7490 counter digital clock buttons...........

[seasame]

hi i have make a digital clock using 7490 counter ics.for mod 6 and mod 10 i hav used 7490.....my clock is completed n showing accurate time i hav also attached the setting buttons for minutes n hours using the OR gate logic....but the problem is that the buttons wont work when the tens counter is at 4 and 5 of both minutes and seconds......plz help............

 

[seasame]

hi i have make a digital clock using 7490 counter ics.for mod 6 and mod 10 i hav used 7490.....my clock is completed n showing accurate time i hav also attached the setting buttons for minutes n hours using the OR gate logic....but the problem is that the buttons wont work when the tens counter is at 4 and 5 of both minutes and seconds......plz help............

 

[alec_t]

Post a shematic of your circuit (as a gif or png or pdf file....not jpg) so that we have some clue as to where the problem might be.

 

[alec_t]

Post a schematic (not jpg) to give us a clue.

 

[Reloadron]

Multiple post do not help, they tend to confuse those who try to help. Not to memberate but please do not make multiple post of the same question / topic. Hopefully a moderator will come along and remove one of them.

Ron

 

[seasame]

@Reloadron.....m new here so i didn't know about it.

 

[seasame]

schematic

hey this ckt is giving accurate time but the only problem is that when the numbers 4 and 5 come on the tens section of section and minutes the buttons for setting minutes and hours dont work that is the problem......... on other numbers like 0 1 2 3 are present on the tens section of minutes and hours the buttons work perfectly meaning by pressing the buttons the count on the minutes and hours units increase by 1......
plz plz help.....

 

[seasame]

tens section of seconds..n minutes

 

[alec_t]

When you say the buttons 'don't work', do you mean (1) nothing happens at all, or (2) they don't work as intended, i.e. there is some unwanted change?
One cause of the problem may be switch contact bounce. I don't see anything in your schematic to overcome this. Switch bounce will cause a series of pulses to be generated instead of a single pulse, giving an erratic change in the count which, because the 7490 is a ripple counter and depending on the switch characteristics, may only occur at certain count values.
Another cause may be interference on pin 2 (with pin 3 already high), resulting in premature resetting of the 7490.

 

[seasame]

see alec .......... i hav used buttons to increase the count of the units section of minutes by 1 i.e. whenever the butoon is pressed the current count is incremented by 1 i.e if it is 4 on the units section of minutes after pressing the button it becomes 5 then then pressing it becomes 6 and so on.........but the problem is that the buttons wont work when the tens section of the seconds section hav the value of 4 and 5 when there is 4 and after 4 ....... when 5 comes the buttons wont work and the count on the units section of minutes remains unchanged nothing happens if it is 4 it rmain 4 if its 9 it remains 9..............i hav heard about switch debounce..............do u hav any idea to overcome this problem.........do my ckt hav any technical problem in the button mechanism.........plz suggest a solution..........thanks...........

 

[seasame]

or suggest any other button mechanism .......... with schematic.........

 

[alec_t]

That has clarified things.
The cause of your problem is the logic used by the buttons. The 7490 increments its count when the clock input changes from Hi to Lo, whereas you are effectively trying to use a button and OR logic to pulse the clock input Hi.
When the tens-of-seconds count reaches 4, the QC output of MOD6_SEC goes Hi, so the output 1Y of the OR-gate will go Hi and will then stay Hi (unless QC changes state) regardless of the operation of the set-minutes switch. Similar reasoning applies to the tens-of-minutes count.
You will need to change the logic so that the 7490 is clocked when QC goes Lo OR the button signal goes Lo.

 

[seasame]

help..............

is there anyone who can help............plz

 

[seasame]

yes alec now u understand my problem that is the exact problem..................can you suggest me some logic regarding buttons........i'll hav to submit my project shortly.......plz suggest some solution.................you hav understand my problem............

 

[alec_t]

One solution to changing the logic (many other solutions are possible) would be to dispense with the OR gate IC and instead make diode negative-going OR gates as follows:-

provide each button with a pull-up resistor (say 1k),
arrange for each button output to go Lo when pressed,
connect each button output to the relevant units 7490 clock input (Ck) by a respective diode (anode connected to Ck),
feed the normal 1Hz clock pulses via respective diodes (anodes connected to Ck) to those counters.

Or you could replace the OR gate IC with a quad NAND gate instead of using diodes.

 

[seasame]

can u explain the procedure of using quad nand gate ic ..........i mean ehich terminal of button is used with nand gate 1st input and what will be the next input that is on 2nd pin .....do i hav to use 1K resistor with this nand ic.........??????

 

[alec_t]

Try this :-



C1 and C2 provide switch debounce to prevent multiple clocking of the counters. C3 and C4 provide a brief pulsed clock signal so that the buttons can increment the count regardless of the count value.

 

[seasame]

thanks alec i will try n tell u the results hav u tested this ckt n is this working perfectly..........

 

[seasame]

hey can i use ttl 7400 nand gate ic instead of 4011 bcoz 4011 is cmos type
?????

 

[alec_t]

No, I haven't tested the circuit and yes, you can use a 7400 instead of a 4011 (I should have suggested it, as it will be better matched to the other TTL ICs). But note that the pin numbering is different for those two ICs.
Instead of using the four gates of a NAND circuit you could simplify the wiring slightly by using just two gates of a quad AND circuit (7408).
BTW, I've just realised that the diode OR arrangement I suggested in post #15 won't work!