Concept of Operation

A metronome is a device used by musicians to help them keep a steady beat. This classic beginner circuit is often called an electronic metronome because it produces a steady stream of clicks.

This type of circuit is called a relaxation oscillator. When the circuit is first activated the capacitor begins to charge. When the charge on the capacitor reaches the threshold required to activate the PN2222A transistor, current begins to flow through the two-transistor portion of the circuit, discharging the capacitor. The charge on the capacitor quickly drops below the threshold required to keep the PN2222A activated. With the transistor circuit now off, the capacitor begins to charge again and the process repeats. The transistor circuit also acts as a small amplifier, driving enough power to the speaker to produce an audible clicking sound.

The rate of the oscillator is controlled by the potentiometer. Rotate the potentiometer clockwise to increase the rate, and counter-clockwise to decrease the rate. You can also experiment with different values for the resistor, potentiometer and capacitor.

Schematic

Breadboard Diagram

Parts

• 2.2K Resistor (x1)
• 50K Potentiometer (x1)
• 22uf Electrolytic Capacitor (x1)
• PN2222A Transistor (x1)
• PN2907A Transistor (x1)
• Speaker (x1)
• Slide Switch (x1)

(All parts for the project are available in our store.)

Notes on Implementation

1. Electrolytic capacitors are polarized and must be mounted in the proper orientation. On standard electrolytic capacitors the cathode (negative lead) is identified by a stripe on the package. Also, the cathode lead is typically shorter than the anode.
2. The schematic shows a single-pole single-throw (SPST) switch between the power supply and the circuit. Our kits often include a three-pin single-pole double-throw (SPDT) "breadboard friendly" slide switch. An SPDT switch can be used here by aligning two adjacent pins with rows 10 and 11 on the breadboard. The breadboard diagram show pins 2 and 3 being used.
3. The circuit was built and tested at 4.8 volts DC (4 AA NiMH rechargeable batteries). Any power source that supplies about 5v DC should work fine.