Name– study of decoder using IC 7447/7448.
Aim – to study the working decoder for conversion of BCD into 7–segment display code.
Apparatus – IC 7447/7448, 7–segment common anode LED display, circuit board, power supply +5V DC, connecting wires, soldering iron, cutter etc.
Circuit diagram
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Procedure –
1) You are given IC 7447/7448 and the 7–segment common anode LED display. Identify them with their proper pin configurations.
2) Switch on the power supply of the circuit.
3) Connect different BCD inputs from 0000 to 1001 and note down the corresponding output on the display.
4) Tabulate the readings
Brief theory – to display the data, we have to convert it from BCD to 7-segment code. The IC makes this process. It has four inputs called BCD inputs and seven outputs to drive the display.
For common anode LED display, the decoders used are IC 7446, IC 74246, IC 7447, IC 74247 etc. They have active low, open collector outputs.
And for common cathode LED display, the ICs are IC 7448, IC 74248, IC 7449 etc. They have active high, open collector outputs.
Working of the circuit –
1) When BCD inputs from 0000 to 1001 are applied at the input terminals, the IC produces equivalent outputs at its output terminals from a–g. The 7–seg display is common anode type.
2) Suppose BCD input is DCBA = 0101. It is equivalent to decimal 5. This input is converted into equivalent 7–seg code by the IC and its outputs abcdefg = 0100100. For the input DCBA = 1000, all segments of the display will glow to show decimal 8 and outputs abcdefg = 0000000 and so on.
Conclusion –
1) When corresponding BCD inputs are applied from 0000 to 1001, we get decimal output on the display from decimal 0 to decimal 9.
2) The tails of ‘6’ and ‘9’ are not displayed on the display.
3) To display the tails of ‘6’ and ‘9’ IC 74247 is required.
4) In common anode FND 507 type display, the anodes of all LEDs are connected to a common line and connected to +5V regulated power supply. The cathode of each LED segment is used as input from ‘a’ to ‘g’.