Semiconductor-fail-alarm

Description: False alarms caused by semiconductor failures are eliminated with this burglar alarm circuit, which is equipped with relays. Additionally, the circuit is nearly immune to false triggering. With a standby current of less than 0.1 mA, it incorporates all essential alarm features, including entry and exit delays, a timed alarm period, and automatic reset following an intrusion. The circuit utilizes a CMOS CD4093B quad NAND gate, designated as IC1, which provides both logic and analog timing functions through Schmitt-trigger switching action. The relays ensure the circuit remains fail-safe when the alarm is activated, even in the event of semiconductor failure. The relays operate at 12 V, with coil resistances of 250 ohms or greater. Activation of the circuit is initiated by closing switch S1.

When switch S1 is closed, capacitor C2 begins charging via resistor R2, and the arming indicator LED1 lights up. Once pin 2 of IC1a reaches its switching threshold, its output drops, turning off LED1 and signaling the end of the exit delay. This output also drives the base of transistor Q1, allowing current to flow if the emitter circuit is connected to the V00 line, thus activating Q1. At this point, the circuit is armed, and the current drain reduces to below 0.1 mA. Upon entry into the vehicle, relay RY1 contacts close momentarily, completing the emitter circuit of Q1 and causing relay RY2 contacts to close. The charging of capacitor C4 through resistor R7 determines the entry delay period. If the system is not deactivated by opening switch S1 during this interval, the oscillator circuit formed by IC1c and IC1d engages, initiating a rapid on/off honking cycle with the assistance of Q2 and relay RY3. The alarm cycle concludes after approximately one minute, when capacitor C2 charges through resistor R3 to the threshold voltage of IC1a at pin 1. This voltage resets the timing circuit, preparing it for the next entry or alarm cycle. Relay RY1 is configured for vehicles utilizing door switches connected to +12 V. For vehicles with grounding door switches, the bottom of the RY1 coil should connect to +12 V instead of ground, necessitating a reversal of the polarity of capacitor C7. For home applications, the time constant of resistor R3 and capacitor C3 should be increased to extend the duration of the alarm.

The circuit design features a robust layout that minimizes the likelihood of false alarms while maintaining a low power consumption profile. The use of CMOS technology in the CD4093B ensures that the logic operations are performed efficiently, with minimal heat generation and power loss. The inclusion of relays not only enhances reliability but also provides a mechanical isolation between the control circuitry and the high-power alarm output, which is critical for protecting sensitive components from voltage spikes or surges.

The alarm system can be further customized based on user requirements, such as adjusting the timing constants for different environments or integrating additional sensors for enhanced security. The circuit can also be adapted for various applications beyond automotive use, making it versatile for home security systems. Overall, this burglar alarm circuit exemplifies a well-engineered solution for reliable security with minimal false alarm occurrences.False alarms produced by semiconductor failure are impossible with this burglar-alarm circuit equipped with relays. What"s more, the circuit is virtually immune to false triggering. With a standby current of less than 0.1 mA, the circuit offers all the features an alarm needs: entry and exit delays, a timed alarm period, and automatic reset after an intrusion.

One CMOS CD4093B quad NAND gate, IC1, supplies both logic and analog timing functions with the aid of Schmitt-trigger switching action. Relays make the circuit fail-safe in the alarm-active mode, even when the semiconductors fail. The relays are 12-V, with coil resistances of 250 0 or more. Closing switch S1 initiates circuit operation. Capacitor C2 begins charging through resistor R2 and arming indicator LEDllights. When pin 2 of IC1a reaches its switching point, its output decreases, extinguishing LEDl and indicating that the exit delay has ended. That output also drives the base of Qllow, so that if the emitter circuit completes to the V00 line, Ql conducts.

The circuit is now armed, and current drain drops to less than 0.1 mA. When the vehicle is entered, relay RY1 contacts close momentarily, completing the emitter circuit of Ql and causing the RY2 contacts to close. Charging C4 through R7 determines the entry-ilelay period. U the system isn"t turned off by opening Sl during this period, the oscillator circuit of IC1c and IC1d activates, and a rapid on/off hom-honking cycle kicks on with the aid of Q2 and RY3.

The alarm cycle ends after about a minute, when C2 charges through R3 to the threshold voltage of IC1a at pin 1. This voltage resets the timing circuit, readying it for another entry/alarm cycle. RYl is connected for vehicles that use door switches connected to + 12 V. For vehicles that use groundiog door switches, the bottom of the RYl coil should connect to + 12 V instead of ground.

In the latter case, the polarity of C7 should be reversed. For home use, the R3C3 time constant should be increased to give a longer alarm.