The circuit is a transistor transistor-logic (TTL) compatible running light display with a maximum of 15 outputs and is capable of driving up to 751 LEDs for display. The circuitry employs TTL devices and ICs. Power dissipation of the logic circuit is in milliwatts.
The circuit is based on digital logic having 4 inputs (hexadecimal) and 15 outputs which drive the output transistors. This logic circuit consists of six ICs — three quad 2-input OR gates and three quad 2-input AND. gates.
If A, B, C and D arc the inputs to this logic circuit and Y1, Y2, Y3 Y15 are the outputs, then the outputs can be expressed in terms of boolean equations as follows:
Y1 = A+B+C+D
Y2 = A+B+C
Y3 = (A+B)+CD
Y4 = A+B
Y5 = (A+BC) +BD
Y6 = A+BC
Y7 = (A+BC) (A+D)
Y8 = A
Y9 = A(B+C)+AD
Y10 = A(B+C)
Y11 = A(B+C) (B+D)
Y12 = AB
Y13 = AB(C+D)
Y14 = ABC
Y15 = ABCD
Here ‘A’ is the most significant bit (MSB) whereas *D’ is the least significant bit (I ,SB).
Clock pulse generator and decade counter. The clock pulse generator and decade counter are shown in the circuit diagram. The clock pulse generator is a timer chip (IC1) with a variable duty cycle, which can be achieved by varying the potentiometer VR1. The mathematical formula for frequency of oscillator is shown in the formula.
1.44 f = ————————
[R1 +2(VR 1 +R2)]C1 where, R1, VR 1 and R2 are in ohms. Cl in farads and f in Hz.
Here IC1 is operating in astable mode. IC2 has been used as a 4-bit binary counter having four outputs A. B, Cand Dwith clock input pin 3. Reset input pins 2 and 3 are grounded, while pin 12 is connected to pin 1. Outputs from IC2 are obtained from its pins 8,9. 11 and 12 respectively.
Main logic circuit. This part of the circuit has three AND and three OR gates. All ICs are TTL compatible. The source and sink current per gate is 0.4 mA and 4 mA respectively.
Driver circuit, Tills consists of a series of 15 npn transistors (one for each output). The circuit is designed in such a way that 5 LEDs can be connected in parallel of a transistor, as shown in Fig- 2.
Power supply unit (PSU). A 5 V regulator chip can be used, if current consumption is less than 200 mA. If it exceeds 200 m A then another IC LM317 can be used with proper heatsink.
All the ICs can be assembled on a single sided PCB with IC sockets. Each of the `Y` output should be connected externally through wires, to the base of a transistor in sequential order on PCB. The potentiometer can be fixed on the front panel in order to control the speed of display. Speed can be varied from 10 milliseconds to 16 seconds.
The cost of the entire circuit will be about Rs 125 which is quite reasonable.