I'm trying to build a Raspberry Pi into an old toy phone and use the GPIO pins to read the numeric pad (this part is working) and attaching the RPi's output to the built-in speaker. The speaker says "8 Ohm" / "0.25 W" on its back.
For interfacing with the keypad matrix (and "off the hook" switch), I'm using these GPIO pins: 15, 13, 16, 11, 12, 18, 22, 8, 19, I'm also using pin 1 (3.3V) and pin 17 (3.3V) for the switch-style connectors.
The audio output so far is working using the built-in ALSA device, but the output volume from the 3.5mm headphone jack is too low. I've researched a bit on the web, and there were some suggestions of amplifier boards or something. Is there an easy way to amplify the headphone output for the speaker in question (8 Ohm, 0.25 W) and can I power this amplification from e.g. the 5V output of the RPi or what would be the best way to get good volume out of the speaker (the quality isn't as important, it's a speakerphone style speaker of a toy). Or would outputting PWM over a GPIO pin and connecting that directly to the speaker be preferable to the 3.5mm headphone jack? I can switch from the Pi 1B that I'm using at the moment to a Pi Zero, if that makes things easier?