re: modifying a circuit design for a touch switch
    ... passed via the 2 capacitors in series to the detector, ... As the capacitors are very small in value, ... The detector voltage will drop, and the D flip flop will trigger, switching ... Up to you now to design the circuit :-) ...
    (sci.electronics.design)