The project under construction here is a DIGITAL STOPWATCH that could count time up to a maximum of 99.9 seconds with a resolution of 0.1sec (or in steps of 0.1 sec) or up to a maximum of 999 seconds with a resolution of 1 second.
This has been possible by having two clock frequency options, a 10Hz clock and a 1Hz clock. This gadget can be used to accurately measure short time intervals. It is portable and operates from four rechargeable Ni-Cd cells.
IC1 (MM5369) is a popular IC used for generating clock pulses in digital time pieces. lC-1 along with resistors (R1), capacitors (C1) and (C2) and crystal generates a 60 Hz clock signal at its output (Pin-1). This 60Hz clock signal is passed on to the clock input of IC-3 (CD4018B) via an arrangement of NAND-gates. It is easy to see that whenever the START switch is pressed momentarily, pin-4 and thus pin-: of IC-2 goes HIGH which enables the NAND-gate designated .IC-2A with the result that the clock signal is
passed onto the IC-3 clock input. When STOP switch (S2) is pressed momentarily, the NAND gate designated IC-2A is disabled due to pin-4 of IC-2 going LOW with the result that the clock pulses are not allowed to reach lC-3. IC-3 and IC4 (CD4018B) are presentable divide by (N) counters where (N) could be 2, 3, 4, 5, 6, 7, 8, 9 or 10. IC-3 here has been wired as a divide byes counter by feeding (Q3) output to Data input of the IC. IC-4 has been wired as a divide-by-10 counter by feeding (Q5) output back to the Data input. With 60Hz clock input to IC-3, we get a 10Hz clock signal at IC-3output which when fed to the lC-4 as the clock signal results in a 1Hz clock signal at the output of IC-4.IC-5, lC-6 and 1C-7 (CD4510B) are presentable UP/DOWN BCD decade counters. These counters are connected in cascade arrangement and can count up to a maximum of 999 clock pulses. When fed with a l Hz clock, the maximum attainable time delay between start and stop operations is thus 999 seconds. Similarly, when the .clock frequency is chosen to be 10Hz, the maximum attainable time delay is 99.9 seconds. The RESET switch can be used to reset the counter to all zeros before start. Usually, the reset terminal is at GND, it is momentarily connected to +5V to reset the counter.IC-8, lC-9 and lC-10 (CD4511B) are BCD to seven segment latch decodes drivers. IC CD4511B can directly drive seven segment LED displays of the common cathode variety.
IC-11, IC-12 and lC-13 (LT543) are seven segment displays of common cathode type. R5 to R25 are current limiting resistors.
Figs. 17.2 and 17.3 receptively show the PCB layout and the components layout. Pin connection diagrams of CD4510B, CD4511B, CD4011B and LT543 are respectively shown in Figs. 17.4, 17.5,
17.6 and 17.7.
Resistors & capacitors
R1 : 22M, 1/4W
R2, R26 : 1K, 1/4W
R3, R4 : 22K 1/4W
R5 to R25 : 680Ω, 1/4W
C1, C2 : 33pF (Ceramic)
C3 : 0.0022µF (Ceramic)
Semiconductor Devices and lCs
D1 : LED
IC-1 : MM5369
lC-2 : CD4011B
IC-3, IC-4 : CD4018B
IC-5, lC-6, IC-7 : CD4510B
IC-8, IC-9, IC-10 : CD4511B
IC-11 , lC-12, IC-13 : LT543 (Common cathode seven segment display) Other comments X-TAL : 3.5795 MHZ crystal
S1, 52 : Miniature push button micro switches
S3, S4 : SPDT switch
S5 : ON/OFF switch
IC Bases (14-pin, 16-pin), Nickel-cadmium Cells (4 Nos.), Multistrand wires, solder metal, suitable mounting cabinet.