Build the Guardian. (silent automobile alarm)

Worried that your car might not be there when you return? Let the Guardian alert you to a break-in without alerting the thief.

You have invested a lot of your hard-earned money into your auto, van, or truck. Understandably, you'd like to protect it as much as you can while it is parked out of sight near your home or where you work. Some type of alarm system would certainly help in that respect. Most alarms with a siren don't bother thieves - they are so prevalent nowadays that a screeching alarm is ignored by almost everyone.

But if a vehicle is equipped with a silent alarm, thieves will take their time, giving the owner time to contact the local police. The Guardian alarm project presented here is just such a system, using state-of-the-art wireless technology. It is especially valuable to those who have recently purchased a new vehicle.

The Guardian consists of two parts: a transmitter and a receiver. The transmitter is a digitally-encoded UHF unit that is completely automatic in operation and permanently installed in the vehicle. Connected only to the 12-volt battery line, its current draw is just 2 milliamps, so it can never run down the battery. The receiver, battery operated for portability, might be carried around with you like a small pager, or placed at a convenient location and powered by a common wall-mounted transformer. The RF section of the receiver consists of a highly-sensitive, one-chip superheteradyne circuit that has been designed for easy construction using a small printed-circuit board. A second board contains the decoder and audio section, and the entire assembly fits neatly into a small enclosure.

Once installed in a vehicle, the Guardian is on duty at all times with no attention to it ever required. When a door on the vehicle is opened, the Guardian senses the current flow to the vehicle's interior light, and automatically transmits a digitally-encoded UHF pulse train. That causes the receiver to respond with an attention-getting audible signal, so that corrective action may be taken before a would-be thief can steal the vehicle or its contents.

Although the Guardian operates at 433.92 MHz, it is easily built and no complicated RF-alignment procedures are required. The entire RF circuitry of the transmitter is contained on a hybrid module that is smaller than a postage stamp. That module has been designed to meet all requirements of Part 15 of the FCC Rules and Regulations that govern the use of unlicensed transmitters.

Digital Encoding And Decoding System. A wireless-security system is useful only when the radio-frequency carrier is modulated by an encoded signal that is decoded at the receiver. That avoids spurious operation or false alarms due to interference or an unauthorized RF signal. The Guardian uses a sophisticated digital encoding/decoding scheme by using o pair of chips developed by Motorola. The transmitter contains the encoder, and the receiver contains the decoder. The two chips are specifically designed for wireless-signaling applications.

The encoder chip, an MC145026P, encodes nine digital bits of information that are programmed by means of hard-wiring to the input terminals of the chip. Each bit can be individually set to one of three states - logic 0, logic 1, or open. That permits up to 19,683 discrete codes. Both transmitter and receiver PC boards for the Guardian are hard wired for address zero, but any other address may be selected by re-connecting the addressing pins to any combination you choose. When the transmitter is enabled, the encoder chip generates a positive going pulse train at pin 15 that contains the encoded address. That pulse train is used to modulate the UHF oscillator.

Serial data is detected by the receiver and presented to the decoder at its data input terminal, pin 9. When that occurs, the address is checked. If two successive transmitted pulse trains contain the correct address, the valid output terminal of the decoder, pin 11, goes to logic 1 condition and remains so as long as the transmitter is operating. At all other times, the decoder output remains at a logic-zero level. The nine address bits of the decoder chip are wired the same as the encoder, allowing the receiver to respond only to its companion transmitter. If two or more independent systems are desired, each system should be wired for different address codes.

About The Transmitter. Figure 1 shows the schematic diagram of the transmitter assembly. The circuit is powered by the vehicle's 12-volt battery. Diode D1 protects the circuit from reverse voltage transients that may appear on the electrical system of the vehicle. A pair of Zener diodes, D2 and D3, create two regulated voltage sources of 10.9 and 6.2 volts.

Integrated circuit IC2 contains a pair of identical op-amps, which are cascaded. The negative input of IC2-a is AC-coupled to the 12-volt bus so that it can detect a sudden sag in voltage caused by …