Audio Visual IC Metronome

With more and more musical sessions becoming louder with the passing of each day the necessity for a metronome that gives a visual indication along with an audible click becomes more apparent. One such circuit is given in Figure 3. Initially capacitor C1 charges up to the applied battery voltage through R1,R3 and R4.As soon as the voltage across C1 reaches about two thirds the supply voltage the IC discharges capacitor C1 through R4 until the voltage across C1 reduces to about 1/3 vcc. At this point C1 starts to accumu-late a fresh charge and the above cycle repeats. This results in a series of short pulses of approximately 7 ms. Duration at the output pin no. 3 of the IC . These pulses are then used to switch on a transistor that has either an LED with current limiting resisting resistor as a load or LED with loudspeaker. 

Audio Visual IC Metronome circuit diagram

Time period between these is dependent on the values of R1,R2,R3,R4 and C1. This time is adjustable between 278 milliseconds and 1.5 seconds (corresponding to 210 to 40 beats per minute). Resistor R3 controls the maximum number of beats per minute and is adjusted on test to compensate for variations in the value of C1. SThe purpose of R2 in parallel with R1 is to reduce the resistance of R1 in parallel from 250 kilo-ohms to a nominal 185 kilo-ohms to obtain the correct range. Resistor R5 is included to prevent a latch up of IC in the dischareged state when the power is first applied to the circuit. A high impedance loudspeaker is used against a standard 8 ohms loudspeaker keeping in mind the limited current capability of the output transistor. If a high power silicon transistor is available with the hobbyist, he may use a standard impedance loudspeaker. Otherwise a 35 ohms to 8 ohms matching transformer may be used. The current limiting resistor in series with light emitting diode limits the diode current to about 20 mA taking into account the saturation voltage drop. The integrated circuit must be carefully soldered on to a suitable circuit veroboard. For those who are not very much familiar with these devices, use of good quality IC sockets is strongly recommended. Make doubly sure that there is a proper contact between corresponding pins of IC and its socket. The cathode side of light emitting dioae has a notch on it and it may be tested on a standard volt-ohm-meter when it will glow on the lowest resistance measuring egran of your multumeter with the positive red probe of the multimeter (which is actually internally connected to the negativeof the battery) connected to the notch or cathode side.