555 + LM386 PWM Speaker
A 555 timer generates a tone, the LM386 amps it up — your first audible analog circuit.
Difficulty: Through-hole. Estimated build time: about 60 minutes. Estimated parts cost: about US$6.00. 12-line bill of materials. Compare supplier offers when available. A two-stage audio circuit that turns a 555 astable into something you can actually hear.
Build the 555 tone generator
Wire an astable 555 like the LED blinker, but with R1 = R2 = 10 kΩ and C = 100 nF. The frequency formula gives f ≈ 1.44 / ((R1 + 2×R2) × C) ≈ 480 Hz — a low buzz. Try 47 nF for ~1 kHz, or 22 nF for ~2 kHz.
Couple the 555 output into the LM386
The LM386's pin 3 (non-inverting input) wants a small AC signal centered at ground. Connect a 10 µF electrolytic capacitor from 555 pin 3 (output) to a 10 kΩ trim potentiometer; wiper goes to LM386 pin 3. The cap blocks DC; the pot sets volume.
Wire the LM386 with default gain of 20
Power the LM386: pin 6 to Vcc (5–12 V), pin 4 to GND. Tie pin 2 (inverting input) to GND. Place a 10 µF capacitor from pin 7 (bypass) to GND — this stabilises the internal bias and is the difference between a quiet amp and a hissy one. Leave pins 1 and 8 open for default ×20 gain (add a 10 µF cap between them later if you want ×200).
Connect the speaker through DC blocking
The LM386 output (pin 5) sits at half-supply DC; you cannot connect a speaker directly. Place a 220 µF electrolytic capacitor from pin 5 to one terminal of the 8 Ω speaker; the other terminal goes to GND. This is the 'output coupling cap' — its job is to pass audio frequencies and block DC.
Add the Zobel network
Place a 10 Ω resistor in series with a 47 nF ceramic capacitor; put the pair from pin 5 to GND. This compensates for the speaker's rising impedance at high frequencies and prevents the amp from oscillating into the inductive load. Most LM386 'doesn't work' problems are missing Zobel or missing pin 7 cap.
Power up and turn the pot
Apply 9 V. Slowly turn the volume pot up — you should hear the tone get louder. Try the 555 capacitor swap (47 nF for higher pitch). Do not push the pot fully clockwise on a 9 V battery — the LM386 will clip at full output and sound gritty.