METRONOME

INSTRUCTION SHEET

 

Price - Stock No 235


Description

This kit mimics the function of a metronome, giving an audio and visual signal for each beat of music.  Like mechanical metronomes, this unit provides for a range of timing intervals and also produces a realistic 'tick - tock' sound effect.

 


Construction
  1. Inspect the tracks for fine breaks and test the continuity of each track with  an  electronic  circuit  tester  or  ohm-meter.
  2. Identify the resistors using a multimeter or the Resistor Colour Chart
  3. Bend  the  legs of the resistors to match their respective holes and push them through from the plain side of the P.C.Board.  Solder in place - refer Soldering Technique   
  4. The 5.6V Zener diode can be bent to shape and  mounted  with the band to the K end of the space. - see the placement sheet.
  5. The 8 pin IC socket can be pushed carefully into place. Solder in place.
  6. The BC 548 can now be inserted in the correct orientation and soldered.
  7. The trimpot can now be inserted and soldered.
  8. Two pins can be pushed into the holes where the piezo transducer leads connect.  Two more pins where the switch mounts.  These can be soldered off. 
  9. Identify the polyester capacitors or 'green-caps' - these are not polarised and can be mounted and soldered.  Greencap markings 100n = 104  , 10n = 103.
  10. The electrolytic capacitors are polarised and care needs to be taken to put the right capacitor in the right location and orientation. 
  11. The LED can be inserted observing polarity, then solder in place.
  12. Solder the switch to the PCB pins.  Center pole of the switch and one outer pole. 
  13. The battery holder is SOLDERED and MOUNTED from the TRACK side of the PCB - see above picture. Solder the holder in place.
  14. Solder the leads of the peizo transducer as shown in the PCB Layout.
  15. Establish the correct orientation of the LM1458 with the locating dot on the IC and on the diagrams.  Insert the IC into the socket with care.
  16. Insert a battery and the metronome is ready for testing.

Testing / Operating

 Insert a battery and turn the unit on.  The 'tick - tock' can be heard with the LED flashing along.  Depending on the initial setting of the trimpot this may be fast or slow.  Adjustment can be made to the trimpot to achieve the desired beat rate.

Trouble shooting

Troubleshooting  if necessary will involve careful checking of locations and polarity of components, the diodes and that the locating DOT on the LM1458 is as the drawing shows. Re-solder all joints and check to make  sure  you  have  not  bridged across  between  any  two adjacent component legs.  The end  of  a  broken  hacksaw  blade  sharpened  on  an  emery wheel is a good tool for cleaning between soldered joints.

Technical notes

Although the schematic diagram may appear complex, if we break down the systems within the circuit, we find that there are only three functions occuring.

First we must understand the IC - LM1458.  It is essentially two 'operational amplifiers' configured as shown in the Block Diagram.


For more information on the LM1458 see Datasheet for the LM1458.

Operational amplifiers have been designed to be used in a wide variety of applications.  By their design, they can acomplish a range of functions depending on the configuration of the components that configure them. 



The main function of the metronome is to be a timing signal generator - to indicate the beat of the music.  (See Schematic #1)  The combination of resistors and capacitors in this portion of the circuit generates a pulse that goes ON OFF ON OFF ON OFF .....  The rate of the pulse is controlled by the 500K trimpot.  So this part of the circuit acts as a DIGITAL pulse generator - either ON or OFF.  

It is also interesting to note that the two 15K resistors create a voltage divider - creating a center voltage - basically dividing the input voltage of 9V by two.  This creates a 4.5V reference that is required by the both the operational amplifiers.


 

The second function is to create the 'tock' sound effect.  (See Schematic #2)  In this case the second operational amplifier is triggered by the output of the first.  It is activated on each change of state - activated when going from ON to OFF, also activated when going from OFF to ON.  Set up as shown above, it creates the 'tock' sound. 

This second operational amplifier is now configured to be an ANALOGUE sound effect generator.

The output of the operational amplfier is then fed via the 22uF capacitor to the piezo transducer. 

The third function is to create the 'tick' sound and illuminate the LED.  (See Schematic #3)  When the output of the first operational amplifier goes ON, the voltage across the zener diode exceeds 5.6V and it begins to conduct, turning the LED on and activating the BC548.  This connects the collector of the BC548 to 0V which adds a subtle change to the configuration of the second operational amplifier.  Creating the 'tick' sound.

When the output of the first operational amplifier is OFF, the the BC548 is not activated - 'tock'

 

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