warbler siren:


parts:

U1, U2	NE555 timer IC
R1, R4	10k ohm resistor
R2, R5	1 mega-ohm resistor
R3	47k ohm resistor
C1	0.22uF ceramic capacitor
C2	0.1uF ceramic capacitor

all resistors are 5 or 10 percent tolerance, 1/4-watt
all capacitors are 10 percent tolerance,
rated 35 volts or higher

this circuit provides a good siren noise suitable for warding off robot-predators! Change the pitch by adjusting R2, and R5.