Christmas Star
 
 This circuit can be used to con- struct 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. 