Nuts and Volts - December 2015

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-