Continuing UFO PCB Construction

The tilt switch is an important component. It allows you to reset, or turn off, the UFO detector by simply turning it upside down.

The mercury tilt switch is soldered upright into the top two holes for the on-off switch, see figure 5. The top two holes are the ones closest to the word on. The LED (D1) and R6 are connected to the pcb. The longer lead on the LED is the positive terminal and ought to be placed in the D1 hole marked with the + sign.

Figure 5

Solder the 10K 1/8 watt resistors in positions labeled R1, R2, R3 and R4 (color bands brown, black, orange). The 4.7K 1/8-watt resistor is soldered and mounted in the R5 position (color bands yellow, violet, orange). Resistor R6 is a 330 ohm 1/4watt resistor (color bands orange, orange brown). This resistor is slightly larger than the other resistors.

A wire jumper is a small piece of uninsulated wire that is place in two adjacent pcb holes and soldered into position. The wire jumps the electrical signal or power from one pcb hole to another, hence the name jumper. You can use the uninsulated wire you clip off the resistors to make the jumper wires. Wire jumpers are placed on the two leads for B0, B1, B2, B3 and GND, +5V and RB4 to RB5 labeled J1 see figure 6.

Figure 6

Figure 7

Also a wire jumper is placed in the B7 position by the R6 resistor, see figure 7.

We need one more jumper between holes RB7 and RB6. This is standard wire jumper, see figure 8.

Figure 8

Next mount and solder the 10 uF capacitor C5 to the pcb. Making sure to orientate the positive terminal of the capacitor to the (+) hole on the pcb. The mini-speaker has 5" inch leads and is mounted off the pc board. The wires to the speaker are mounted to the left of the compass. The leads are labeled SPK. The red wire is soldered to the (+) pcb hole. The black wire to the (-) hole. The speaker should not be placed near the 1490 compass sensor when assembling the UFO detector or it will adversely affect the performance of the sensor and circuit. This will be discussed later.

Figure 9

Connect the 9V aluminum battery holder with a few inches of red and black 22 ga. wire. The bottom terminal on the battery holder is the positive terminal and the red (+) wire is soldered to it, see figure 9. The black wire is attached to the top negative (-) terminal.

Mount and solder the 18 pin socket for the microcontroller in the 16F84 / 16F628 position. Make sure to match the indent in the socket to the indent shown on the silkscreen outline. This helps orientate the microcontroller when you need to put it into the socket. Mount and solder C1 and C2, the 22 pF capacitors. Mount and solder the 4.0 MHz crystal in the XTAL position between C1 and C2.

9V Lithium Battery

Although the 9V lithium battery looks identical to a standard 9V transistor battery, the 9V lithium battery has up to three times the power density of a standard 9V battery. But that IS NOT the primary reason we use a lithium battery. The primary reason is that the Lithium battery uses a non-metallic, non-ferrous case. So the battery case will not adversely affect the sensitivity of the digital compass. On the other hand, standard carbon and alkaline 9V batteries do use a metallic ferrous material and will adversely affect the sensitivity of the digital compass sensor.

While there is a small amount of ferrous material used in the construction of a lithium battery, the terminal clips on top of the battery, if the terminal clips of the battery are placed away from the sensor, they do not affect the sensitivity of the digital compass to any great extend.

While we are on the subject of keep any ferrous material away from the sensor. If you did not buy the kit, whatever 9V battery holder you use ought to be made of a non-magnetic material like, plastic, aluminum or brass. As stated, the 9 Volt battery holder used in the kit and shown in the photographs is made of aluminum. In addition battery holder provides a place to mount the small 8-ohm speaker that should be placed away from the sensor.

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