You park your car and go in the house, and probably the last thing that you want to think about is someone stealing your vehicle. We all know, however, that it can happen. This simple circuit consists of a transmitter/receiver pair that functions as an automobile antitheft alert system.
The premise for the design is straightforward. You place the transmitter in a vehicle parked outside your home and leave the receiver in your house. When the vehicle leaves the area, this circuit sounds an alarm because the distance between the transmitter and receiver increases and the received power level falls below a predetermined level (threshold). The circuit is, in effect, warning of a potential auto theft. This circuit can also be applied to other applications for security purposes, such as securing USB flash drives or monitoring a child’s presence.
Figure 1, below, shows the ASK (amplitude-shift keying) transmitter system with a carrier frequency of 315 MHz. IC1 is a 555-based timer that provides a bi-level oscillation signal. IC2, the ASK transmitter itself, has an adjustable output power level of up to +10 dBm. This circuit is placed in the owner’s car.
Figure 2, below, shows the receiver and alarm system that is placed in the home. IC3 is the 315 MHz ASK receiver with -115 dBm receive sensitivity. The integrated received signal strength indicator (RSSI) and peak detector are used to determine if the transmitter is located within the specified distance. The RSSI signal is filtered by an internal second-order Sallen-Key filter. The filter’s poles are configurable with two external capacitors at pins OPP and DSP/DF. This filter limits the frequency to the maximum ASK data rate and the filtered data signal is internally connected to a data slicer and peak detector.
The data slicer converts the received RSSI signal to a logic level. It uses the average voltage level to determine if the received signal is logic high or low. The self-adjusting average voltage level is determined by the RC (5 kO, 0.47 µF) filter, which maintains the average RSSI value. Here the RSSI signal is used to determine its average level.
Therefore, when a long sequence of 0s or 1s is transmitted, then the average level will shift. This is not an issue in this application, since the data signal is not used and is driven by the oscillation of IC1, a 555 timer with equal occurrences of logic 0 and 1 states. Instead, the focus is on the integrated peak detector.
IC3 contains an integrated peak detector (op amp and diode) with its output at pin PDOUT. The RC circuit (1 MO, 0.1 µF) at pin PDOUT provides an RC tank filter to the RSSI signal, which is needed because of ASK modulation. With a large RC constant, the voltage level will hold its peak value through the 555 timer’s oscillations. When the time constant is compared to human speeds, it will still respond quickly and instantly trigger the alarm after the threshold is reached.
IC2 and IC3, the ASK transmitter and receiver pair, are also available in designs optimized for 433 MHz operation for the ISM band in Europe and most of the world. The ISM band allows for unlicensed applications. The 315 MHz frequency used in this design is for use in North America.
When the car’s transmitter moves away from the receiver in the house, the received power is reduced with the growing distance. The RSSI level then drops below the threshold voltage defined by an adjustable resistor-divider connected to IC4, a comparator.