Flip-Flop). The Q-bar (pin 12) of U3B is connected to the Data input (pin 9) forming a
toggle flip-flop. When counter U4 completes one cycle of counting, a positive edge at
Cout (U4 pin 12) causes U3B outputs (pin 12 and 13) to toggle, switching the two LED
banks alternatively.
The outputs of counters U4 and U5 are delivered to U6, U7, and U8 (ULN2003
Darlington Array) to drive the 20 LEDs. The positioning of the LEDs is important. LED9
of U4 Q9 should be next to LED10 of U5 Q0 so that the light spot will run in sequence.
Each bank of LED takes one half of the ring driven by Q0 through Q9 of the counters. In
this configuration, you will notice that (for example) LED11 corresponding to U4 Q1 will
be directly opposite to LED1 of U5 Q1.
Even though the two counters are running in parallel, they may not count in sync as the
initial count during power up is uncontrollable. This is solved by sending the Cout pulse
of U5 at pin 12, through C6 and R16 (to shorten the pulse width) to reset U4 pin 15 so
that they will count in sync after the first reset pulse has arrived.
U3A is configured as a one-shot circuit. When SW1 is pressed, the Q output of U3A
flips to the high state. Through the delay action of R2 and C3, the flip-flop is cleared by
pin 4 after about 2 seconds. The one-shot pulse triggered by SW1 is delivered to U2A
pin 4 to stop the clock oscillator and the counting action of U4 and U5. The pulse also
lights up LED20 through U6 pin 7. Counting resumes when the pulse ends.
U2B is another 555 equivalent oscillator driving a piezo buzzer. Q3 acts as a logic
decision gate. To trigger a winning buzz, three criteria need to be established through
R8, R9 and R11. R8 signals that the one-shot circuit is triggered (SW1 pressed), R9
signals that the winning bank of LED is activated and R11 signals that the winning LED
is lit. Any other state will turn on Q3 and send a low signal to U3B pin 10 to suppress
the buzzer. The buzzer sound pitch is adjustable by trimmer resistor VR2.
The power is supplied by a wall power adapter that can provide 8 - 12V DC or AC.
Diode D1 protects the system against wrong power polarity and also serves as a
rectifier for AC power input. DC battery power of the same rating is also acceptable. The
LM78L05 voltage regulator pr5V power supply to drive the electronics and
LEDs. The current consumption is very low with each LED consuming ~5mA and
~10mA for the buzzer. The total current consumption should not exceed 30mA at any
given time.
