help in digital clock project using 555 timer

Thread Starter

mr.harm

Joined Nov 24, 2008
29
Hello,
I have been asked to design a Digital clock using a 555 timer as clock generator and a 7-segment display to display clock.

I have no idea how to start, could u give me some hints.

many thanks,,,,,,
 

Thread Starter

mr.harm

Joined Nov 24, 2008
29
the gates to be used were not specified, if it is possible, I need to use the simplest available gates. The thing is that I don't even know how to use 555 timer in this case,
 

Wendy

Joined Mar 24, 2008
22,206


Use one of the circuits on the bottom row, pick a cap (say 1µF), then adjust the resistors to match. Ra should be 1/10 value of Rb.
 

SgtWookie

Joined Jul 17, 2007
22,220
You will need to use some logic ICs.

TTL and 4000-series CMOS have been around for years, and are still pretty readily available. You will need to chose one logic family to work with.

TTL (54xx/74xx series) requires a voltage supply of 5v; minimum 4.5v max 5.5v.

CMOS can work over a voltage range of around 3v to 16v, and requires much less current than TTL.

This link has pages that describe in general terms a good portion of TTL (74xx series) and CMOS (4xxx series) ICs:
http://www.kingswood-consulting.co.uk/giicm/

A couple of handy ICs that you will need are BCD to 7-segment drivers.
There are TTL versions in the 7446, 7447
CMOS has the 4033, 4511 and a few others.

You will need some decimal/BCD counters (NOT Decade/Johnson counters). I suggest that you avoid HEX/binary counters, as that will make extra work for you.

You may also need AND, OR, NAND, NOR, XOR gates and inverters.
The 4093 is a very handy IC to know about; it is a quad Schmitt-trigger NAND gate. You can construct any other logic gate using combinations of NANDs.
http://en.wikipedia.org/wiki/NAND_logic
The Schmitt-trigger input is very useful for "squaring up" slowly transitioning waveforms.
 
Last edited:

Wendy

Joined Mar 24, 2008
22,206
There are lots of chips that count either to 6 or to 10, with mates that can decode that to a digital display. With TTL it is a 7492 and 7490, not sure of the CMOS numbers (but I know they are out there).

It occured to me after seeing Wookie's reply that you meant the project as a whole, not just the 555 clock.
 

Thread Starter

mr.harm

Joined Nov 24, 2008
29
Thanks a lot Bill_Marsden, I started to get the idea, I am stuck now to get an output pulse from the 555 with a width of 1s.

-am I right in the 1s width?-
 

Wendy

Joined Mar 24, 2008
22,206
Not really, 1 second frequency. Width is ½ second, assuming a symetrical square wave (which isn't really important).

You have the formula's on the sheet I posted, try using the procedure I posted. Start with a 1µF cap, 1KΩ for Ra, and work out the resistor value for Rb.

As for the rest...



This is a generic block diagram I drew up a while back. The first 2 counters are replaced with your 555 timer time base.
 

Thread Starter

mr.harm

Joined Nov 24, 2008
29
mr.Bill, I have succeeded in connecting the six 74ls160 ICs to the 7-segment displays. I tried giving clocks with different frequencies from a function generator. The seconds, minutes and hours are being showed correctly. --:--:--. My problem now is to get the right input frequency to the first counter, what's the right frequency? I want to try supplying it firstly using the function generator then I will design the 555 timer according to this frequency.

(first counter is for seconds --:--:--'the one in red')
 
Last edited:

Wendy

Joined Mar 24, 2008
22,206
The first counter is 1 Hz. You have all the information in front of you, I've given you two of three components to make a 555 1 Hz oscillator, the rest is algebra.
 
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