Elenco Christmas Tree Kit - Manual (Page 3)

INTRODUCTION
The Electronic Christmas Tree Kit is a fun project which also gives you the opportunity to learn about the wonderful field
of electronics. The heart of the Electronic Christmas Tree is a 556 integrated circuit. This chip contains two 555 timers which
are very popular in the electronic circuit blocks. The sound of the Electronic Christmas Tree is a special integrated circuit
with a piezoelectric buzzer. You’ll enjoy three charming Christmas melodies: “Jingle Bells”, “Santa Claus is Coming to
Town”, and “We Wish You a Merry Christmas”.
CIRCUIT OPERATION
The 556 timer uses two 555 ICs. Since the Electronic Christmas Tree Kit
contains two 556 timers, there are a total of four 555 timers used in our
circuit. We will confine our analysis to the 555 circuit. The pinout of the
555 is shown in Figure 1.
The basic circuit of the 555 timer as used in the Electronic Christmas
Tree Kit as shown in Figure 2. This circuit is used four times with a slight
variation of components. Refer to the schematic diagram on page 6 to
get a better idea of the total circuit.
The 555 operates in the astable or clock mode. This means that the IC
puts out a series of pulses or oscillations as shown in Figure 3. These pulses are
at the output pin 3, and go high and low in voltage. Note that when the output goes
high, LED D2 will light since there will be about nine volts across R4 and D2. When
the output goes low, LED D1 will light. Thus, the two LEDs will alternately be ON
at the rate determined by the frequency of the oscillator. This frequency is
controlled by the values of capacitor C1 and resistors R1 and R2. This combination
is called an RC time constant. This determines the time constants, the lower the
frequency of oscillation. Thus, if you make the capacitor C1 or resistors R1 and R2
larger in value, the LEDs will take a longer time to flip ON and OFF.
Resistors R1 and R2 determine the duty cycle of the output square wave. The
current charging capacitor C1 goes through resistors R1 and R2. At this time, LED
D2 will be lit. At a certain charge level the IC will react and start discharging C1. The
discharge current will flow only through resistor R2. At this time, LED D1 will be lit.
The difference in charge and discharge time will change the shape of the square
wave as shown in Figure 4.
LED D2 will light a little longer than LED D1. Having the LEDs ON for slightly
different time intervals adds to the flashing effects.
To review what we’ve learned, the values of R1, R2 and C1 determine the output
frequency or how fast the LEDs flash. The relationship between R1 and R2
determines the duty cycle or how long LED D1 will be lit in comparison with LED D2.
The other three 555 timers operate the same except for different values for
R2. Resistors R6, R10 and R14 are each made higher in value, thus each
timer will oscillate at a slightly higher frequency. The blinking rate will be
different for each timer. Capacitors C2, C4, C6 and C8 are added to stabilize
the circuit.
The sound circuit of the Christmas Tree Kit is shown in Figure 5. U3 is a
special integrated circuit. Its memory has three Christmas melodies. These
melodies will repeat all of the time when the switch (S1) is in the ON position.
IC U3 needs to have a power supply of 1.5 - 2V, the same drop voltage as
the LEDs. The piezoelectric buzzer utilizes the principle that crystal material
vibrates when an electric current is imposed upon it.
Figure 1
Figure 2
Figure 3
Figure 4
1
2
3
4
8
7
6
5
1. Ground
2. Trigger
3. Output
4. Reset
5. Control Voltage
6. Threshold
7. Discharge
8. VCC
Square Wave
High
555
Output 3
Low
Clock Circuit
Figure 5
On Time of LED-2
On Time of LED-1
1 Cycle
P1 P4
P2
P3
U3
RB
1.2k
S1
SOUND
INTEGRAL
CIRCUIT
D9 9V
BZ
-2-