Q&A
■ WITH RUSSELL KINCAID
In this column, I answer questions about all
aspects of electronics, including computer
hardware, software, circuits, electronic theory,
troubleshooting, and anything else of interest
to the hobbyist. Feel free to participate with
your questions, comments, or suggestions.
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Q&A@nutsvolts.com
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basics of electronics as applied
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✓Fly Zapper
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RAILROAD CROSSING
LIGHT CONTROLLER
QI am a volunteer
worker/helper at a small
local history museum
located in Tullahoma, TN.
We have a two arm RR crossing light
that has been modified with small
110V holders and lamps. I would
like to buy/build a controller to
alternately light the lamps so that
when one goes out, the other will
come on. Do you happen to have
any ideas?
— Greg Klein
AThis calls for a flip-flop. I have
used a 555 oscillator driving
an SN74LS74 and two solid-state switches (see Figure 1).
The oscillator is designed to run at 2 Hz,
but you may want to make the
frequency variable by replacing R2
with a rheostat connected 500K pot.
The SN74LS74 is a dual D type
flip-flop and I have connected them
in parallel to give more drive to the
solid-state switch. The output is
arranged to pull down because TTL
does not pull up very well. The switch
is an opto triac rated at two amps, so
it will operate a 100 watt lamp with
no problem. The rocker switch
mounts in a rectangular hole; you will
have to file it to fit, and the toggle
switch mounts in a round hole — just
drill it. A five volt power supply is
needed and the power is low, under
1/2 watt. If you don’t have a spare
kicking around, this $8.70 unit from
Mouser will do: 826-DA4-050US.
AUTOMATIC ON/OFF
SWITCH
QMy project is to design and
build an automatic on/off
circuitry for a remotely
located audio amplifier. The
■ FIGURE 1
26 August 2008