Digital Electronics

[Gary Dent]

To get myself acquainted with digital electronics I am going to build myself a digital clock. I have a basic overview of how its going to work. The power supply will drive a 555 timer in astable mode generating a square wave. To get the seconds out I am going to need to get a 1Hz signal and feed this into a counter to get the counter ticking over every second. From here I can drive a decoder to get a 7 segment display output. For the minutes I will need a once a minute signal and for hours a once a hour signal.

Can anyone recommend some good resources for learning about counters, 555 timer, decoders and other things. Basically some good hobbyist resources.

 

[Harald Kapp]

E.g.:
http://www.williamson-labs.com/555-tutorial.htm
http://www.sentex.ca/~mec1995/gadgets/555/555.html

By the way: The NE555 is not a good choice for a stable clock. It is an anlog circuit with all tolerance issues inherent to analog circuits. Not unmanageable, but not well suited for a clock.
I suggest you use
- either a digital clock source (quartz oscillatzor + divider)
- or a mains derived clock source

If your AC mains is e.g. 60 Hz, tap the secondary of the power supply transformer, generate a square wave (using a simple comparator) and divide by 60.
An example can be found here: http://sound.westhost.com/clocks/sync.html

Harald

 

[Gary Dent]

Thanks Harold,

The links you have sent me do help to understand operation of the 555, however I was thinking of the following, the seconds hand should tick over every second. Therefore getting a square wave with a period of 1s and setting the on time to be .8 and leaving the other .2 for off will give the illusion of the display ticking over every second. Does this make sense? I guess .2 of a second is quite hard for most people to detect!

 

[BobK]

Thanks Harold,

The links you have sent me do help to understand operation of the 555, however I was thinking of the following, the seconds hand should tick over every second. Therefore getting a square wave with a period of 1s and setting the on time to be .8 and leaving the other .2 for off will give the illusion of the display ticking over every second. Does this make sense? I guess .2 of a second is quite hard for most people to detect!

I am not sure what you are thinking here. The clock will change (depending on the logic you use) either on the rising edge or the falling edge of the 1 Hz square wave. The change will appear instant to human eyes.

Bob