December, 2014, CTT

[lionelsoni]

The "Questions-Answers" feature threw up their hands on a question asking for a substitute for the original controller for a 6019 remote-control track.  The questioner needs a momentary DPDT-CO switch, like this one:  http://www.radioshack.com/product/index.jsp?productId=2062530&filterName=Type&filterValue=DPDT  (A fixed accessory voltage can be substituted for the center rail.)

           Center rail---o   o---Center rail

Electromagnet---Rail A---o   o---Rail B

                Common---o   o---Center rail

The article "Yard lights:  From bulbs to LEDs" is much more complicated than it needs to be.  A lamppost with two lights cries out for an anti-parallel LED connection.  Instead of connecting the LEDs anode-to-anode and cathode-to-cathode, connect them anode-to-cathode and cathode-to-anode.  Then, with a suitable ballast resistor in series, the pair of LEDs can be powered directly from an AC source.  The way it works is that one LED lights on one half-cycle of the alternating voltage waveform, and the other one lights on the other half-cycle.  Each LED protects its companion from the damaging reverse voltage that would otherwise be a concern when using AC power.  (Note that, because of the symmetry of the anti-parallel connection, there is no need to keep track of the LEDs' polarity after they are connected together in pairs.)

For information about ballast-resistor selection, see my article in the September, 2008, CTT.

Speaking of ballast resistors, looking at the article again, there's some stuff about them in a box on page 77 that has with a couple of problems.  One is the number which is 100 in the initial formula, but seems to get corrected to 1000 later in some dodgy algebra or arithmetic.  The other is the number 1.7, which is meant to account for the forward voltage drop of the LED.  But 1.7 is typical for a red LED.  For white LEDs, 3 or 4 volts is a more reasonable number.

[fifedog]

HUGE CONGA for our own Bill Dimenna! (page 56)  Nice layout.  Love it when I see our Forum members in print.