The circuit can be used as a clock. In this circuit the counter will advance after one minute or after every one second depending on the value of resistor selected. It can be used while cooking or while doing exercise or by students preparing for competitive exams.
This circuit is based on NE555 and CD4026, which is a Johnson counter IC commonly used in digital display. It has a 5 stage Johnson decade counter with decoder which converts the Johnson code to a 7 segment decoded output. It means input will be converted into numeric display which can be seen on 7 segment display. It can be used in various applications like in 7 segments decimal display circuit, in clocks, timer etc. Advantage of 4026 counter-
● It contains counter and 7 segment decoding in one package.
● It can be easily interfaced with 7 segment types.
● Ideal for low power display.
● Operated at wide range of temperature from 5V to 20V.
● The big advantage of the 4026B counter IC is that it can drive a 7-segment display without needing a decoder driver IC.
In this circuit NE555 astable generates a clock for the circuit, which provides an oscillating wave to the output pin 3 of IC1. The time period of counter can be calculated by following formula-
T = 0.7 (R1 + 2*R2)*C1
For counter to advance after every minute use following value-
R1 = 560K
R2 = 150K
C1 = 100uF
For counter to advance after every second use following value-
R1 = 100K
R2 = 15K
C1 = 10uF
To understand its working on how we can interface 4026 with 7 segment display we have to understand it’s pin diagram.
Fig. 1: Pin Diagram Of 4026 IC
Now let us understand the working of individual pins-
1. Pin 1 or clock pin- It receives clock signals. At every positive clock counter advances one by one. You can provide clock with the switch, 555 timer or with the help of logic gates. In short high pulse on this input increments the counter.
2. Pin 2 or disable clock (clk inhibit) pin- 4026 counter advances one by one by receiving positive pulse. At this time for this clock inhibit pin should be grounded. If it is connected to supply than counter advancement will be inhibited means there will be no meaning of clock pulse.
3. Pin 3 or enable display (En in) pin- It enable the 7 segment display to display the numeric value. It should be kept high for enabling the display. It means output goes high when only when display enable is high.
4. Pin 4 or enable out- It Enables the carry out pin. In our circuit we have left this pin unconnected.
5. Pin 5 or divide by 10 output – It is used to complete one cycle for every 10 clock input cycle and it also used to cascade more IC’s.
6. Pin 6, pin7 and Pin9 to pin 13 – These are 7 decoded output from A to G used to illuminates the corresponding segment of 7 segment display to display the digit from 0 to 9.
7. Pin 14 or not 2 output (UNGATED “C” SEGMENT) signals- They are not gated by the Display clock and therefore are available continuously. This feature is a requirement in implementation of certain divider function such a as divide by 60 and divide by 12.
8. Pin 15 or Reset pin- It is used to reset the counter. When it receives high it clears the counter and counting again starts from zero. One important thing reset pin should again made low to start the counter once again.
9. Pin 8 or ground pin and Pin 16 known or Vdd. It should be connected to power supply.