Building the mechanism by which numbers climb on a clock, counter, or scoreboard yourself lets you understand digital circuits at a glance. This article explains, pin by pin, a circuit where a 74LS90 decade counter counts the 1 Hz clock made by a 555 timer, a 74LS47 decoder converts the number to seven-segment signals, and a common-anode FND repeatedly shows 0 to 9. Patterndo's built-in ICs include the NE555, 74LS90, 74LS47, and the FND-507, so you can put the same arrangement in the editor and simulate it.
Block layout
| Stage | Part | What it does |
|---|---|---|
| Clock | NE555 | Makes one pulse about every second |
| Counter | 74LS90 | Counts pulses and outputs 0 to 9 as a 4-bit binary number (BCD) |
| Decoder | 74LS47 | Converts the 4-bit number to seven-segment lit signals |
| Display | FND-507 | A common-anode seven-segment display |
Parts list
| Part | Qty | Value |
|---|---|---|
| NE555 | 1 | 8-pin socket |
| 74LS90 | 1 | 14-pin socket |
| 74LS47 | 1 | 16-pin socket |
| FND-507 | 1 | 10-pin, common anode |
| Resistors R1 / R2 | 1 each | 10 kΩ / 68 kΩ |
| Capacitor C | 1 | 10 µF electrolytic |
| Segment resistors | 7 | 330 Ω |
| Decoupling capacitors | 3 | 100 nF |
| 5 V supply | 1 | The 74LS needs 4.75–5.25 V only |
The 74LS family needs a 5 V ±5% supply. Do not connect a 9 V battery directly; use the 7805 power board or the USB 5V power board.
Stage 1: The 555 clock (about 1 Hz)
It is the same as the 555 astable circuit. With R1 = 10 kΩ, R2 = 68 kΩ, and C = 10 µF, the frequency is 1.44 ÷ ((10k + 2×68k) × 10µ) ≈ 0.99Hz.
| Pin | Connection |
|---|---|
| 1 | GND |
| 2, 6 | Tied together, to the + side of C and one side of R2 |
| 3 (OUT) | Pin 14 (CP0) of the 74LS90 |
| 4, 8 | +5V |
| 5 | GND through 10 nF (can be omitted) |
| 7 | The node between R1 and R2 |
R1 goes between +5V and pin 7, R2 between pins 7 and 6, and C between pin 6 and GND.
Stage 2: The 74LS90 counter
The 74LS90 is 14 pins, but its power pins are pin 5 (VCC) and pin 10 (GND). You must not connect +5V to pin 14 (Reading DIP IC Pin Numbers).
| Pin | Name | Connection |
|---|---|---|
| 5 | VCC | +5V |
| 10 | GND | GND |
| 14 | CP0 | Pin 3 of the 555 (clock input) |
| 1 | CP1 | Connected to pin 12 (Q0) |
| 2, 3 | R0(1), R0(2) | GND |
| 6, 7 | R9(1), R9(2) | GND |
| 12 | Q0 | Pin 7 (A) of the 74LS47 |
| 9 | Q1 | Pin 1 (B) of the 74LS47 |
| 8 | Q2 | Pin 2 (C) of the 74LS47 |
| 11 | Q3 | Pin 6 (D) of the 74LS47 |
Connecting CP1 to Q0 links the internal binary and divide-by-five counters into a BCD counter that counts 0 to 9. Tying both the R0 and R9 inputs to GND leaves the normal counting state, with reset and set-to-9 turned off. The clock input increments on the falling edge (High→Low).
Stage 3: The 74LS47 decoder
| Pin | Name | Connection |
|---|---|---|
| 16 | VCC | +5V |
| 8 | GND | GND |
| 7, 1, 2, 6 | A, B, C, D | Q0, Q1, Q2, Q3 of the 74LS90 |
| 3 | LT | +5V (lamp test off) |
| 4 | BI/RBO | +5V (blanking input off) |
| 5 | RBI | +5V |
| 13, 12, 11, 10, 9, 15, 14 | a, b, c, d, e, f, g | To the FND through 330 Ω resistors |
The 74LS47 outputs are open collector that pull Low, which suits a common-anode FND. The segments whose outputs go Low light up.
Stage 4: Connecting the FND-507
The FND-507 is common anode, so connect its COM pins (3 and 8) to +5V (Driving Seven-Segment Displays).
| 74LS47 output | Segment | FND pin |
|---|---|---|
| 13 (a) | a | 7 |
| 12 (b) | b | 6 |
| 11 (c) | c | 4 |
| 10 (d) | d | 2 |
| 9 (e) | e | 1 |
| 15 (f) | f | 9 |
| 14 (g) | g | 10 |
Put one 330 Ω in each connection. Leave the decimal point (pin 5) unconnected. The segment current is about (5 − 2) ÷ 330 ≈ 9mA.
Perfboard layout tips
- Place them from left to right as 555 → 74LS90 → 74LS47 → FND, so the signal flows in one direction.
- Solder the three sockets and the FND first, then wire the power rail and ground.
- Put the seven resistors between the 74LS47 and the FND in a row right next to the FND.
- Put a 100 nF next to the power pins of each IC (Power Rails, Grounding, and Decoupling Capacitor Placement).
- There are many wires, so print the tables above as a checklist and solder while crossing off one line at a time.
Checking operation and troubleshooting
- Before applying power, do a continuity test between +5V and GND.
- Apply power and check that the voltage on pin 3 of the 555 changes about every second (attaching an LED makes it easy to see).
- If the FND digit climbs from 0 to 9 in order and returns to 0 after 9, it works.
| Symptom | Cause |
|---|---|
| Nothing displays | FND COM not on +5V, a 74LS47 power problem |
| Digit shapes look wrong | Segment pin order mix-up (check the table), or a common-cathode FND was used |
| Digits skip or are irregular | Jitter on the clock, the 74LS90's R0/R9 pins left floating |
| Counting stops | Check the 555 wiring (pins 2 and 6 tied, pin 4 to +V) |
| Misbehaves even at 5 V | Not enough supply (needs about 100 mA or more), bad ground contact |
Extension: make it two digits
Connect Q3 (pin 11) of the first 74LS90 to CP0 (pin 14) of a second 74LS90, and on the 10th pulse Q3 falls from 1 to 0 and the second counter goes up by 1, becoming the tens digit. Attach a second 74LS47 and FND the same way and it counts 00 to 99.
Summary
The 555 makes a one-second clock, the 74LS90 counts 0 to 9, the 74LS47 converts that to seven-segment signals, and it connects to a common-anode FND through 330 Ω. Pay special attention to the 74LS90's power being on pins 5 and 10 and to putting the FND's COM on +5V. A circuit that sets the number with switches instead of a clock can be built in A Seven-Segment Display Selected with a DIP Switch.