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Four Transistor Alarm Circuits: Design & Build Your Own!
Alarm A
The circuit known as “Alarm A” represents a straightforward implementation of the design showcased in my “Introduction To The Complementary Latch.” It incorporates both normally-open and normally-closed contacts. Upon activation, the buzzer will commence emitting sound until the alarm system is deactivated. The minimal standby current for this design is less than 100uA, solely due to the value of R1. Adjustments to the value of R1 may be made depending on the amplification characteristics of the utilized transistors, potentially extending the battery life.
Alarm B
“Alarm B” constitutes a sophisticated modification of the aforementioned “Alarm A” circuit. It also includes both normally-open and normally-closed contacts. Similarly, once activated, the buzzer will continue to produce sound until the alarm system is switched off. However, the reduced standby current is considerably lower – approximately 2uA. Consequently, battery operation is anticipated to be augmented by a factor of fifty. Again, variations in the amplification of the transistors can be exploited to further optimize battery longevity by modifying the value of R1.
Alarm C
“Alarm C” represents an advanced adaptation of the “Alarm B” circuit. It maintains the inclusion of both normally-open and normally-closed contacts. Furthermore, the Buzzer output has been superseded by a Siren output. The presence of the relay allows for the safe drawing of approximately 500mA from the output terminals. This permits the connection of a significantly louder sounder to the alarm system.
Alarm D
“Alarm D” presents an alternative instantiation of the “Alarm C” circuit. It retains the inclusion of both normally-open and normally-closed contacts. Additionally, the mechanical elements of the relay have been replaced by a transistor switch. The BD136 power transistor will reliably switch 500mA without the need for a heatsink.
