Here the wailing alarm circuit diagram:

Component parts List:

R1,R5___________ 4.7K
R2______________ 47K
R3______________ 10K
R4______________ 100K
Rx______________ *see text

C1,C4__________ 100uF/25V, electrolytic
C2,C3__________ 0.01uF (10nF), ceramic
T1_____________ 2N3702 (NTE159, TUP, etc.)
IC1,IC2________ LM/NE555, MC1455P, etc
LS_____________ Loudspeaker

This is the sound generator which will simulate British police car siren. The circuit is built using 2 pieces of timer IC 555 to generate sound frequency.

How the circuit work:

This is a simple fire alarm circuit based NE555 timer and use thermistor as temperature sensor. This sensor will activate the transistor when the temperature is in high value.

The thermistor offers a low resistance at high temperature and high resistance at low imperature. This phenomenon is employed here for sensing the fire.

The IC1 (NE555) is configured as a free running oscillator at audio frequency. The transistors T1 and T2 drive IC1. The output(pin 3) of IC1 is couples to base of transistor T3(SL100), which drives the speaker to generate alarm sound. The frequency of NE555 depends on the values of resistances R5 and R6 and capacitance C2.When thermistor becomes hot, it gives a low-resistance path for the positive voltage to the base of transistor T1 through diode D1 and resistance R2.

This is a low cost circuit which is  simple and easy to build. The main components of this  circuit is based on dual op-amp MC1458 and LM 380. The two op amps inside the MC 1458 are used to produce square and triangular waves. LM 380 is used to amplify the output. The first op amp IC1a is wired as an astable multi vibrator and second op amp IC1b is wired as an integrator, to make the square wave triangle.

Download the complete TUTORIAL of 2×90W Stereo Tube Power Amplifier with EL34. The document in PDF version contain of  schematic diagram, part list, component guide, step by step how to build the circuit. This circuit is for advance hobbysts…