by pebe » Fri May 27, 2011 8:58 pm
Your problem is that the circuit does not provide enough power to drive a loudspeaker. You need to connect it to the input of an amplifier like an LM386, and connect your loudspeaker to the output.
The frequency (pitch) of the output depends on R and C. The BC557 in your circuit is configured as a constant current source that charges C, so frequency will also depend on the supply voltage. With a 2.7V zener the voltage across R will always be 2V when the transistor is conducting, so if you make R = 1K then it will pass 2/1000 = 2mA. If you make supply = 9V then C will have to charge 3V each time.
Assuming the discharge time is short and can be ignored, then the frequency can be calculated from
F= I/(C x V) = 2mA/(1millionth x 3) = 500Hz. Increasing R or C will reduce the frequency and vice versa. Don't reduce R below 100ohms.
I hope that helps.