There aren't many digital metronomes on the market, either amateur or professional, unless incorporated into instruments like the Synclavier. This is in a way surprising because it is surely easier to set a particular tempo using a digital readout rather than an inherently less accurate calibrated dial. A problem arises when building such a unit, because the user obviously requires pulse-rates per minute, but for a reasonable response time the counter needs to read at the most pulses per second. A possible technique is to have an oscillator running between approx 30-150Hz, and displaying this, before dividing the frequency by 60 to give pulses per minute. However unless care is taken, inaccuracies occur in the counting and gating sections.

This design aims to overcome these problems by using the Intersil gating IC, the ICM 7207A, controlled by a 5.24288MHz crystal. This will give either a 0.1 or a 1 second gate, so by using a 300-1500Hz oscillator and the 0.1 sec gate, the display will read 30-150 with a very high degree of accuracy, and fast resolution. The oscillator is then divided by 600 to give the required 'bleeps'. Readers will no doubt realise that if the oscillator range was expanded it would make a useful squarewave signal generator/tuning aid... particularly since with the 1 sec gate there would be a direct digital readout of Herz. It could be set to 440Hz for example with confidence, knowing that concert pitch 'A' is coming out of the speaker.

Operation

A metronome needs to cover approx 20-150 beats per minute, but a signal generator should range from approx 150Hz to at least 5kHz to be of any real use, so two variable ranges are provided by SW2a, controlling the 555 oscillator. The effect of this switch is to alter the top of the frequency range by either R2 or R3, reducing the range for the metronome function. The output is a squarewave which is fed via the NOR gate IC7d to the 4 digit counter/driver ICM7217A.

The other input to the NOR gate is provided by the 'gating' output of the ICM7207A, which goes low for either 0.1 sec, if pin 11 is high, or 1 sec, if pin 11 is left floating. The 7207A also produces the necessary store and reset pulses in the correct sequence to give a constantly updating display. The output from the 7217A controls a 4 digit multiplexed CC display, mounted on a separate PCB.

The output from the 555 also goes to IC4, a dual synchronous divide-by-10 counter. The two sections are cascaded together to divide the incoming frequency by 100, and the output feeds IC5, a 4017 which, because output '6' is connected to the 'reset' pin, acts as a divide-by-6 counter. Pin 7 thus gives one high pulse for every 600 fed into pin 9, IC4. The width of this pulse varies as the frequency changes and, at the bottom of the scale, is not short enough to give the characteristic 'bleep' of the metronome. To overcome this, the pulse goes via IC6c and d, connected as a short period monostable, whose output is largely independent of the width of the incoming pulse. This is inverted by IC7a and b, (with an approximate frequency of 1kHz with the R9 and C6 values given). The resulting 'pips' are connected via SW2c into the audio amp constructed around TR1.

When the unit is switched over to 'signal generator' mode, the divide by 600 section is bypassed by SW2c, the audio amp receiving the signals direct from the 555. The range of the oscillator is extended by SW2a and the gating period of IC2 is changed to 1 sec so that the display reads Hz direct. For readers who wish, a switch can be inserted between pin 20 (IC3) and +ve to blank the display when not required, but this was not included in the prototype.

The power supply consists of a PCB mounting transformer for easier construction, bridge rectifier, smoothing capacitor C7, and a 5 volt regulator with C8 and C9 included to aid stability.

Construction

This should not cause too many problems provided care is taken. A PCB design is given in figure 2, and the overlay in figure 3.

Mount all the components except the ICs and display, inserting sockets in readiness if these are to be used. Make sure the polarised capacitors are inserted the correct way round, and use veropins and insulated leads for all interconnections. Bolt the regulator to the small PCB heatsink. C1 has two possible dimensions, depending on whether it is polyester, or Siemens polyester and so a spare pad has been provided on the PCB.

Switch on and check that 5 volts is coming out of the regulator. If it is, switch off and insert IC1. Restore power and check that a squarewave emerges from pin 3. Switch off again and insert ICs 2, 4, 5, 6 and 7. Switch on and measure the output at pin 10 of IC6, which should show a positive-going pulse train with the frequency variable by VR1. Pin 3 of IC6 should show a gated oscillator waveform and this can be switched in via SW2c to the audio amp. Pin 13, IC2 should show a negative-going gate signal lasting for either 0.1 or 1 second, and pins 2 and 14 should consist of very thin negative-going pulses. If all is well, switch off, insert IC3 and connect the display, mounted on its own PCB, using ribbon cable. Power up and you should be rewarded with either a Hz display, (in sig/gen mode) or a display of pulses per minute. R2 and R3 can be varied to alter the tops of the ranges if required. The prototype was intended for use in an area of subdued lighting, but readers may want to change the LED for a high-brightness type.

For absolute accuracy C3 can be replaced with a 5-54pF trimmer, and adjusted against a known frequency source, but this was not found to be necessary in the prototype. Assuming everything is working, the unit can be mounted in the DX2 case. (It will fit in the smaller DX1 case, but of course the front panel details will be different). Drill and cut the front panel as shown in figure 4 to take the switches, LED, display, etc., which are mounted with the appropriate hardware. The mains lead is fed via a strain relief grommet on the rear panel, which also contains the 20mm fuse holder. Use sleeving or heatshrinking for all mains connections for obvious safety reasons. After giving the unit a final check the case can be bolted together and a useful bit of gear added to your equipment stock!

The Digital Metronome is not available in kit form.