Protecting Your Speaker (and MCU)
Before you connect your circuit to the 8 Ω speaker, it is critical to ensure that the amplifier output and speaker load will not draw excessive current.
Driving the speaker directly from your MCU will damage both the MCU and the speaker, so always check your circuit before connecting the speaker.
Always verify your output current using a dummy load (resistor) or multimeter before connecting a real speaker!
Step 1: Measure the Output Current
- Build your circuit without the speaker — instead, connect a dummy load such as an 8 Ω resistor to simulate the speaker.
- Use a multimeter to measure the RMS voltage drop across the resistor while your MCU is generating the audio signal.
- From the measured voltage, compute the expected current using Ohm’s law:
\[ I = \frac{V}{R} \]
If you prefer, you can measure current directly by inserting the multimeter in series with the load.
Step 2: Stay Below Safe Limits
An LM386 amplifier can safely drive small speakers, but you must ensure that:
- The output voltage does not exceed approximately 1 V RMS (≈ 2.8 V peak-to-peak).
- This corresponds to a maximum RMS current of about:
\[ I_{\text{max}} = \frac{1\,\text{V}}{8\,\Omega} = 0.125\,\text{A (125 mA)} \]
If your measured voltage produces a current greater than 125 mA, you risk damaging the speaker.
To reduce current: - Lower the amplifier gain - Reduce the input signal amplitude - Add a small series resistor
Step 3: Never Connect the MCU Pin Directly
Remember:
- MCU GPIO pins can safely source no more than 10–20 mA.
- An 8 Ω speaker draws over 100 mA even at low volume.
If you connect the speaker directly to the MCU, you will fry the MCU output and possibly the speaker.
Always use the LM386 amplifier to buffer the signal and supply sufficient current.
Quick Reference
| Parameter | Recommended Limit |
|---|---|
| Speaker impedance | 8 Ω |
| Max safe RMS voltage | ~1 V |
| Max RMS current | ~125 mA |
| MCU GPIO output current | <20 mA |
By following these steps, you’ll protect both your hardware and your hearing while getting clean audio output from your MCU.