Home > Circuit Bricks > Decoder - BCD to 7 segment UPDATED:.21:04 21 October 2013
 FUNCTION BLOCK DESCRIPTION This decoder will convert a 4-bit BCD binary input value and generate signals to display the value on a 7-segment display. It can also latch any value which freezes the outputs.
 CIRCUIT DETAILS The circuit is based around the 4511 counter IC. It is a CMOS type and can work from a wide voltage range. The circuit shown will convert a 4-bit value and generate signals for a 7 segment display. Leave inputs as they are if not using them. It is a digital device and works with logic level inputs. A high refers to +v and low refers to 0v. Inputs Circuit Diagram BCD Input - 4 inputs for the binary value. Seg Test - A low input will turn on all the outputs (to test a display) Blank - A low input will turn off all the outputs (for zero blanking) Strobe - A high input will latch the BCD value and freeze the outputs Outputs The outputs must have resistors in series with the LEDs if using a standard seven segment display. (see below) The type used must be a common cathode type, meaning all the cathodes are connected together. The outputs can provide up to 25mA but at this current, you must use a supply of 5 volts or less.
 MODIFICATION DETAILS Resistors R4-10 are important to regulate the current to the LED segments. The value depends on the voltage supply you are using. The value can be calculated using Ohms Law. R = V / I or in this case Resistor = ( Supply Voltage - Vf ) / If Assuming that you are using a standard display requiring a current ( If ) of 20mA per segment and the LEDs have a forward voltage ( Vf ) of about 2.5volts then then with a 9 volt supply the value is Resistor = ( 9 - 2.5 ) / 0.02 = 325 ohms (or 330 ohms). A good rule of thumb is to use 330 ohm resistors anyway. This will still keep the current within the tolerance of most displays.
 PACKAGE DETAILS The 4511B counter comes in an 16pin DIL package. The pin connections are below. Note: Test, Blank & Strobe are active low. If possible use an IC holder so the IC can be removed easily and is not damaged from excessive heat while soldering. IMPORTANT: Try not to handle the pins of the IC too much. They do have protection diodes connected inside but CMOS types are very sensitive to static charge.
 LINKS Links to projects using the 4511B ...GCSE Projects/Race Timer ...Projects/Frequency Meter ...Circuits/Bingo Number Generator ...Circuits/24 Hour Clock ...Circuits/Dual Digit Counter ...Circuits/Lottery Number Generator .
 Written by Phil Townshend - 2008 www.edutek.ltd.uk - Working Electronics For Students & Teachers