The circuit is built around the CD4553 3-digit BCD (binary coded decimal) Counter which provides the necessary functionality to count and store input pulses and enable display control of the count. The CD4553 works in conjunction with the CD4511 BCD-to-7 Segement LED driver. The CD4511 provides the functions of a 4-bit storage latch, an BCD-to-seven
segment decoder, and an output drive capability. Three individual common-cathode 7-segment LED displays are used for the output display. Switch debouncing is provided by NAND Schmitt triggers (CD4093 provides four NAND gates of which two are used).
7-Segment LED Display's
A single 7-segment LED display consists of seven LED's (each shaped as a short bar) arranged in a rectanglar fashion, which when illuminated together in certain patterns, form the digits from 'zero' to 'nine' (i.e a single digit of a 'number'). There is an eighth LED to give the decimal point if necessary (not used in this application).
Each of the 7-segment LED displays have a common-cathode in the version used in this particular circuit. The following diagram (from the datasheet of the particular LED displays used) shows how the various segments (labelled 'a' to 'g') on the LED display are arranged and connected to the various pins. For example, to form the digit representing '7' segments 'a', 'b' and 'c' would need to be illuminated simultaneously (ie, pins 10, 9 and 8).
The CD4553 and CD4511 provide the necessary functionality that enables an input digit (in the form of binary coded decimal format) to be translated into the necessay segments being illuminated on a connected 7-segment LED display.
BCD to 7-Segment Driver (CD4511)
The CD4511 converts an input 'number' (binary coded decimal in the range 0 to 9) to the correct 'segments' that need to be illuminated on a 7-segment LED display that represent the number as a 'digit' on the display. The truth table given in the CD4511 datasheet demonstrates how this is done. For example, for an input number '7', which in BCD is 1110, i.e. 22 + 2 1 + 20 which equals 4+2+1=7, and corresponds to inputs C, B and A being '1' in the truth table or equivalently pins 1, 2 and 7 being 'high' (~5V).
Note that the CD4511 also has inputs for Lamp test (LT), blanking (BI), and latch enable (LE) inputs are used to test the display, to turn-off or pulse modulate the brightness of the display, and to store a BCD code, respectively.
The CD4553 provides the necessary circuity to convert a 'number' into BCD format necessary for connection to the CD4511 (as well as event/pulse counting functionality).
3-Digit BCD Counter (CD4553)
The CD4553 consists of three BCD counters that are cascaded synchronously with a latch to store the result. This means a value in the range of 0 to 999 can be counted and stored (on overflow returns to 0). The signal lines Q0 to Q3 (pins 5 through 9) output a BCD value that represents each digit. Since three digits are required to respresent the full possible range of values, the output lines DS1 to DS3 (pins 1, 2 and 15) enable multiplexing of the BCD signal output lines to output one BCD digit at a time.
The frequency of the multiplexed output is given by the capacitor connected across pins 3 and 4 (which with 1nF is approximately ~1kHz). The output lines DS1 to DS3 are therefore rapidly switched in sequence, which in turn connects the cathode of each 7-segment LED display via a BC557 transistor. This then enables each 7-segment LED display to be enabled at the correct time to display the correct digit.
The momentary push-button switches used on the clock and reset lines are 'noisy' and will cause multiple 'counts' if not debounced. There are numerous ways to provide 'debouncing' for switches (an appropriately valued resistor + capacitor can perhaps be sufficient), but in this case the '100%' effective option of using NAND gates (from CD4093) was taken.