The seven-segment display is the indicator that shows digits in clocks, calculators, and instruments. In Korea the name FND (Flexible Numeric Display) is more familiar. It shows digits and some letters with seven bar-shaped LEDs and one decimal point. This article covers the structure of a seven-segment display, how to tell common cathode from common anode, calculating the resistor for each segment, and the pinout and driving of the 10-pin FND-500/507.
Structure
A seven-segment display has six segments, a, b, c, d, e, and f, forming the outline clockwise from the top, a g in the middle, and a separate decimal point (dp). The digit 8 is all seven lit, and the digit 1 lights only b and c.
One terminal of the LED of every segment is tied together, and depending on which side is tied, there are two types.
| Type | Tied side | How to light it | Typical part |
|---|---|---|---|
| Common cathode (CC) | Cathode (−) is common | Common pin to GND, segment pins High | FND-500 |
| Common anode (CA) | Anode (+) is common | Common pin to +V, segment pins pulled Low | FND-507 |
If you mix them up, it lights inverted or not at all, so check the part name and type first.
Pinout of the 10-pin FND
The widely used 0.56-inch 10-pin FND (FND-500, FND-507, and so on) has 5 pins on the top and 5 on the bottom, and two common pins (COM). The two COM pins are connected inside, so connecting just one is enough.
| Pin number | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
|---|---|---|---|---|---|---|---|---|---|---|
| Role | e | d | COM | c | dp | b | a | COM | f | g |
With the digit face toward you and the decimal point at the lower right, the bottom row is pins 1 to 5 from the left, and the top row is pins 6 to 10 from the right. The gap between the two rows is 6 holes (15.24 mm) and the pin spacing within a row is 2.54 mm, so it plugs into perfboard in a 5-hole × 2-row area. The FND-500 and FND-507 in Patterndo use the same layout.
Telling common cathode from common anode
When the part name has worn off or you are not sure, check with the multimeter's diode measurement mode.
- Set the multimeter to diode (▶|) mode. The red terminal is + and the black terminal is −.
- Touch the red probe to a COM pin (pin 3 or 8) and the black probe to the other pins one by one. If a segment lights faintly, it is common anode.
- Conversely, if it lights when the black probe is on COM and the red probe is on the other pins, it is common cathode.
Blue and white LEDs may not light because the multimeter's voltage is low. In that case, check directly with a 5 V supply and a resistor.
Segment resistors
Each segment is an LED, so it needs a current-limiting resistor. The calculation is the same as in LED Current-Limiting Resistors. With 5 V, Vf about 2 V, and about 9 mA per segment, (5 − 2) ÷ 0.009 ≈ 330Ω. You can use the LED resistor calculator for the calculation.
- Put one resistor per segment. If you put a single resistor on the COM side, brightness varies with the number of segments lit (1 is bright and 8 is dim).
- The digit 8 lights seven segments, so the total current is
7 × 9mA ≈ 63mA. Also check the allowed current of the driver IC.
How to drive it
Using a decoder IC
A dedicated IC that converts BCD (a 4-bit binary number) to seven-segment signals greatly reduces the wiring.
- 74LS47: for common anode. The output is open collector (pulls Low), so connect a segment resistor to each FND pin and connect COM to +5 V.
- CD4511: for common cathode. The output is driven High and connects to a common-cathode FND.
- CD4543: a decoder with a latch and a phase input that can be used with both common cathode and common anode.
The built-in IC list in Patterndo includes the 74LS47, CD4511, and CD4543, so you can check the same circuits by simulation. The 74LS47 input pins are A (pin 7), B (pin 1), C (pin 2), and D (pin 6), and LT (pin 3), BI/RBO (pin 4), and RBI (pin 5) are normally tied to +5 V. Real connection examples are in A 0–9 Digit Counter with a 74LS90, 74LS47, and Seven-Segment Display and A Seven-Segment Display Selected with a DIP Switch.
Driving it directly from a microcontroller
If you drive the seven segments (+dp) with individual I/O pins, no decoder is needed. As the number of digits grows you run out of pins, so for multiple digits use multiplexing. Turn on each digit's COM in turn quickly (hundreds of Hz or more) and output only that digit's segment pattern, and to the eye all the digits look lit at once. In that case, the COM needs a circuit that carries a large current with a transistor (Transistor Switch Circuits).
Using it on perfboard
- The orientation with the decimal point at the lower right is normal. If you plug it in backwards, the digits look upside down.
- If you line up the seven resistors in a row near the FND and connect them straight to each segment pin, the wiring is shorter.
- There is a lot of wiring between the FND and the decoder, so write the pin numbers in a table and check them off one by one.
Summary
For a seven-segment display, first tell common cathode from common anode, put a resistor on every segment, and use a decoder IC to cut down the wiring. Remember the pin order of the 10-pin FND (1 = e, 2 = d, 3 = COM, 4 = c, 5 = dp, 6 = b, 7 = a, 8 = COM, 9 = f, 10 = g). Next, you can get used to digital ICs with 74LS and 74HC Logic Gate Basics, or build a project example right away.