74LS145 BCD to Decimal Decoder, Pinout and Examples

The IC 74LS145 is a TTL based BCD to Decimal decoder used to drive the ten binary outputs on the base of four binary inputs. The IC consist of eight inverter and ten NAND gates which have four inputs. These total eighteen gates come up with a smart circuit that gives ten output through the NAND gates. The inputs go through four NAND gates. The four inputs have a minimum value equal to 0000 and maximum value to 1111.

The IC ensures that it gives HIGH output on invalid inputs and also it gives LOW output on a single output on specific inputs. The IC is compatible to use with any TTL, DTL or and MOS integrated circuits. The IC was designed to use as open collector logic circuits and to use as indicators/drivers for relays. It has low Power dissipation and in the IC clamp diodes control the termination on high speed.

Other equivalents of this IC are SN74LS145, SN54145, SN54LS145, SN74145.

SN74LS145 Pinout

The pin diagram of DCB to the decimal decoder is shown here. Different color patterns are used to differentiate input, output and power supply pins.

74LS145 BCD to Decminal decoder pinout diagram

74LS145 Pin Diagram Details 

PINSDETAILS
Q0Pin 1Pin 1 to Pin 7 will be used as the first 7 output pins on first 7 inputs. These pins will be in a HIGH state when there is no input or invalid input on the input pins. In case of input that will affect the output, the pin will change the state from LOW to HIGH until the input changed to a new one or invalid. Only one output pin will be LOW at specific input.
Q1Pin 2
Q2Pin 3
Q3Pin 4
Q4Pin 5
Q5Pin 6
Q6Pin 7
GNDPin 8Pin 8 will be used as a common ground. The ground of Power supply and other compatible devices ground will be common with Pin 8 to make it usable with IC.
Q7Pin 9Pin 9 to Pin 11 will also be used as an output pin. In these pins, the output will also act like the first seven pins. Only one pin (from first seven and these three pins) will be affected by inputs at a time.
Q8Pin 10
Q9Pin 11
P0Pin 12Pin 12 will be used as the first input pin for the IC 74LS145.
P1Pin 13Pin 13 will be used as a second input pin for the IC 74LS145.
P2Pin 14Pin 14 will be used as the third input pin for the IC 74LS145.
P3Pin 15Pin 15 will be used as the fourth input pin for the IC 74LS145.
VCCPin 16Vcc pin will be used as the power pin to make the IC functional.

74LS145 Decoder FEATURES

  • It can be used for full decoding of all input logic.
  • The IC comes up in multiple packages, DIL and SOIC.
  • It has a low power consumption of about 35mW.
  • All the outputs of the IC will be off for all invalid inputs.
  • The IC can be used easily with TTL, DTL, MOS, and any other IC or microcontroller.
  • The inputs consist of clamp diodes to control the speed.
  • At a single time, the only one output will be LOW others will be HIGH.
  • The outputs will mutually exclusive.
  • It has a capacity of 1 of 8.
  • The outputs will be invalid when input will be above nine.

BCD to Decimal Decoder Specifications 

  • The voltage range for 74LS145 is 4.5 to 5.25 Volts. The voltage for power supply should not be more than 5.25.
  • The temperature range for IC is 0 to 70 degrees.
  • The HIGH input state voltage will be 2 and the maximum input voltage for the LOW state will be 0.8.
  • The high to low and low to high propagation time is 50ns for IC 74LS145.
  • The IC 74LS145 can sink up to 80mA current.
  • The input current for the HIGH state is maxed 0.1mA and the LOW state is -0.4mA.

More information on specifications and features available in the datasheet of 74LS145. Download datasheet.

74LS145 datasheet

How SN74LS145 Decoder works?

In IC 74LS145 the circuit consists of 8 NOT gates and 10 NAND gates. The two NOT gates are connected with each input and a single NAND gate is connected with each output. Every NAND gate receives four inputs and every output gets inverted from the NAND gate. There will be a total of 11 conditions on which the circuit will operate and we need to understand how each and every input will give the specific output.

75LS145 internal circuit

Internal Circuit 74LS145

In IC 74LS145 we have a total of 16 inputs but only from 0000 to 1001 will be able to make the output LOW other will have no effect on the OUTPUT, which makes the state from 0101 to 1111 invalid states. The IC will always follow the following truth table. Every input condition will make the output LOW and those who will not be able to change the output state will be known as invalid states. The invalid state won’t be able to change even a single NAND gate from HIGH to LOW due to the Circuit. The only 10 states will be able to change the outputs which make the circuit 1-10 decoder too.

INPUTSOUTPUTS
P3P2P1P0Q0Q1Q2Q3Q4Q5Q6Q7Q8Q9
00000111111111
00011011111111
00101101111111
00111110111111
01001111011111
01011111101111
01101111110111
01111111111011
10001111111101
10011111111110
10101111111111
10111111111111
11001111111111
11011111111111
11101111111111
11111111111111

74LS145 decoder Example in Proteus  

In this example, we will understand the IC functionality on three states “0000” which is the first state, last state 1001 and the invalid state. First design the IC circuit in the Proteus. Use the logic gates to understand the IC functionality. Design the circuit according to the following diagram:

75LS145 internal circuit

Figure 3: PROTEUS CIRCUIT

First, give the input as “0000”. You will notice that the input will only make the Q0 active which will make the output LOW and other output will remain HIGH.

74LS145 proteus example input 0000

Figure 4: WHEN INPUT IS 0000

In another case when the input will be 1001 then Q9 will be LOW and others will remain HIGH

74LS145 proteus example input 1001

Figure 5: WHEN INPUT IS 1001

In case of values from values between 0000 to 1001, the logic will follow the different paths to make the other gates active but in case of values above 1001, there won’t be any path and outputs will remain HIGH for all the gates.

74LS145 proteus example input 1010

Figure 6:When input is greater than 1001

The logic states will not active any gate. That is why these states are known as Invalid states. The IC has multiple uses due to its fast speed and usage with another controller. Whenever we will need to use the controller, we will need to use only valid inputs otherwise IC won’t give any output, all will remain HIGH until there’s a valid input.

APPLICATIONS S4LS145

  • It is used to drive multiple LED lights and for seven-segment displays.
  • It also helps to control the incandescent displays.
  • IC also helps in gas discharge displays.
  • IC helps in to control Florescent displays and LCD displays.

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