LED Metronome Circuit

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The LED Metronome is a modern interpretation of a classic device which is a staple of music teachers, students and composers everywhere. This circuit uses 12 LEDs to simulate the sweeping motion of the pendulum and a speaker with a simple amplifier to generate a tick as the LEDs at the end of the arc are struck. It is adjustable from about 40 BPM to just over 200 BPM. When made in a Lexan or Plexiglass case it can add a fun ultra-modern touch to music practice (though most music teachers agree that you should never rely on a metronome to keep your beat).

Schematic

Parts

Part	Total Qty.	Description	Substitutions
R1, R4, R6	3	10K 1/4W Resistor
R2	1	1.5 Meg Linear Taper Pot
R3	1	120K 1/4W Resistor
R5	1	220 Ohm 1/4W Resistor
C1, C3	1	0.1uF Ceramic Disc Capacitor
C2, C4, C5	3	0.01uF Ceramic Disc Capacitor
D1, D2, D13, D14	4	1N914 Signal Diode
D3-D12	10	Jumbo Red LED
Q1, Q2	2	2N3904 NPN Transistor
U1	1	555 Timer IC
U2	1	4029 CMOS Up/Down Counter IC
U3, U4	2	4051 CMOS 8 Of 1 Switch IC
U5	1	4011 CMOS Quad 2 Input NAND Gate IC
SPKR	1	Small 8 Ohm Speaker
S1	1	SPST Switch
MISC	1	Board, Wire, Case, Sockets for ICs, Battery holder, Knob for R2

Notes

The circuit will reliably run from 6 to 12V. A typical 9V battery will be drained in several days by the LEDs with constant use. A 6V pack made from AA, C or D cells will last much longer.

The circuit will need some calibration before use. Calibration is fairly easy and accomplished by using a stopwatch to count the number of beats. You'll want to calibrate for 60 BPM (one per second), 120 BPM (two per second) and 180 BPM (3 per second). Just use your stopwatch to find the points of R2 that correspond to those beat rates and the mark the dial appropriately. Next find 90, 150 and 210 BPM. By that time you should then be able to evenly mark the remaining positions on the scale.

Blue LEDs need about 5V to light, so if you intend to use blue you will need to remove R5.

For an ideal metronome effect, the LEDs should be arranged in an arc.

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