System operation and other applications
Adjust the threshold voltage to correlate with the ideal distance based upon the complete system and transmitter/receiver location. Note that the threshold should be adjusted based on where the system is located in the dwelling. Also, differences in the density of house construction materials affect signal attenuation. Finally, IC4 triggers the buzzer in the receiver, sounding an audible alarm. At this point, the owner of the vehicle can verify if the car has gone missing.
There is a way to overcome low signal levels: Add a low-nose amplifier (LNA) such as the MAX2634
. The LNA placed at the receiver increases the receiver’s effective range, and it is lower cost than a full power amplifier. This LNA can be optimized for use in the 315 MHz and 433 MHz ISM bands. With a higher signal-to-noise ratio (SNR), moreover, there is more freedom in setting an optimal threshold.
The audio level of the alarm will ramp up as the transmitter’s power decreases below the threshold level. This increasingly loud alarm is intended to heighten the alert level. The rising audio level is a result of ASK modulation varying the RSSI above and below the threshold value. The effect is similar to applying a pulse-width modulated (PWM) signal at 50% duty cycle to drive the buzzer.
Once the RSSI is completely below the threshold, the buzzer’s volume will be at its maximum. To increase the ramp-up volume time, decrease the RC-tank circuit value. Verify that the threshold value is set correctly so that the buzzer is fully enabled when the vehicle is beyond the user’s defined distance away from the receiver.
Gone in a flash
This antitheft system can also be implemented on USB flash drives containing classified or confidential information. In this application, users have USB drives attached to their keychain and kept in their pockets or purse. When the USB drive is removed from its cover, it can activate a battery-powered receiver integrated in the USB cover. The transmitter is located on the flash drive itself, drawing power directly from the computer. When the user steps away, the alarm on the keychain will trigger. The transmitter and receiver pair offers low-power shutdown or standby modes, which can also be used to implement battery-saving features. By sensing when the USB cover is removed, the shutdown modes can be used to prolong battery life.
There is yet another application for this automotive alert system: To remind a parent of a child left in a car. A child left unattended in a vehicle at any time is a cause for alarm. In this case the parent secures the transmitter onto the car seat and keeps the receiver. The devices function exactly as described above. If the adult moves too far from the car, the transmitter and receiver become out of range of each other. The alarm will sound, serving a as reminder that the child is still in the car.
For this application adjust the potentiometer to trigger the alarm when the distance is more than a few feet. You can also reverse the location of the transmitter and receiver for the case when the child wanders off. The alarm assists in locating the position of the child, although the effect is lessened in noisy environments. Another positive side effect is that the alarm will draw attention from bystanders to help in reuniting parent and child.
About the Authors:
Tom Au-Yeung is a director at Maxim Integrated Products where he manages of team of analog and RF customer applications engineers. Tom has been with Maxim for more than 11 years. He holds a BSEE from Cal Poly State University in San Luis Obispo and an MSEE from Santa Clara University. Wilson Tang is an applications engineer at Maxim Integrated Products and has been with the company since 2010. He holds an MSEE and BSEE from California Polytechnic State University in San Luis Obispo.
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