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This
circuit can be used to construct an attractive Christmas Star.
When we switch on this circuit, the brightness of lamp L1
gradually increases. When it reaches the maximum brightness level,
the brightness starts decreasing gradually. And when it reaches
the minimum brightness level, it again increases automatically.
This cycle repeats. The increase and decrease of brightness of
bulb L1 depends on the charging and discharging of capacitor C3.
When the output of IC1 is high, capacitor C3 starts discharging
and consequently the brightness of lamp L1 decreases. IC2 is an
opto-isolator whereas IC1 is configured as an astable
multivibrator. The frequency of IC1 can be changed by varying the
value of resistor R2 or the value of capacitor C1. Remember that
when you vary the frequency of IC1, you should also vary the
values of resistors R3 and R4 correspondingly for better
performance. The minimum brightness level of lamp L1 can be
changed by adjusting potentiometer VR1. If the brightness of the
lamp L1 does not reach a reasonable brightness level, or if the
lamp seems to remain in maximum brightness level (watch for a
minute), increase the in-circuit resistance of potmeter VR1. If
in-circuit resistance of potmeter VR1 is too high, the lamp may
flicker in its minimum brightness region, or the lamp may remain
in ‘off’ state for a long time. In such cases, decrease the
resistance of potmeter VR1 till the brightness of lamp L1 smoothly
increases and decreases. When supply voltage varies, you have to
adjust potmeter VR1 as stated above, for proper performance of the
circuit. A triac such as BT136 can be used in place of the SCR in
this circuit. Caution: While adjusting potmeter VR1, care should
be taken to avoid electrical shock
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