Build a BCD-to-7-segment display circuit with a CD4511 driver IC. Pinout, parts list, breadboard steps, and an interactive digit you can drive yourself.
Digital electronics · 1 driver IC
The CD4511 takes a 4-bit binary number and lights up exactly the segments needed to show it as a decimal digit — no code, no microcontroller, just logic gates.
Toggle D0–D3 to set a BCD value · hold LT or BL to test · flip the view above to see it as a circuit
Parts bin
One driver chip does all the decoding — the rest is current-limiting and a clean way to feed it a number.
Reference
Sixteen pins, four jobs: take power, take a number, decide what to show, and hold it there if asked.
Operating principle
It's a lookup, wired directly in logic gates instead of software. Flip to the circuit view above and toggle D0–D3 to watch it happen live.
Press buttons on D0–D3 to set a 4-bit binary value.
The CD4511 latches that value and decodes it internally.
The matching output pins swing HIGH; the rest stay LOW.
Current flows through the resistors and lights the matching digit.
Inside the CD4511, each of the four input pins feeds a small memory cell called a D latch. Whatever binary value sits on D0–D3 gets held there the instant it arrives.
A block of logic gates then reads all four latched bits at once and works out, for that specific combination, which of the seven segments need to be on to draw the matching digit — this is the "BCD to 7-segment" conversion the chip is named for.
Because the mapping is fixed in hardware, there's no counting, no scanning, no code involved. Change the input and the correct segments switch on the same instant.
LT (Lamp Test) — pull it LOW and every segment lights, regardless of the input. Good for confirming none have burned out.
BL (Blank) — pull it LOW and every segment turns off, regardless of the input. Useful for PWM brightness control.
LE (Latch Enable) — pull it HIGH and the chip freezes on whatever it's currently showing, ignoring further changes at D0–D3 until it goes LOW again.
Assembly
Five steps — the same footprint works for a single digit or, later, one stage of a multi-digit counter.
Straddle the chip across the centre gap of the breadboard, notch to the left, so pins 1–8 and 9–16 land in separate rows.
VDD (16) to the positive rail, GND (8) to the negative rail. For ordinary counting, tie LT and BL straight to VDD and LE straight to GND — that keeps the chip in normal, live-decoding mode.
Each of D0–D3 gets its own pushbutton to +V and its own 10 kΩ resistor down to GND. The resistor keeps the pin reading a clean LOW until the button is actually pressed.
Run outputs a through g to the matching segment pins on the display, each through its own 1 kΩ resistor. Tie the display's common cathode pin to GND.
Press combinations of the four buttons to step through 0000–1001 and watch the digit change. Anything from 1010 upward isn't a valid decimal digit, so the display simply blanks.
If it misbehaves
Almost always one of the three control pins, or one swapped wire.