At the instant power is applied to the circuit, the voltage across C1 and hence at the anode of Q1 would be zero. C1 would then be slowly charged by R3 towards +12V. Once the voltage across C1 exceeds the value of the voltage appearing on the gate of Q1 as set by R1 and R2, the PUT (Programmable Unijunction Transistor) fires, that is, it will conduct, causing C1 to quickly discharge.Once C1 is discharged, Q1 will turn OFF, causing another charge - discharge cycle to be initiated. The manner in which the voltage across C1 varies, when plotted against time, resembles a sawtooth, hence the name sawtooth wave.
|Figure 1: Electronic Circuit of a Typical Warbling Alarm|
In the process, the warbling sound is generated, the output sound can be varied through trimmer resistor R5.
While testing, R7 can be adjusted until the desired sounding is obtained. C2 can also be changed to modify the pitch of the tone, but should not be higher than 0.047 uF or lower than 0.01 uF.
Q1 - 2N 6027 Programmable Unijunction Transistor
Q2 - NA11E NPN Transistor
Q3 - 9466 PNP Transistor
All Resistors are 1/4 watt
R1 - 5.6 kΩ
R2 - 1.8 kΩ
R3 - 120 kΩ
R4 - 22 kΩ
R5 - 470 Ω (Trimmer Resistor)
R6 - 100 Ω
R7 - 15 kΩ
C1 - 4.7 uF / 50V Electrolytic Capacitor
C2 - 0.047 uF / 50V Mylar Capacitor
1 piece 2-1/4 inch 0.25 Watts, 8 Ω Speaker