R1,R3____________1K 1/4W Resistors R2,R5___________10K 1/4W Resistors R4_____________220R 1/4W Resistor R6_____________220K 1/4W Resistor R7______________22K 1/4W Resistor R8_____________100K 1/4W Resistor C1,C2___________22µF 25V Electrolytic Capacitors C3______________10nF 63V Polyester or Ceramic Capacitor C4______________47µF 25V Electrolytic Capacitor Q1,Q2_________BC557 45V 100mA PNP Transistors Q3____________BC337 45V 800mA NPN Transistor SW1___________SPST Toggle or Slide Main Switch (See Notes) P1____________SPST Pushbutton Operating Switch SPKR__________8 Ohm Loudspeaker B1____________12V Battery
This circuit was requested by several correspondents. Its purpose was to obtain more power than the siren circuit already available on this website since many years (One-IC two-tones Siren) and to avoid the use of ICs.
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.
- 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.
- Maximum current drawing at full output is about 200mA.