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........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....can anyone plz suggest a solution for this..............

 

plz help.............

 

Your schematic is too small to make out easily.

One problem, though, is the mix of 74LSxx logic and 4000 series CMOS. The 74LS logic does not produce a HIGH level compatible with the input requirement for CMOS logic. Use all CMOS or all 74LSxx logic.

 

Hi Seasame,

Welcome to the forum. I think I see what you are doing - using the 7432 to allow you to switch between using the clock and a button to increment the digits.

I'd suggest removing the 7432 and directly connecting the clock to the CKA inputs of the 7490s. Then use a Schmitt-trigger debouncing circuit such as: http://www.google.com/imgres?q=schm...tart=0&ndsp=24&ved=1t:429,r:0,s:0&tx=78&ty=70

Connect a diode such as 1N4148 or 1N914 between the outputs of the 4093 and the inputs of your minute and hour 7490s. This should allow you to increment the minutes and hours regardless of the seconds' value.

 

hey i want to ask 0ne thing i.e. can u explain how this schmit trigger debounce circuit works and how this ckt is used as button ......one more thing what will be the polarity of the diode connection i.e. which pole of the diode i have to connect with the output of schmitt trigger ckt and with which pin i have to connect the other part of the diode with the 7490 counter...................thanks////...........what will be the values of r1 and c1 in schmitt trigger ckt

 

Seasame,
Have a look at Jack Ganssel's Guide to Debouncing:
http://tinymicros.com/mediawiki/images/8/8a/Jack_Ganssel_Guide_To_Debouncing.pdf
There is a lot of good information in that PDF. You will need Adobe Reader installed to view the PDF.
If you prefer to view the information online, then here is the start of Mr. Ganssel's online Guide to Debouncing:
http://www.ganssle.com/debouncing.htm

 

Hi Seasame,

The link I sent earlier explains how the Schmitt trigger works. In essence, it removes any contact bounce from the switch sending a single, pure clock pulse to the output when you press the switch. I've attached a quick drawing (using the one from the link) showing you how to connect the diode. One IC has four NAND logic gates giving you up to four separate outputs. You only need two. The diode is used to prevent the clock signal from the 555 from hurting the 4093 output.

Beenthere makes a good point, CMOS and TTL usually don't play well together. If your circuit works so far, you're probably okay, but you'll be safer using a 7447 instead of the 4511 OR a 4510 instead of the 7490.

If you decide to stick with the 7490 (and recommended 7447), use a 74132 Schmitt trigger NAND IC instead of the 4093.

So use either:
A: 7490, 7447, 74132
or
B: 4510, 4511, 4093

A will only work with 5VDC, B will work with anything between 3-15VDC.

 

hey i hav a ckt for button mechanism plz check this .................

 

Seasame,

I think you're still trying to merge the clock signal with the switch. Maybe that is the way to do it, I'm not sure. I don't know if this will work much better than your first design or not. I still think you should run the clock signals directly to the clock inputs and put the switches in parallel with diodes.

In reviewing your original circuit again, I don't think just tying the Qc output from the tens place from one set of digits to the clock of the ones place for the next set of digits will work. The tens place at, say 59, will be 1 (0101). In going to 60, it will still be 1 (0110). It won't go to 0 until the tens place goes to 8 (80 -> 1000). Since you mentioned the clock input will clock the output only when the signal goes from high to low, I'm not sure the current design will work. However, if it does, more power to you. Let me know as I'm curious . . .

Hope this helps.

 

hey elec_mech.....you are say write about the Qc thing but in order to make it 0 i have tied the 7490 ina certain way such that when the 60 count came i.e. 6 at the mod_6 counter instead of counteing further the conuter will be resetted to 0.........as far as button mechanism is concerned i am also not sure about my new design..........i'll apply n let u know.........

 

@elec_mech
hey......i hav applied the logic that you hav suggested........but when the button is clicked or pressed the count of the previous counter and the current counter to with the nand logic is connected both changes simultaneously...........???????.........where the problem is.......i m attaching a ckt plz see does this logic will work or not and made corrections if possible...thanks ...........

 

Hey Seasame,

I'm not sure what you mean by the previous and current counter change simultaneously. Do you mean the ones (0-9) and tens place(0-6) of the minutes change at the same time?

I also do not know what NAND logic you are referring to. Could you better explain by calling out the IC and display digit labels you are referring to?

In looking at your circuit, I'm not sure why you are feeding the 1 Hz clock signal through the minute- and hour-set switches. Try disconnecting the 1 Hz clock from the switches (on pin 4). Instead, connect them (pin 4 of each switch) to your 5V source. This way, when you press and release a switch, you will send a 5V pulse to the clock input which should increment your counter by one.

You should add some type of debounce to the switches using either an RC circuit or a dedicated IC such as the 4093, otherwise you may see the display do strange things.

You will need to repeatedly press the switch to increment each time. If you want the ability to hold down the switch and have it automatically increment the display, I can show you a 555 circuit that will do this.

Also, add a 0.1uF capacitor to each IC across Vcc and GND to help eliminate any noise.

 

hey can you post that 555 ckt you are talking about

 

Here you go.