as you walk through the beam, we employ a second 555
timer IC2, this time configured as a “one-shot” monostable
multivibrator whose output pulse width is set to a little
over one second by R16 and C5. If a longer or shorter
delay between detection pulses is desired, R16 can be
replaced with a higher or lower value or — for more
versatility — a trimmer pot. LED2 is just for diagnostics and
shows that the timer output is indeed going high and the
length of that pulse.
IC2 requires a negative going pulse to trigger a
positive output, and since the output of the junction
between Q3, R8, and R9 goes high when the beam is
broken, we need to invert this. Therefore, this signal is fed
through R10 to Q4 — another small signal NPN. R15
holds the input of IC2 high until the beam is broken,
The output of IC2 is then fed to the input of IC4 — a J-K flip-flop. With PRE, CLR, and J tied high and K tied low,
for every positive pulse received on the CLK input from
timer IC2, pin 7 of the flip-flop alternates between high
and low. R18 ensures that the clock pin is held low when
not being triggered by IC2.
The output of IC4 is finally fed through R19 to Q5 —
our final NPN — which, in turn, toggles SPST relay K1 on
and off.
The Fun Part or
Hooking It All Up
I prefer re-purposing the things that other people
would throw out whenever I can, so most of my projects
are built around whatever I happen to have on hand at
the time — the IR pairs being the sole exception in this
case. With that in mind, I built my system (Figure 2 and
Figure 3) into an old gutted Westell DSL modem case.
Hot glue holds the power supply and the screw terminals
down, while two 4-40 stand-offs hold the main board. As
you can see from Figure 4, everything fits perfectly and
neatly. Of course, any plastic housing would be fine —
building this into a two- or three-gang wall box mounted
in the wall with the original light switch is a wife-friendly
option.
Rather than reinvent the wheel and spend more
money, I have found that splitting orphaned wall wart
transformers open is an excellent source of cheap, ready-
24 December 2015
■ FIGURE 4. Everything mounted in the modified
modem housing.
■ FIGURE 2. The completed circuit board.
■ FIGURE 3. A Westell modem waiting to breathe
new life.