Welcome STEM kids to old school electronics!
The midget tone generator is as simple as useful. With just a couple of transistors and a few other parts, it is fairly easy to build the belittled circuit.
First, here is the circuit diagram:
Next, the parts list:
- T1: S8050 NPN Transistor
- T2: S8550 PNP Transistor
- LED1: 3mm Red LED
- R1: 100KΩ ¼ w Resistor
- C1: 10nF Ceramic Capacitor
- LS1: 8Ω/0.5w Magnetic Speaker
- S1: SPST on/off Switch
- BAT: 3V (1.5V AA or AAA x2)
How does it work? Simply put, this is a simple relaxation oscillator!
The capacitor C1 charges through resistor R1 until transistor T1 turns on. Eventually this will turn transistor T2 on as the collector current increases which will short the capacitor C1 through resistor R1 thus completing the cycle (after completing one cycle the same process is iterated all the time).
This particular circuit as-is works great and as a single (but changeable) frequency tone generator. Needless to say, this is an easy do-it-yourself project that has some potential to make a serious signal injector for bench testing, also a good jump off point to a series of simple tone or noise makers.
As usual, I built a quick test setup on the mini breadboard. It worked as expected (and it still works). As you can see, it generates an output tone with a frequency close to 1.4kHz (yours may vary slightly based on the parts used).
Note that this circuit can be modified for use as a low-power hooter, buzzer, or Morse code practice oscillator, etc. The R-C components R1 and C1 set the tone and can be adjusted for a distinct or desired output. Also note that the reverse-wired LED1 will not look as bright when lit.
There are several things that may evolve from this little circuitry. If I take this basic concept seriously and extend it into something else, I will share the idea(s). See you next time. Good luck!
