There are many display circuits already published in BEP. But this circuit is very special in its own way because it contains all the facilities required for any type of decoration.
Lighting decoration broadly consists of two main categories-the running effect and the spellar effects. Running effect is well known to every electronics hobbyist. The spellar effect consists of many output driving the load-lamps; all of them go high one after another, and eventually all outputs becomes high simultaneously showing a complete display. Thereafter all the outputs go low and the cycle is repeated. Both these effects can be obtained by using thus circuit with just a flap of a switch!
As shown in the circuit, IC1 is wired as a free-running astable multivibrator whose frequency can be controlled through VR1. This changes the speed of running or spelling effect. The output of IC1 is directly connected to the clock input pin 6 of IC2 which is a CMOS shift register.
IF switch SW1 is kept in position 1, it connects pin 3 and 4 of IC2 to pin 12 of the same IC. Also, pin 9 is held high. So binary number 1000 is loaded in the register. This causes initially T1 to go high and T2, T3 and T4 low. Now as T4 is connected back to pin 3 and 4, the number 1000 is circulated continuously through the register in IC2 and as a result the four outputs sequentially go high. Thus a running effect is obtained.
Now keeping switch SW1 in position 2, pin 7 of IC2 is connected to pin 13. Pin 3 and 4 are held high and pin 9 is grounded. As pin 3 and 5 are high, all the outputs of IC2 go high one by one on each consecutive clock pulse. When T4 goes high, pin 7 of IC2 also goes high. This makes all the outputs low because all the parallel inputs are now grounded. Thus the operation repeats (ad infinity), and the spellar effect is obtained.