The PIC 16F685 is a re-programmable microcontroller used to coordinate the rest of the Tactile Metronome. It runs firmware which was written in the C programming language.
Ceramic Resonator The ceramic resonator provides the clock frequency for the microcontroller. Compared to a crystal, it doesn't need external capacitors and is less expensive, but it isn't quite as accurate.
Battery HolderThe battery holder holds the batteries and connects them together to the rest of the circuit.
Seven Segment LED DisplayThe seven segment LED display is used to display the beats per minute on the metronome. These are common anode displays. This means that on each display, each LED (segment) is connected together at the power side and connected to an individual pin on the ground side. To make a segment glow, a voltage is applied to the common anode pin, and a lower voltage is applied to the segment pin.
A resistor is commonly used when powering LEDs. This is to avoid damaging the LED. However, many LEDs can be powered without resistors if the power is pulsed. This puts the "average" power within safe limits and the LED isn't damaged. The appropriate information can usually be found on the LED datasheet.
This pulsing technique is used to eliminate the need for the resistors. Transistors are used to multiplex the display so fewer pins are required to display all three digits.
NPN TransistorsThe NPN transistors are used to enable and disable each digit of the seven segment LED displays.
The human eye is much slower than a microcontroller and an LED. If the LEDs are switched on and off quickly, we don't perceive the gaps, just like we see fluid motion when watching a movie made from still frames. We can use this to our advantage to reduce the pin count required to connect all the seven segment LED displays to the microcontroller. If we use the transistors to switch the seven segment displays on and off, we can make sure only one display is on at a time. Once we have this guarantee, we can join all the similar segments together and connect all three similar segments to the microcontroller just once. For example, all the decimal point pins would be connected to each other, and connected to only one pin on the microcontroller. We need one pin per digit to use the transistor to turn the digit on or off. After that, we just need 8 pins to control the segments, and we have full control over the whole display. This takes a total of 11 pins, compared to at least 24 pins if each segment was directly driven by a microcontroller pin.
Piezo SpeakerThe piezo speaker is used as both input and output in the Tactile Metronome. It does the beeping in the circuit as well as senses the taps. When the circuit beeps, it's due to the microcontroller applying a square wave of 0 and 5 volts across the leads of the piezo speaker.
The piezo speaker contains a tiny piece of ceramic that exhibits the piezoelectric effect. This means that when the ceramic is flexed, it generates a voltage, and when a voltage is applied across it, the ceramic flexes. This crystal is attached to leads and placed inside a small plastic resonant chamber. This resonant chamber makes the sound louder.
This piezo speaker is direct-drive, which means that something outside of the speaker has to generate the frequency that the piezo will buzz at. Self-driven piezo speakers have some circuitry in them that lets them buzz themselves when powered.
Power SwitchThe power switch is used to control the power to the circuit.
Push ButtonsThe push buttons are used to provide input to the microcontroller. In this application, they're used to change the tempo of the rhythm, and to control the pitch of the beeps.
10 uF Electrolytic Capacitor and .10 uF Ceramic CapacitorThe 10 uF electrolytic capacitor and the .10 uF ceramic capacitor are used together to smooth the power. When the power switch is used, the voltage can "get a little jumpy." By adding these capacitors, this jumpiness is reduced. When the circuit is first turned on, the capacitors charge and smooth that transition. When the circuit is turned off, the capacitors discharge and smooth that transition as well.
10k Ohm 1/4W ResistorThe resistor in the circuit is used to provide a path from the piezo speaker to ground. Without it, when the piezo is tapped, charge builds up on the element and readings become inaccurate. The resistor provides a path to ground for that charge.
20-DIP SocketThe socket is a way to connect the chip to the PCB without soldering the chip directly. By soldering the socket on the board instead, there's no possibility of overheating the chip with the soldering iron. The chip can also be removed or replaced without soldering.
Tactile Metronome PCBA professionally-made PCB is the key to many successful projects. The PCB is a fully-custom design with two signal layers and no vias. All parts are through-hole for easy soldering, with full soldermask and complete silk-screen labeling of all parts.
Not quite perfect, but still excellent. 
