Single IC design with visual beat indicator — ideal for home musicians

Originally, metronomes were purely mechanical devices which used a pendulum and a clockwork mechanism to produce a 'click' sound at regular intervals to help musicians play at the correct tempo; sheet music often has metronome marks which indicate the required number of beats per minute. The device was invented by Maelzel, a friend of Beethoven, well over 150 years ago. Apart from the click sound, the pendulum also gives a visual indication of the beat rate. This is important when it is likely that the sound of the metronome will be drowned by the volume of the instrument.

Although electronic metronomes are considered novel, they have been in existence for quite a few years. Most, like the unit featured here, provide a sound which is similar to that provided by their mechanical counterparts, and give some form of visual indication of the beat rate as well. In this case a light emitting diode (LED) flashes each time a click is produced by the unit.

The Circuit

Refer to Figure 1 for the complete circuit diagram of the metronome. The circuit is based on a TDA2006 integrated circuit (IC1) which is really designed for use as an audio power amplifier. It is quite a versatile device though, and could be regarded as an operational amplifier having a class AB output stage capable of delivering output currents in excess of 1 Amp. It has inverting and non-inverting inputs, and these are pins 2 and 1 respectively.

R1, R2 bias the non-inverting input of IC1 to half the supply voltage but the effect of R3 must also be taken into account, and will normally modify this voltage. If the output of IC1 is fully negative, R3 is effectively connected across R2 and reduces the bias voltage to one third of the supply.

If the output of IC1 is fully positive, R3 is effectively shunting R1, and the bias voltage is raised to two-thirds of the supply.

At switch-on, C2 is completely uncharged and the inverting input of IC1 is therefore at the negative rail potential. IC1 is being used here as a sort of voltage comparator — its output will go negative if the inverting input is at a higher voltage than the non-inverting input, or positive if the comparative input states are reversed. Thus IC1's output initially goes fully positive.

This results in C2 being charged from the output of IC1 via D1 and R4 and, to a lesser extent, through the relatively high resistance path provided by R5 and RV1. When the charge on C2 exceeds two thirds of the supply voltage the inverting input is at a higher voltage than the non-inverting one. IC1's output therefore starts to swing negative and the coupling through R3 to the non-inverting input results in the voltage at the latter being reduced as well. This increases the difference between the two input voltages, sending the output and non-inverting input further negative. This positive feedback causes the output to rapidly go fully negative, leaving the non-inverting input at one third of the supply voltage.

C2 now starts to discharge but it can only discharge through RV1 and R5 since the path through R4 is blocked by D1. Thus the discharge time is relatively long and is determined by the value of RV1, which acts as the tempo control. When the charge on C2 falls below one third of the supply voltage, the output of IC1 starts to swing positive and the regenerative action results in it almost instantly going fully positive. C2 then rapidly charges through D1, R4 and R5, RV1, as was the case at switch on and the circuit goes through this cycle continuously, with a series of brief positive pulses being generated at the output of IC1. C3 couples these to the loudspeaker which produces the required clicks. The pulses are also used to drive LED indicator D2 via current limiting resistor R6 so that each click is accompanied by a flash from D2.

The current consumption of the circuit is about 12mA.

Construction

Figure 2 shows details of the Veroboard panel and the wiring of the unit, which is all straightforward.

A scale, calibrated in beats per minute, should be marked around the control knob of RV1 using rub-on figures, and it is advisable to use a large control knob and scale because the scaling is non-linear and will otherwise become excessively cramped at the top end.