Hi Bryan. Good question! I think this sort of modification can be easily done (with one minor exception).
Your first reference point should be the circuit schematic:
(Click image for full size)
Notice that the piezo is connected to two signals: pwm_out and adc_in which are connected to pins 5 and 13 respectively. Notice also that adc_in is connected to P9, which is a separate pad on the circuit board. pwm_out is also connected to pin 6 on the P2 bank of pins.
These two signals are tied together on the circuit board, but we designed the PCB so that they are relatively isolated, and you can cut one trace to fully isolate them.
Here’s a view of the PCB layout. Teal represents the silkscreen artwork. Red traces are top-side traces (component side), and green traces are bottom-side traces (battery holder side). Gold pads are on both top and bottom.
<img src=’/uploads/default/34/b05facb99be6fc8b.png’ width=690" />
Follow the red and green traces connected to pins 5 and 13. Notice that the traces from pin 13 (adc_in) only go to piezo resistor R1, pad P9 labeled “P”, and the piezo itself. The traces from pin 5 (pwm_out) connect directly to the pin labeled “P” on the EXT bank of pins. Then there is a trace with an odd-looking horizontal piece that connects pins 5 and 13. If you cut the that odd horizontal trace on the back side of the PCB, that will disconnect pins 5 and 13, and will enable you to use separate hardware for the input and the output functions. You can use an exacto or other sharp knife to cut that trace.
Once you have cut that trace, you can connect your own piezo input to the P pad by the build-in piezo. You could also just replace the on-board piezo with two wires to your external piezo.
Then, you can use the EXT pin labeled “B” (for “beat”) for your external LED blinking visual. This signal is high whenever the metronome is beeping, and low otherwise. This signal will work better for you for controlling an LED than the pwm_out, because the pwm_out signal is a pulse-width modulation (PWM) signal, meaning it is a square-wave that cycles a few thousand times per second while the metronome is beeping. Here’s a quick comparison between the pwm_out (EXT “P” pin) and the beat (EXT “B” pin):
Hopefully that makes sense, and shows how those two signals differ. Use the Beat signal for an external LED, use the pwm_out signal for an external piezo.
Keep in mind that the weak little pin on the microcontroller that generates the beat signal won’t be able to drive more than a single LED. If you want to drive more LEDs, then you’ll need to use a transistor. We can discuss these options later if you want.
The only issue that might be tricky would be to output a signal to an 1/8" jack. Were you thinking of connecting a pair of headphones or other audio equipment? The pwm_out signal is great for driving a piezo, since the physics of the piezoelectric materials produces a reasonably-pleasant sound, but the raw square wave sounds pretty bad to human ears http://en.wikipedia.org/wiki/File:Square_wave_1000.ogg One reasonable approach may be to build a little oscillator circuit that produces a nice-sounding sine wave to your headphones, that is controlled by the beat output. We can discuss this further if you like.
I hope that answers your questions, let’s discuss further to see what you’re looking to do.