Nuts and Volts - June 2010

2,400 kHz ± 200 Hz. I wrote a program that will warble around 2,400 kHz for maximum sound. There are two other alarms written in the software code. If you want to use them, you will need to change the software GOTO to the different locations. One is a French ambulance alarm; the other is a single tone. This will require you to have a programmer for the chip and understand writing code. (A pre-programmed chip is available from the author.) If you’re feeling ambitious, you can even write little tunes that it can play.

The magnetic transducer is activated by turning it on and off using a square wave generator. The resonate frequency (being 2,400) needs a square wave of 417 micro-seconds.

■ Top and bottom of board.

1 = .0004167 2,400

The output from the op-amp goes to a PIC12F508 which acts as a timer and tone generator for the speaker (magnetic transducer). The 10 µF capacitor passes only AC to the speaker and blocks any DC.

The PIC508 is inexpensive and is about as simple as microcontrollers can get. When it is put to sleep, it draws less than two micro-amps. The op-amp draws about 230 micro-amps, so in total the two AA batteries should last for over a year. I put in a one minute delay so that the unit can be set down and arm itself. To save power, I use the 508’s sleep function.

When the micro is sleeping, there are a couple of things that can wake it up. I used a watch dog timer (WDT) and a pin change on one of the input pins (GP0). I set the WDT to wake up every 2. 3 seconds; when this happens, it increments a counter. When the counter reaches 26 ( 60 seconds), it arms the alarm. If the sensor detects any movement, it changes pin 7/GP0’s voltage and this causes it to alarm.

How does the micro know what woke it up or if it just turned on? With the status register. By checking the bits in the status register, the micro will either jump to the program area to increase the count or jump to sound the alarm.

The magnetic transducer has a resonate frequency of This is controlled by a counting loop in the software in Listing 1.

Construction

■ Listing 1.

TONE
MOVLW
.068

MOVWF  LOOP1
BCF SPEAKER
DECFSZ LOOP1,1

GOTO
MOVLW

$-1
.068

If you’d like to make your own boards, the files are available on the Nuts & Volts website ( www.nutsvolts .com) along with the software code, tips, and suppliers of parts. Go to www.expresspcb.com and download their free software for making your own boards and schematics. This software will open the files on the website.

Pull the two wires coming out of the battery box back into the battery box. Slide a piece of heat shrink tubing over the black wire. Cut them to a length so that you can still solder them, and then parallel solder them together. Slide the heat shrink tubing over the solder joint and push them inside the box next to the switch.

Use the board as a template and place it into the center battery compartment of the battery box. The +

mark should be next to the spring of the battery compartment. At first, you might think that’s wrong but it’s not. Move the board toward the spring about 1/16”. Using a small drill, make a hole in the center of the speaker area into the box. Turn it over and using the small hole as a guide, drill a 1/4” hole in the battery box. Be careful not to drill through the spring. This hole is for the output of the speaker. Depending on how you are going to use the alarm, you can glue a magnet on the back or pop-rivet a lanyard so that you can hang it on a doorknob.

MOVWF
BSF
LOOP1
SPEAKER
;LOADING A NUMBER TO BE DECREMENTED FOR FIRST
;HALF OF SQUARE WAVE (THIS SETS THE TONE)
;MOVE IT INTO LOOP 1
;TURN ON SPEAKER (GROUND THE SPEAKER)
;COUNT DOWN LOOP 1 UNTIL ZERO AND SKIP NEXT
;COMMAND
;JUMP BACK ONE UNTIL ZERO
;RELOAD  NUMBER TO BE DECREMENTED FOR OTHER
;HALF OF SQUARE WAVE
;MOVE IT INTO LOOP 1
;TURN OFF SPEAKER (BOTH TERMINALS OF SPEAKER
;POSITIVE)
;COUNT DOWN LOOP 1 UNTIL ZERO AND THEN SKIP
;NEXT COMMAND
;JUMP BACK ONE UNTIL ZERO
;KEEP DOING UNTIL POWER IS SHUT OFF