How to build an LED electronic counter

Written by jonathan pfeiffer
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How to build an LED electronic counter
An LED counter is used to display numbers on a LED sign. (Jupiterimages/ Images)

An LED (light emitting diode) electric counter allows a person to create circuits such as stop watches and digital clocks. As a basic arrangement, a binary coded decimal (BCD) counter drives a seven-segment display driver, this in-turn attaches to a seven-segment LED. The counter value goes up every time a voltage pulse is applied to the input of the counter. The digital voltage levels on the display driver outputs will be changed, allowing the different segments of the seven-segment LED to show numbers 0 through 9.

Skill level:

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Things you need

  • Seven 470 Ohm, 1/4 Watt resistors
  • One 74LS90 TTL BCD Counter
  • One 74LS47 TTL Seven Segment Display Driver
  • One 7 Segment LED Display
  • Electronic Breadboard
  • DC Power Supply
  • Pulse Generator or Digital Oscillator

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  1. 1

    Assign the label part "A" for the 7490 BCD counter, part "B" for the 7447, and part "C" for the seven-segment LED display. Put parts A, B, and C in the electronic breadboard. Inside the electronic breadboard, place all seven 470-Ohm resistors between parts B and C. Assign each resistor a label using the names Ra, Rb, Rc, Rd, Re, Rf and Rg.

  2. 2

    Attach pins 2, 3, 6, 7, and 11 to one another and use the term "ground" to label this point. Affix pin 14 of part A to the positive terminal of the pulse generator. To ground, connect the pulse generator's negative terminal. Connect pin 7 from part B to pins 1 and 2 from part A. Attach pin 1 from part B to pin 9 from part A. Attach pin 2 from part B to pin 8 from part A. Connect pin 6 from part B to pin 11 from part A. Affix the supply voltage pin from part C, pin 16 from part B and pin 5 from part A to the pulse generator's positive terminal. To ground, attach the power supply's negative terminal.

  3. 3

    Attach the left lead of Ra to pin 13 from part B. Connect the left lead of Rb to pin 12 from part B. Affix the left lead of Rc to pin 11 from part B. Attach the left lead of Rd to pin 10 from part B. Affix the left lead of Re to pin 9 from part B. Attach the left lead of Rf to pin 15 from part B. Attach the left lead of Rg to pin 14 from part B. To ground, attach pin 8 from part B.

  4. 4

    Attach the power supply's positive lead to the anodes from each of the seven-segment LED. Attach the right lead of Ra to the cathode from the first segment of the seven-segment LED driver. Attach the right lead of Rb to the cathode from the second segment. Attach the right lead of Rc to the cathode from the third segment. Carry on in this manner until Rg is attached to the seventh segment of the LED driver.

  5. 5

    Activate the power supply and position it to 5 Volts. Activate the pulse generator and position the low output voltage to 0 Volts and the high output voltage to 5 Volts. Set the LED to shift the numeric value every ten seconds by adjusting the pulse generators frequency to 0.1 cycles per second. Reset the frequency to 1 cycle per second to allow the LED display to change numeric value every second.

Tips and warnings

  • It's important to wire the outputs of the display driver to the LED segment display in correct order. If incorrect, the display will function, but you won't display numbers correctly.
  • Improper use of electronic equipment and components can result in fire, serious injury or death. Always work under the supervision of a safety certified electronic technician or electronic engineer.

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