By Mark Stansberry — Updated March 24, 2022

In electronics, an LED counter is the backbone of digital time‑keeping devices such as clocks, stopwatches, and scoreboards. This tutorial walks you through a proven BCD (binary‑coded decimal) counter circuit that drives a seven‑segment LED display. By the end, you’ll have a fully functional counter that increments on each pulse from a pulse generator.

Step 1: Gather and Mount the Components

Label the three main ICs: 74LS90 BCD counter (A), 74LS47 seven‑segment driver (B), and 7‑segment LED display (C). Place them on a breadboard and insert seven 470 Ω, ¼ W resistors between the driver (B) and the LED display (C). Name the resistors Ra through Rg for clarity.

Step 2: Wire the Counter to the Driver

Ground: Tie pins 2, 3, 6, 7, and 11 of all ICs together. Connect the pulse generator’s positive output to pin 14 of the counter (A) and its negative output to ground. Wire the counter’s output pins to the driver’s input pins as follows:

• Pin 1, 2 of A → Pin 7 of B
• Pin 9 of A → Pin 1 of B
• Pin 8 of A → Pin 2 of B
• Pin 11 of A → Pin 6 of B

Power: Supply +5 V to pin 5 of A, pin 16 of B, and the supply pin of the LED display (C). Connect the supply negative to ground.

Step 3: Connect the Driver to the Resistors

Ground pin 8 of B. Then map the driver’s output pins to the resistor leads:

• Pin 13 → left lead of Ra
• Pin 12 → left lead of Rb
• Pin 11 → left lead of Rc
• Pin 10 → left lead of Rd
• Pin 9  → left lead of Re
• Pin 15 → left lead of RfPin 14 → left lead of Rg

Step 4: Finish the LED Connections

Connect each LED anode to the +5 V rail. Wire each cathode through the corresponding resistor to the driver’s output:

• LED segment A → right lead of Ra
• LED segment B → right lead of Rb
• LED segment C → right lead of Rc
• …continue in order until segment G → right lead of Rg

Step 5: Power On and Test

Set the DC power supply to 5 V. Configure the pulse generator to output 0 V (low) and 5 V (high). Start with a low frequency (0.1 Hz) to see the display change every 10 seconds, then increase to 1 Hz for a one‑second tick. Verify that the LED counter increments correctly.

Components Checklist

• Seven 470 Ω, ¼ W resistors
• 74LS90 TTL BCD counter
• 74LS47 TTL seven‑segment driver
• 7‑segment LED display
• Electronic breadboard
• DC power supply (5 V)
• Pulse generator or digital oscillator

TL;DR

Wiring the driver outputs to the LED segments in the correct order is critical—otherwise the display will show incorrect numbers.

Safety Notice

Working with electronic circuits carries risks of fire, injury, or death. Always operate under the guidance of a certified technician or engineer and ensure you hold an up‑to‑date electronic safety certificate before beginning.