I was thinking for some projects for the pocket size contest so I came up with two. One is this mini metronome and the other one is a pulse clock light box (I'll explain in another instructable).
Moving on... a metronome is a device that produces a ticking sound which is a metrical beat. The first type of them (mechanical) were created early 19 century. They are used to study a music sheet. You can adjust the beats they produce throughout a certain time so you can study to different rhythms. The one I made is electronic. I have a mechanical one and an electronic one because I find them quite amazing objects and I play piano. You can still get the mechanical ones.

Step 1: Materials

You will need:

X1 555 chip
X2 LED
X3 1K Resistors
X1 250K -220K Potentiometer
X1 Small PCB
X1 Small Altoids
X1 Little speaker (or big)
X1 On-On Switch (I used a slide switch)
X1 Jack for 9v
X2 22uF electrolytic capacitors

And your ordinary tools.

Step 2: The Circuit

It is really straight forward.
Pin 1 form the 555 goes to ground, 2 goes to pin 6, 6 goes to ground on the pot which goes with a 22uF cap to ground. 7 goes to the other two pins of the pot which goes with a 1K resistor to 9V. Pin 4 and 8 go to 9V. Pin 3 goes goes to an LED that goes to a 1K resistor on 9V and 3 also goes to a 1K resistor that goes to an LED on ground. And pin 3 also goes to a 22uF cap connected to the speaker thats on ground.

(Remember the led's polarity, the flat part of the LED is negative)

Step 3: The Housing

First you may need to cut the PCB so it can fit in the altoids. Im sure you can cut neater than me.
I made 6 holes.

A hole for the Pot
A hole for one LED
A hole for another LED
A hole for the switch
A hole for the 9v power jack
A hole for the speaker (optional)

The way you make holes is up to you. I suggest using a drill and a wooden working table.

Step 4: Wrapping Up

I may post a video if I have time, Im making another project so I'll get to it when I finish.
So metronomes are very useful to study. You don't always need them but sometimes music composers are strict with timing in music pieces.
Have fun, any questions in the comment section below or just sent me a message. :)