I would like to build a stopwatch / countdown timer circuit, but I am having difficulty in finding the right IC to start with. Any suggestions?

 

4510 can do the job of down counting and you have to also use the 4511 for 7seg out put...

 

Depending on the accuracy needed, you will need a 555 timer (for pulses - 100 per second)

About 4 to 6 decade counter chips with Binary Converted Decimal output pins (CD4510 as suggested above). Then 4 to 6 CD4511 chips to convert the BCD to a 7 segment display digit LED. Finally, you will need 4 to 6 LED digits for the display.

I say 4 to 6 because it depends on the number of digits of accuracy you want to display. it gets more difficult if you want to display times over 60 seconds as minutes instead of keeping everything in seconds. if you want tenths of seconds or hundreths of seconds, you will need a decimal point wired into the display but that is easy.

The datasheets for the chips mentioned will help you too. Start a design and post it for comments. The chips are available from jameco.com for about $0.30 each last time I checked. There is a nice tutorial on the CD4510 and when you are done, it goes into the CD4511 (for the LED display). Finally, it has another one for CD4060 for a stable time base to drive the whole thing. HERE

Another option is to use a microcontroller but you will need to learn programming, buy a programmer and likely learn about multiplexing a display. Google each of those terms if needed. Only one chip will be needed in this case, a PIC16F628a is an old stand-by and will cost under $2.00. If you are just starting, go with he first suggestion. It will be lots of wires or PCB traces but you will learn a lot.

 

I have been looking at the electronic stores inventories and I think I would used the following ICs.

Since I want a fairly accurate time base for the stopwatch (to 100th of a second), I will use a crystal and divide the frequency with a 4040.

I also found a clock circuit at http://www.bowdenshobbycircuits.info/clock.htm which uses 74HC390s to count the seconds, minutes and hours.

For the display, I found ICM7212 from Maxim that will drive a four digit 7-segment LED display.

Now to wire them up properly.

 

GopherT,
Excelent tutorial on the BCD counter. I did a quick scan and it is very informative and well written. Thanks for the good link.

Wheelchair Bob

 

Since I want a fairly accurate time base for the stopwatch (to 100th of a second), I will use a crystal and divide the frequency with a 4040.

The CD4060 does essentially the same thing as the 4040, except that the 4060 allows a 32.768 kHz crystal to be directly attached and drive the crystal. Perfect for a timer with a watch crystal. The only difference is that the 4060 does not output two levels of divides to pins. Both chips put out a final of divide by 2^14 power for a 2 Hz signal. Just no need to crate you own crystal oscillator circuit.

 

As Gopher says, do it first with 4000 series CMOS ICs.

After you have done that, try it again with a MSP430G2553 microcontroller chip. See my recent blogs. Coming soon I will show how to interface to LED seven-segment displays.

 

OK, I got the major parts in my circuit and wired together. I still have a few controls to add (start/stop stopwatch/timer and reset to zero).

The crystal vibrates at 3.2768 mHz, so I used a fast inverter to drive it. Does the crystal frequency vary if the capacitance change?

I had to use a 16 bit flip flop to divide by 2^15. This gives me a 100 Hz clock frequency. I use this clock for the stopwatch and allow the counter to go to 99.99 seconds. For the timer, I divide the 100 Hz clock by 100 with U3 and use the 1 Hz clock for the timer. The timer will roll over at 99 min 59 sec.

The BCD to LCD driver does not tell me the use of the PH pin (except that it inverts the outputs). I don't understand it's purpose. Also some other LCD drivers indicate that it is connected to the LCD display. Also, is the common on the LCD display connected to ground or Vcc? The LCD display datasheet only gives the pinout and dimension of the part.