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Modified: 21:02, 26 October 2013

Circuit Designed by Phil Townshend c.

This is a development of the basic metronome but with some added logic to separate the first beat of each bar and also to make the bar length adjustable, 2, 4, 6, 8 or 10 beats to the bar. In fact you can have any length if you select a different output but there are no copper tracks for them and you would have to solder directly to the PCB underside - not too difficult though.

Download Circuit Diagram and PCB details - (PDF - 68K)

Download Block Diagram - (PDF - 5K)

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PCB being tested (no rotary switch attached)

Circuit description
The low frequency astable sets the tempo adjusted from around 90 to 200 beats per minute (bpm) using VR1. The square wave output has roughly a 50/50 duty cycle. This is converted into a brief pulse by the pulse shaper which can be varied using PR1. This will adjust the length of the beat. This pulse triggers another astable set to an audible frequency to produce bursts of sound. The frequency (pitch) of the tone can be adjusted by PR2. The output is amplified using a darlington driver and sent to the speaker. T3 (BFY51) transistor can get a bit warm after prolonged use so take care. There is just space to add a small push heatsink if you have one.

The tempo output is also fed to a decade counter which switches 10 outputs on one at a time as each pulse occurs. The decoded outputs can be made to reset the counter after a set number of beats using SW2 rotary switch. The first output is the first beat of each loop (bar) which is fed to an LED and also adjusts the frequency of the tone astable. This gives a different tone on the first beat of each bar. The pulse from the pulse shaper is also gated with the first output from the counter and its inverted logic, giving the first and remaining beats of the bar signals for the LED's.

Another version made.
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