Nuts and Volts - December 2015

are the power supply connections to the AC line (if you opt for the internal supply method as I did) and the connection from the light switch to the relay.

When making these connections, make SURE that the circuit breaker feeding the light switch is turned off. The safest way to do this is to turn the light switch on where you want this installed, and flip the breakers one by one until that light goes out. Place a piece of masking tape across the breaker to prevent anyone from accidentally resetting the breaker while you are working. If you have a no-contact voltage detector, double-check that the line is dead using that. It is always better to be safe than sorry — especially when working with line voltage. Once the circuit is fully enclosed, there is no danger of electric shock. With all of that out of the way, let’s get back to the installation.

All of this was mounted directly below the light switch (the modem case conveniently provided two wall mounting screw slots) and the AC lines were run through the wall under the case. I tied my power supply directly to the AC wiring since it was handy, but if you are using an external supply, you can just plug it into an existing outlet. The contacts of K1 are wired across the existing light switch with a short length of standard lamp cord.

Wiring across the existing switch instead of replacing the original switch with this circuit allows the system to be overridden and allows the light to be kept on regardless of the state of the circuit. Using screw terminals for the AC connections makes wiring easy and safe. After the AC lines are connected and the switch is mounted back in the wall, you can turn the breaker back on in preparation for the alignment procedure (Figure 7).

The Hard Part or Lining Up the Sensors

The Optek sensors are designed to be snapped into mounts facing each other, but since I am mounting them to a wall, there really isn’t an easy way to attach them.

What I did (Figure 8) was to hot glue the sensor to a small piece of perfboard with a hole drilled in the top. Screwed into a sheetrock anchor, this allows stability as well as a place to solder the current-limiting resistor (in the case of the LED) and the connector. Once you have determined which doors you’d like to cover, mount the LEDs on one side of each door about waist high.

Now, it gets a little more difficult.

JP1 allows the initial alignment of the beams without triggering the circuit continuously, causing the light to flicker on and off. With pins 1 and 2 jumpered, the 555 trigger IC2 is held high, and instead LED1 lights when the receiver and the emitter are aligned and extinguishes when there is a misalignment.

Only connect one receiver at a time across X5-1 and X5-2 while performing the setup, otherwise you’ll never get the sensors aimed properly. Both would have to be perfect for the LED to light. To avoid any external light source from giving a false positive, turn off all the lights while performing this step and work by flashlight. Adjust R4 to about its middle position. Now, hold the detector (mounted on its perfboard) directly across from the LED and slowly move it around until LED1 shines the brightest. Mark that spot, secure the board to the wall, and repeat for the other detector. This may take a little time and patience. I know it did for me!

Once both receivers are correctly mounted (don’t do what I did — align the sensors BEFORE mounting or you’ll wind up with Figure 9!), connect both receivers in series across X5-1 and X5-2 using a small wire nut to connect the emitter of one to the collector of the other. With both phototransistors connected in series, you may find that LED1 no longer lights. You can adjust the sensitivity by