4026 IC Counter Circuit

The 4026 is a decade counter integrated circuit (IC) with decoded outputs for driving a common-cathode seven-segment LED display. An advantage of this IC is that it has decade counter functionality together with 7-segment decoder driver. The CD4026BE, manufactured by the Texas Instruments Corporation, is the IC currently used for simple GCSE type project circuits. It comes in a 16-pin plastic dual-in-line package for use with strip boards and breadboards. This IC is based on CMOS technology and will operate from as little as 3 V (minimum), to 18 V maximum, and the absolute maximum rating is 20 V. It can therefore interface with CMOS (3.3 V) and TTL (5 V) systems directly.

Pin Function

Since 7-segment LED displays consume much power, this chip has special function Display Enable In (DEI) through pin 3, which is has to be HIGH when a display is required. This is useful if you had many display panels, because you could switch them ON only when required thereby saving power. For a simple GCSE type project with one 7-segment panel, it is best practice to tie this pin HIGH to the positive voltage rail.

The "Clock Inhibit" function at pin 2 is used when you want to stop the count by blocking the clock signal at pin 1. Since in this application we will not be stopping the counter this way, it is best to tie it to the ground to keep it LOW. If you tied it HIGH, then it would inhibit the clock signal at pin 1.

The "Carry Out" pin 5 produces one pulse at the count of ten, and this is usually used as a clock pulse for the next counter stage. Since we have only one stage in this application, we leave the pin unconnected.

The Display Enable Output (DEO) pin 4, and Un-gated C-Segment (UCS) pin 14, are used in special divider circuits, and therefore not connected in this simple application.

Circuit Diagram

Your friend, "The Doc", wants to build a special LED display for his time machine, which displays the current time and date, and the time and date of the location, where he plans to visit. He wants to use a 4026 IC counter chip to control each of the seven-segment panels. He wants one push to make (PTM) button, which advances the count by 1, and also another PTM button to reset the count.

In this circuit, we use 1 kΩ resistors, also known as pull-down resistors, to keep the logic states of pin 1 and pin 15 low. Without these resistors, the electrical state of these pins would remain floating making the circuit unstable. We also use PTM switches, which provide a positive pulse for a short moment. A short positive pulse to the clock pin would make the counter increment by 1 and the 7-segment display would then display the numeral 1. Further presses of PTM S1 increments the clock and the display shows the count. However, pressing the switch PTM S2, causes a positive pulse to go into pin 15 initiating a reset of the count.

7-Segment Display Series Resistor Value

A series resistor is usually used to produce the required voltage drop when driving an LED. Since each segment of the 7-segment display is an LED, we connect resistors at pins 10, 12, 13, 9, 11, 6, and 7 to produce the necessary voltage drop. Whether you decide to use these series resistors to produce a voltage drop depends upon two main criteria; the voltage you are driving the IC at, which is the voltage you feed at pin 16, and the forward drop of the LEDs in your 7-sement display. Since the IC is capable of being used on a wide range of voltages, and currently in the market there are a wide range of 7-segment displays with differing colours and forward voltages, it is good practice to show series resistors in the circuit diagram.

Since the output voltage that drives the LEDs is close to the supply voltage of the IC (assuming room temperature typical values), operating the IC on the lower voltage range may allow one to eliminate the use of series resistors, however it is not a practice I would recommend. If you are using the cheap and commonly available red coloured display, then they usually have a voltage drop of 1.8 V. Here is a table showing resistor values for each power supply option for such a display.

If you are a GCSE student, then ideally you should consult the LED article to calculate the value of the series drop resistor. Once you have calculated a value, consult the E24 article to choose the nearest available resistor value in the E24 series.

Cost/eBay

The CD4026BE is usually for sale on eBay for approximately ten chips for a pound, however be careful and very cautious when buying from places such as China as there are many fake versions that do not work.

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