Hi all,

I am trying to create a decade counter by using the below schematic. When I switch on the circuit, by default it displays 5 or 7 (mostly 7) on the display. When I connect pin 14 of the counter to the -ve, I expect it to count the next number. But if it is 7, it changes to 5 and vice versa. But does not change after the first time.

http://www.next.gr/uploads/82-b66141494b.png

Currently I am giving the clock input manually (you can see a black wire hanging from pin 14). I connect this to the ground to give the clock input.

I tested the chips individually and it looks to be working fine. I tested 7447 by giving the -ve input on the pin 3(i think) and the seven seg display showed 8 . Then I connected LEDs on the 4 output of the 7490. I looked for different leds to glow for every time i connected pin 14 to ground and that worked too.

I checked the connections, it looks good. For your expert eyes, do you see any obvious mistakes i have done ?

Note: I do not have 220 Ω resistor handy. So taking up the risk of burning the components, I am proceeding with building the circuit. I hope not using resistor cannot be the reason for the improper display.


With Regards,
S.Sudharsan

 

Put in the resistors. They don't have to be exactly 220Ω.
What values do you have?
Anything from 220Ω to 1kΩ will work, with decreasing brightness as you increase the resistance value.

You cannot clock the counter manually unless you debounce the input.

Use a 555 timer circuit set to a low frequency such as 1Hz.

 

I do not have any resistance with me handy. I got to purchase.. But not using the resistance is the reason why the 7 segment is not showing the numbers ?

If so, why is it showing 5 or 7 alone ??

 

the 7490 has a not really feature that is somewhat forgotten these days, if you pull an output up or down, it stays and counts from there. this was used in the days berore the presettable ttl counters were available to preset the count. if you pull too much current or pull it up too high, it stays till counting.

 

if so, how to fix this problem ?

 

You will probably fix the problem by first following MrChips' advice and put in the resistors.

Its not only a matter of the danger of damaging components. The large current spikes that occur when switching voltage across the diodes without benefit of current limiting, can cause your power supply voltage to dip and that can cause the internal states of the chips to change.

If you don't put in the resistors, then trying anything else is a waste of time.

By the way, triggering the clock input by manually grounding it will probably get the counter to advance by more than one count sometimes because the contacts, whether in a switch, button, or just a wire touching another, will bounce of if I may, "stutter" giving you several low-to-high transition every time you close the circuit.

Below is the Google search for button debounce circuits.
http://bit.ly/1krdTSF

 

I tried adding the resistors(220Ω).. Still no luck.. But now it gives some symptoms of working. But still it is not displaying a proper number. The display keeps changing for every key press. But it is not showing any readable characters..

Attached is the circuit. Do you see any obvious errors in my circuit. ?

(All three pix are same except that they are from different angles for better visibility.)

Thanks,
S.Sudharsan

 

I simulated your circuit and it worked in proteus.

Are all your segments connected correctly? What's the part number for your display? Instead of using a wire to send clock to the input did you try using a proper push button switch like in my schematic?

Though TTL input has a default high, it would be better to put a 10K resistor to Vcc. Try also put a 0.1uF cap on the input to GND for debouncing.

Allen

 

Guys.. Atlast the circuit showed symptoms of working !!!!! Butbit worked for only twice.. I think the culprit is circut debounce. I am working on resolving that..

 

Not "I think the culprit is circuit bounce".
I know it is circuit bounce. I told you in post #2.