[pearljammer]

I saw this circuit on some electronics websit and built it for a project of mine.
Now my professor wants me to explain in detail how exactly it works...

All I know is that the basic principle of the circuit is that when SW1 is closed the circuit dissipates all the current in the 330K resisot.

When SW1 is open, the circuit supposedly begins to oscillate... but i have no idea which is the feedback path and which is the oscillator output...

any help?

Parts:

R1____________330K 1/4W Resistor
R2____________100R 1/4W Resistor

C1_____________10nF 63V Polyester or Ceramic Capacitor
C2____________100µF 25V Electrolytic Capacitor

Q1____________BC547 45V 100mA NPN Transistor
Q2____________BC327 45V 800mA PNP Transistor

SW1____________Reed Switch and small magnet (See Notes)

SPKR___________8 Ohm Loudspeaker (See Notes)

B1_____________3V Battery (two A or AA cells wired in series etc.)

 

[Resqueline]

Why, output=speaker, feedback=C1..
With SW1 closed Q1 doesn't get any base voltage (or current) and so neither does Q2 which means speaker sees no voltage (or current) either. No voltage across C1.
Open SW1 and C1 starts to charge via R1. Eventually Q1 starts to conduct and so also Q2 which thus delivers a rising current (and voltage) to the speaker.
The rising speaker voltage transfers to Q1 base (via C1) making it (& Q2) conduct even more, rising the speaker voltage to the max.
Eventually Q1 consumes all the charge in C1, making Q1 (& Q2) drop out of conduction, lowering the speaker voltage. The lowered voltage transfers to Q1 base (via C1) thus making them conduct even less - all the way to shutdown - and the process repeats.
I think I remember it being called a relaxation oscillator, but you may want to check up on that.

 

[pearljammer]

Thanks.
Yup it's a relaxation oscillator alright!