This simple 4 digits stopwatch circuit for counter number down to zero as dynamic drive LED segment. Use a package IC-Cmos TC5001P or 74C926 is main and 5-6v dc supply.
We use a package-IC number is TC 5001 P (Toshiba) or No. 74C926(NS) as main. Both are CMOS-IC has the 4 digit counter circuit chip that consists of the Multiplexer and pulse generator 4 types are inside neatly. This circuit can be reduced in small size. When is the 4 digit counter so have the pulse generation on 4 types are T1, T2, T3 and T4, as principal of LED driving on dynamic types there.
When T1 pulse is “1” at the A, B, C and D output. Which is output of multiplexer will have BCD code of numeric unit digits. from the counter inside. When T2 is “1” will have the BCD code of numeric ten digits. When T3 and T4 is “1” will provide the hundreds and thousands digits respectively.
We connected the output A, B, C and D to the decoder IC number MC14511B to drive the seven segment LED next. That 74C926-IC there are within the BCD decoder (BCD to 7 segment decoder/driver) so have output are seven segment code ready to LED. For T1 to T4 polarity which generated pulse that we will connect to drive the four transistors to drive each numeric LED.
If use IC1 as TC5001P we can determine the frequency of these pulse by choosing the R and C that is connected into IC1. In circuits will see R6, C2 and IS1588 diode. (can use 1N4148 silicon diode instead) is the parts determine frequency of pulse that use to the multiplexer.
The working principle.
In Figure 1 the upper left we use NAND Gate-I1 and I2 of IC(MC14011B) is connected as the multivibrator to product pulse on frequency of 100 Hertz(Hz). Used as a time base for the timer pulse period is 1/100 Second (S). When enter into the 4 digits counter circuit it will count number up one unit every 1/100 S. The time is maximum 99.99 S.
For gate I3 and I4 will connect are RS-FF flip flop to use interbank with micro switch-SW1. Which we will use to starting and stop this stopwatch, This output of RS-FF will control multivibrator on pin 2 of gate T1.
If the signal is “1” will causes the pulse oscillator on multivibrator to the counter circuit.
If is “0” will stop pulse , the clock also stop. Normally SW1 will touch at NC polarity, makes RS-FF has output is “0” .
When want to use the stopwatch again, to push SW1 will be NO , so RS-FF have the output is “1” . The clocl will start continue.
But when want to stop, Release the press switch, The RS-FF will have output is “0” as before.
The SW2 switch connect with R0 terminal of IC-TC5001P, use be the reset switch. The counter circuit inside IC. When press this switch, The LED will display number are 0000 fully. as start before counting.
The segment LED is used on the the one panel. So wiring circuits in less, since pin a, b to g of each LED are connected. With common pin on each LED. In figure 1 we use the cathode common.
We connect small as the dicimal point at LED Numeric in the hundreds digits. Lights up at all times by connecting R 560 Ohm limit current between the power supply and Pin Dp of LED.
For LED on the left hand connected to pin OF of that IC, to display counting over flow. When TC 5001 P count numbers over 9999. This LED will glow light to indicate exceed the maximum count it.
From this circuit will see that be a reasonably complex. But the principle works easier. Need sedation, should check the wiring as well.
The power supply of this circuit use about 5-6 volts at pin T1 or T2 or T4. The frequency this size cause we see as light and LED all together. Try connect the VR1-1 Mohms series with R6. If rotate the volume will can adjust frequencies of pulse on multiplexer. Then try rotate volume to a lot of resistance will see LED flashing. And Switch sorted it. From the units digits to tens or hundreds of thousands of sequences and so on. If adjust volume to lower resistance down, will see that LED switch faster until the visible LED lights all together, can not flash again. From circuit will see that R7-R13 resistance are set current drive LED each segment is 180 ohms only.