I like to build a simple audio alarm circuit using transistors. Although it is an old circuit, it still useful. The circuit uses just two transistors and a few other components. We will try to build it to learn a simple oscillator circuit.
How it works
In the circuit diagram above. When we press the switch-S1. It will have the electric current flows through R1 to charge at the C1. While the voltage at the Base(B) of Q1 rises until makes Q1 work.
When Q1 is working, also the Q2 is working together. It causes a large current to flow through leads E into C of Q2, and R4 to drives the loudspeaker.
Next, the electric current begins to charge into C2, until it full. While the voltage at the B of Q1 will begins to go down until Q1 does not work.
Also, the Q2 stop work. It causes C2 to discharge to R4, SP1, and R3 to ground. Until the voltage at lead B of Q1 rises up. It will make Q1 begin working again, Q2 works too. This working generates the pops to SP1.
Which Q1 and Q2 will work together with high frequency. until merging many many pops into a continuous buzz or tone to the speaker.
When we release switch S1, C1 begins to discharge. It makes the voltage at B of Q1 go down. The output is a low frequency until the sound comes to silent.
Parts you will need
Q1 – 2SC1815—NPN transistor,
Q2 – 2SA1015—PNP transistor,
Resistors (All 0.25 watt,5% metal/carbon film)
R1 – 22K. Quantity: 1.
R2 – 68K. Quantity: 1.
R3 – 56K. Quantity: 1.
R4 – 10 ohms. Quantity: 1.
C2 – 0.068uF 50V—Ceramic. Quantity: 1.
C1 – 100uF 25V—Electrolytic. Quantity: 1.
SP1 – 8 ohms speaker. Quantity: 1.
B1 – 9V battery. Quantity: 1.
How to build it
This is small circuit. You can assemble them on universal PCB Board.
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