Nuts and Volts - June 2017

switch and I/O connectors into the space behind the sheetrock, they can be plugged into the circuit board, and the PVC module can be pressed into the sheetrock hole.

Switches

Figure 9 shows how to install a switch for a dead bolt lock. A switch like that in Figure 8 is mounted in a slot in a 1” section of 7/8” wood dowel. The dead bolt hole is then extended about an inch with a 1” hole saw or auger bit, and then drilled all the way through with a 3/8” bit.

The mounted switch goes into the dead bolt hole with the connector and wires passing through the 3/8” hole to connect to the circuit board module in Figure 5. The face of the dowel stops the dead bolt from traveling too far and smashing the switch. Figure 10 shows how to attach a magnetic proximity switch for a garage door. If your home has multiple garage doors, connect the door switches in series to a single circuit module located at a convenient inside location. Additional switches for windows can be added in series to any circuit module to make the LED indicate the security status of a zone with multiple entry points. Patio switches are a challenge because the typical latch moves out, down to engage the door, and then back to pull the door tight. Here, I extended the switch lever arm so that the downward motion of the latch pushes the switch as in Figure 11. If you want an additional House-Secure indicator in an upstairs bedroom, for example, just add to the series loop an additional green LED in parallel with a reverse-facing diode without the extra diodes and switch. This indicator will show green when all doors are locked.

Parts for a Four Door System

The Parts List shown here is for a four door system (stock numbers are from Jameco Electronics; www.Jameco.com). Every home is different, so you need to adjust this list for your particular design. Various components for this project including assembled printed circuit boards (refer to Figure A) can be purchased from the Nuts & Volts webstore ( store.nutsvolts.com).

Taking It to the Next Level

If you are an inspired DIY enthusiast, you can — with the addition of separate red and green LEDs and one extra resistor in each door circuit — build a base station that can indicate which doors are unlocked. The resistors go in series with the red LEDs in a binary sequence; for example, 10, 20, 40, 80, 160 ohms in a five door system. This makes the positive voltage Vp developed across the series string a unique function of the 32 possible combinations of locked and unlocked doors.

With appropriate offset and gain adjustments to Vp,

26 June 2017

■ FIGURE 10. A magnetic proximity switch installed on a garage door. The steel on the right side is the garage door track. The brighter white box holds the magnet, and is attached to the edge of the door and moves vertically with the door. The white box with the two wires coming out the bottom is the magnetic switch. Its contacts close when the two boxes are within about 3/4” of each other.