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Fun and easy to build buzzer circuit!
Me and this circuit met when I was around 14 years old and I played with it for years. This was probably the circuit that most fascinated me in my early years and that awoke in me the passion for electronics. I saw it for the first time in Brazilian electronics magazines, some of them still in print today, but I tracked its origins it to a circuit from a book written in 1960 by Lou Garner - Transistor Circuits.
This circuit is an oscillator, and is able to generate a sound wave, a tone. The tone’s frequency (if it’s a high or low sound) is controlled by the variable resistor. The sound volume of this circuit is considerable so, don’t put your ear too close to the speaker on the first run 
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The circuit can be used as a bell, a sound warning, small alarms, simple sirens, etc (leave your application ideas on the comments section at the bottom). When I was in high-school I made a game as a project for the Electricity course. I made that game where you have a twisted wire that runs inside a metal ring; the goal is to take the ring from one extreme of the wire to the other, without ever touching the wire with the ring. If the wire and ring touched, a bulb lamp would turn on. This was what my teacher allowed me to do, but a few hours before project delivery I built this circuit inside a cheese box and connected it in parallel with the lamp, adding a quite audible buzz to any game playing failure. My teacher didn’t comment on my work, but he spent about 15 minutes looking inside the cheese box :).
The circuit is quite simple, consisting of only 7 components, including the loud-speaker (FTE1). The speaker must have an impedance of at least 8ohm (it is usually written in the speaker itself). You can get one of these speakers easily, but also scavenge them from old small pocket radios (maybe even found on the street) or a “PC speaker” taken from an old PC. The speaker you can see in the photo below was taken from a toy given with a MacDonald’s Happy Meal! It’s a nice speaker, 8 ohms and 0.25 W maximum power, excellent for small projects as this one.
Any voltage between 3V and 6V will power this circuit; I used a pair of AA batteries, resulting in 3V.
The values of variable resistor P1, capacitor C1 and resistor R1 define the tone’s frequency and are not critical. You can try other values for C1, bigger or smaller than 100nF, as long as they are not polarized; a bigger capacitor like 680nF with a big R1 like 2.2M will give you a “heart beat”-like sound, whereas a small value like 47nF with the original R1 value will make unbearable ear cracking sounds 
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You can try any non-polarized capacitor that you can put your hands on, even if you don’t know its value (like capacitors taken from old radios or TVs). If you train your ears with known capacitor values you can even be able to guess an unknown capacitor value just by its sound in this circuit.
With small modifications it’s possible to turn this circuit in a small amplifier.
The component list:
Q1 - BC548 (or BC547, BC549, with suffixes A, B or C)
Q2 - BC558 (or BC557, BC559, with suffixes B or C)
C1 - Capacitor 100nF for 10V or higher
R1 - 33K (orange, orange, orange, gold or silver)
R2 - 1K (brown, black, red, gold or silver)
P1 - Variable resistor 100K to 470K
FTE1 - 8 ohms loud-speaker
Misc. - batteries box, solder-less breadboard, wires, etc.
See below a diagram to help you identify the transistor’s pinout.
We now have a slightly improved version for this circuit for sale as a KIT to build it yourself, click here to check it out.
Well, and now… have fun
Nuno
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I want your feedback!
your circuit diagram has some error on it, the other end of r2 that should only be connected to the capacitor, is also connected to the emitter of the BC548 transistor. hope u’ll consider my comment .. but all in all, its a great help.. tnx. .
William, thanks for your comment, but no, there’s no error on the diagram :). You are right in that R2 should not be connected to BC548’s emitter, and it is not. If you look closely at the schematics, you’ll see that all nodes with more than 2 components have a tiny gray square on them, which means they are connected; you will not see that mark where R2 crosses the NPN transistor’s emitter, meaning that the wires are *not* connected.
i have a question sir? where the lower pin of variable resistor connected in your schematic? all i can see is the middle pin is connected to R1 and upper pin is connected to power
It’s not connected anywhere, Batman. It’s just working as a variable resistor and not as a voltage divider. You can probably do a “drawdio” with this circuit.
How could you use this circuit to sound the buzzer based on a light source? Basically, no sound if light is on; buzzer on at 1 KHz if the lights go off. I understand that you need a phototransistor (that’s what I have) but not sure if the same circuit above can be used?
Well, if you connect Q1’s base to ground the sound will stop. I guess you could use a phoyotransistor for that, depending on its exact characteristics. Using a NPN phototransistor you can connect the emitter to ground and the collector to Q1’s base and do some experiments.
Not sure if it’s the particulars that are getting me or the overall picture, but I can’t seem to get a circuit that works. The sound either continuously stays on whether I shade the phototransistor or not.
Well, I don’t know too :). If the sound it’s always on, should be because the phototransistor is not posing any relevant resistance between, looks like it’s always on!