BCD Sayıcı Devresi / BCD Counter Circuit

A BCD counter is a special type of digital counter that can count up to 10 in a clock signal application.

We have seen before that transitional T-type flip-flops can be used as separate splitting counters. If we connect several transitive flip-flops in a series of chains, we can produce a digital counter that stores or displays the number of occurrions of a particular counting sequence.

Clocked T-type flip-flops act as binary split counters, and in asynchronous counters, the output of a counting phase provides clock pulse for the next stage. Then a flip-flop counter has two possible output states, and we can add more flip-flop stages to make a2 nsplit counter. However, the problem with 4-bit binary counters is that they count from 0000 to 1111. This is de de desed from 0 to 15.

To make a digital counter that counts from 1 to 10, we only need the numberer to count binary numbers from 0000 to 1001. That is, it must be counted as de desed from 0 to 9. Fortunately, some counting circuits are available as single integrated circuits. For example: Asynchronous 74LS90 is a tener.

Digital counters count upwards from zero to a predetermined counting value in the implementation of a time signal. When the counting value is reached, the counter resets and returns the counter to zero to start over.

The tenner counts in a series of ten, and after counting to nine, it returns to zero. That is, to count to a binary value of nine, the counter must have at least four flip-flops in its chain to represent each decimal place, as shown.

bcd counter
BCD Counter Status Schema

The tenner has four flip-flops and 16 potential situations, if we use only 10 of them and connect a series of counters together, we can count to 100 or 1,000, or to the last number we choose.

The total number that a counter can also count is called MOD. The counter that returns to zero after the number n is called the mod-n counter, for example, the modulo-8 (MOD-8) or modulo-16 (MOD-16) counter, and so on. The full range of the count is from 0 to 2n-1.

However, as we have seen in the training of asynchronous counters, a number that is reset after ten counts with a dividing count sequence of 10 from binary 0000 (decimal "0") to 1001 (decimal "9") is called "binary". The "binary encoded decimal counter", or BCD Counter for short, or MOD-10 counter, can be created using a minimum of four-pass flip-flop.

The name of the BCD counter comes from: ten state sequences are an array of BCD code and do not have a regular pattern, unlike a flat binary counter. Then, a single-stage BCD counter, such as 74LS90, counts decimal 0 to decimal 9 and therefore can count to a maximum of nine pulses. Also note that depending on an input control signal, a digital counter can count up or down, or count up and down (duplex).

Binary decimal code is an 8421 code consisting of four binary digits. The 8421 representation refers to the binary weight of the four digits or bits used. For example, 23 = 8,2 2 = 4, 21 = 2 and 20 = 1. The main advantage of BCD code is that it allows easy conversion between dex and binary number formats.

74LS90 BCD Counter

The 74LS90 integrated circuit is basically a MOD-10 counter that generates an BCD output code. The 74LS90 consists of four master-slave JK flip-flops connected internally to provide a MOD-2 (counting up to 2) and a MOD-5 (count to 5) counter. In 74LS90, there is an independent-pass JK flip-flop driven by clk A input and a three-pass JK flip-flop that creates an asynchronous counter run by clk B input as shown.

bcd counter

The four outputs of the counters are represented by a numeric subscript equal to the binary weight of the corresponding bit in the BCD counter circuits code and the letter Q symbol. For example, QA, QB, QC, and QD. The 74LS90 counting array is triggered at the negative outbound edge of the clock signal, that is, when the CLK clock signal switches from logic 1 (HIGH) to logic 0 (LOW).

Additional input pins are "reset" pins of the R1 and R2 counters, while inputs S1 and S2 are "set" pins. When logic is connected to 1, the R1 and R2 Reset entries reset the counter to zero, 0 (0000), and when set entries S1 and S2 are connected to logic 1, they set the counter to the maximum or to 9 (1001). Regardless of the number or location of the current count.

As we have said before, the 74LS90 counter consists of a dividing counter of 2 and a dividing counter of 5 in the same package. Then we can use the two together to produce only the frequency counter divided into 2, or to produce only the frequency counter divided by 5 or our desired BCD counter of 10.

When the four flip-flops that make up the 5-by-5 split counter section are disabled, if a clock signal is applied to input pin 14 (CLKA),the output from pin 12 (QA)is a standard splitting signal.

74LS90 2-By-2 Dividing Counter

bcd counter

To produce a standard 5-by-5 split counter, we can disable the first flip-flop above and apply the clock input signal directly to pin 1 (CLKB)by taking the output signal from pin 11 (QD),as shown.

74LS90 5-By-5 Dividing Counter

bcd counter

Note that with this 5-by-5 split counter configuration, the output waveform is not symmetrical, but has a line-to-space ratio of 4:1. This means that the four-input clock signal generates a LOW or logical "0" output, and the fifth input time signal produces a HIGH or logical "1" output.

To make a 10-by-10 BCD decimal counter, both internal counter circuits are used with a division value of 2 times 5. Since the first Q A output from flip-flop"A" is not internally connected to the next stages, this QA output can be extended to form a 4-bit BCD counter by connecting to the CLKB input as shown.

74LS90 10-by-10 Split Counter

bcd counter

We know that BCD counters are binary counters that count from 0000 to 1001 and are reset because they have the ability to clean all flip-flops after the ninth count. If we connect a button switch (SW1)to the CLKA clock entry, the counter will count one by one each time the button switch is released. If we connect light-emitting diodes (LEDs) to the output terminals QA,QB,QC and QD as shown, we can see the binary dexeration as it occurs.

74LS90 BCD Tennial Count

bcd counter

The more I press the button, the more the count will increase to nine, 1001. In the tenth application, ABCD outputs will be reset to zero to start a new counting sequence. With such a MOD-10 pulse number, we can use the tenth counter to drive a digital display.

If we want to display the counting order using a seven-segment display, the BCD output must be decoded accordingly before it can be displayed. Digital circuits that can decode the four outputs of our 74LS90 BCD counter and illuminate the necessary parts of the screen are called decoders.

7-Segment Display Usage

Fortunately for us, someone has already designed and developed a BCD to 7-segment Screen Decoder integration such as 74LS47 to do this. The 74LS47 has four inputs for BCD figures A, B, C and D, and one output for each segment of the seven-segment display.

Keep in mind that a standard 7-segment LED display usually has one input connection for each LED segment and eight input connections that serve as a common terminal or connection for all internal display segments. Some screens also have a de de-delusk (DP) option.

bcd counter

The 74LS47 display decoder receives the BCD code and generates the necessary signals to activate the appropriate LED segments responsible for displaying the number of pulses applied. Since the 74LS47 decoder is designed to drive a common anodized display, a LOW (logic-0) output illuminates an LED segment, while a HIGH (logic-1) output will put it in a "OFF" position. For normal operation, LT (Lamp test), BI/RBO (Blinding Input/Surge Dimming Output) and RBI (Surge Dimming Input) must be open or connected to logistics-1 (HIGH).

Note that although the 74LS47 has active LOW outputs and is designed to decode a common anodized 7-segment LED display, the 74LS48 decoder/drive integration is exactly the same except that it has active HIGH outputs designed to decode a common cathode 7-segment display. Therefore, depending on the type of 7-segment LED display you have, you may need to integrate a 74LS47 or 74LS48 decoder.

74LS47: Anod Displays , 74LS48 For Cathode Displays.

Decimal entries with binary code 74LS47 can be connected to the corresponding outputs of the 74LS90 BCD Counter to display the counting order on the 7-segment screen, as shown each time the SW1 button is pressed. Counting can be done by changing the position of the push button and 1kΩ resistance, and making changes to the activation or release of the push button switch SW1.

bcd counter

Keep in mind that a 7-segment display is made of diode that emits seven separate lights to create the screen. The best way to limit the current through a seven-segment display is to use a current limiting resistance in series with each of the seven LEDs, as shown. But we can do this two ways.

Current Limiting Resistors

With Single Resistance

bcd counter

Here a single series of current limiting resistance, R is used. This method is the easiest and simplest option to control the 7-segment screen, especially if a fixed screen brightness is not important to you.

The amount of light emitted by an LED varies according to the current passing through the device, with the current flowing from the resistance shared between the number of screen segments. Then the brightness of the screen now depends on how many segments are illuminated at the same time. In short, the more segments active, the more currents are attracted and the brighter they become.

With Multiple Resistance

bcd counter

Here each segment has its own current limiting resistance, as shown in our simple BCD meter circuit above. Usually 7-segment displays require about 12 to 20 milli-amps to illuminate segments, so the resistance value of the current limiting resistance (all of which will be the same) is selected to limit the current within these values.

The advantage here is that the brightness of a particular LED segment does not depend on the condition of the other six LEDs, which give the screen a constant brightness. Since the amount of ambient light will also determine the required LED density, the values of the current limiting resistors can be selected to provide the right amount of brightness.

Our circuit shows a simple 0 to 9 digital counter that uses the 74LS90 BCD Counter and the 74LS47 7-segment display driver. To count above 10 and produce a 2-digit ten-based counter and display, we need to cascade two separate ten counters together. A 2-digit BCD counter is counted as a decimal point from 00 to 99 (0000 0000 to 1001 1001), and then resets back to 00. Although it is a 2-digit counter, the values representing hexadecimal numbers from A to F do not apply in this code.

Likewise, if we want to count from 0 to 999 (0000 0000 0000 – 1001 1001 1001), then a three-digit decimal counter is required. In fact, multi-de tenth counters can be simply created by combining individual BCD counter circuits, one for each ten, as shown.

BcD Counter Counting from 00 to 99
BcD Counter Counting from 00 to 99


In this tutorial, we found that a BCD Counter is a device that passes through ten state sequences when the clock is set and returns to 0 after counting to 9. In our simple example above, the input time pulses are from a push button. however, counters can be used to count many real-world events, such as counting moving objects.

However, since these events can occur at discrete time intervals or may be completely random, the appropriate circuit may be required to produce electrical pulses for each event to be counted.

In many digital electronic circuits and applications, digital counters are implemented using Toggle flip-flops or any other type of flip-flop that can be connected to give the necessary switching function, or with the use of custom counting integrateds such as the 74LS90. Binary counters are counters that pass through a binary array, and an n-bit binary counter consists of a "n" number of flip-flops that count from 0 to 2n-1.

BCD counters follow a ten-state array and count using BCD numbers from 0000 to 1001, and then return and repeat to 0000. Such a counter must have at least four flip-flops to represent each decimal point, because a decimal place is represented by a binary code that contains at least four bits that give a MOD-10 count.

We also found that BCD-coded output can be viewed with four LEDs or digital indicators. However, to display each number from 0 to 9, a decoder circuit is required that converts a binary-encoded number representation to the appropriate logic levels in each screen segment.

Display decoder circuits can be created from combination logic elements, and there are many special integrated circuits on the market, from 74LS47 BCD to 7-segment decoder/drive integration to perform this function.

Most 7-segment displays are typically used in multi-digit counting applications, so by combining more BCD counters, 4-digit counters can be created that give maximum 9999 read screens.

The 74LS90 BCD Counter is a very flexible counting circuit and can be used as a frequency divider or can be made to divide any integer count from 2 to 9 by feeding the appropriate outputs back to the reset and set inputs of the integrated.