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4026 Decade Counter & 7-Segment Display Driver

Introduction

4026 Pinout

The 4026 Decade counter is a counter and display driver all in one chip. The counter is a decade counter which means that it counts from 0 to 9 and then starts again. Internal logic converts the count to the seven outputs (a - g) necessary to drive a 7-segment display. The outputs can provide a few mA of current, enough to directly drive a small 7-segment LED display.


Basic Operation

7 Segment Dsiplay

The are 7 outputs labelled a to g which are used to drive a standard common cathode 7-segment display. The outputs go HIGH to illuminate the various segments of the LED display.

The CLOCK input (Ck) makes the output change each time the clock input goes from LOW to HIGH. The counter output changes on the rising edge of the clock input.

The RESET (RST) returns the counter to zero and, as a consequence, the display shows zero. The RESET is usually LOW. The counter is reset to zero when RESET goes HIGH. Whilst RESET is HIGH, the counter does not count clock pulses.

CLOCK ENABLE (CE) is usually held LOW. When CE is LOW the counter responds to the clock pulses and the count advances on each rising edge of the clock. When CE is HIGH, the counter does not respond to the clock pulses and the display does not change. Whilst CE is HIGH, clock pulses are ignored.

DISPLAY ENABLE (DE) is usually held HIGH. When DE is HIGH the outputs a to g source current to the display and the 7-segment display is illuminated. When DE is LOW all outputs a to g are LOW and the 7-segment display is blank, no segments are illuminated. DE does not affect the counter and, even when no segments of the display are illuminated, the counter continues to count the clock pulses. A square wave with a variable mark-space ratio can be connected to DE to vary the display brightness.

The divide by 10 output (÷10 OUT) is a square wave that is HIGH for counts 0 to 4 and then LOW for counts 5 to 9. The frequency of the ÷10 OUT is the frequency of the clock pulses divided by 10. Counters can be cascaded to count 10s, 100s, 1000s etc by using the ÷10 OUT as the clock for the following counter.


Simple Circuit

4026 circuit

Pin 2 (CE) is connected to 0V so that it is LOW and the counter will count the pulses from the astable at Pin 1 (Ck).

Pin 3 (DE) is connected to +Ve so that it is HIGH and the display will be illuminated.

Pin 15 (RST) is held LOW by the pull down resistor until the RESET button is pressed.

Outputs a to g are connected to the 7-segment display. The common cathode of the display is connected to 0V.


Output Considerations

The 4026 IC can source around 5mA per output when used with a 10V supply (and a little less when used at lower voltages) which is enough to directly drive a standard 7-segment LED display without the use of current limiting resistors. However, when used with higher supply voltages, current limiting resistors might still be needed for each segment of the display to avoid the IC having to dissipate too much power.

Higher power 7-segment displays will need separate drivers, such as MOSFETs or bipolar transistors, for each segment. The same applies for any other type of display such as neon or liquid crystal displays.

Note: If the 4026 IC is connected without current limiting resistors, the voltage at the output pins will be around 2V (or the forward voltage drop of the LED segments) with the rest of the voltage being dropped across the internal resistance of the IC. Therefore, when the output is HIGH, the output will not register as logic 1 when connected to any form of logic circuit.

Truth Table

Count ÷10 OUT a b c d e f g
0 HIGH 1 1 1 1 1 1 0
1 HIGH 0 1 1 0 0 0 0
2 HIGH 1 1 0 1 1 0 1
3 HIGH 1 1 1 1 0 0 1
4 HIGH 0 1 1 0 0 1 1
5 LOW 1 0 1 1 0 1 1
6 LOW 1 0 1 1 1 1 1
7 LOW 1 1 1 0 0 0 0
8 LOW 1 1 1 1 1 1 1
9 LOW 1 1 0 1 1 1 1