Wednesday, February 9, 2011

Low Cost Siren Using 3-Transistor

This is a low-cost siren circuit to add to our range of alarm modules. This circuit generates a tone that sounds very similar to a siren.
Low  Cost Siren Using 3-Transistor Circuit
A complementary transistor pair (Q2 & Q3) is wired as a high efficiency oscillator, directly driving the loudspeaker. Q1 ensures a full charge of C2 when power is applied to the circuit. Pressing on P1, C2 gradually discharges through R8: the circuit starts oscillating at a low frequency that increases slowly until a high steady tone is reached and kept indefinitely. When P1 is released, the output tone frequency decreases slowly as C2 is charged to the battery positive voltage through R6 and the Base-Emitter junction of Q2. When C2 is fully charged the circuit stops oscillating, reaching a stand-by status.

Notes:
  • A good sized loudspeaker will ensure a better and powerful output tone.
  • As stand-by current drawing is zero, SW1 can be omitted and B1 wired directly to the circuit.
List Component of Low  Cost Siren Circuit

R1,R3 : 1K         C1,C2 : 22µF/25V
R2,R5 : 10K        C3    : 10nF
R4    : 220R       C4    : 47µF/25V
R6    : 220K       Q1,Q2 : BC557
R7    : 22K        Q3    : BC337
R8    : 100K
SW1   : SPST  Toggle or Slide Main Switch (See Notes)
P1    : SPST  Pushbutton Operating Switch
SPKR  : 8 Ohm Loudspeaker
B1    : 12V Battery

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