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7 seg. display problem

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bluegirl

New Member
hello

I'm doing a project which has a 3 digit 7 segment display in it. I'm using IC 7447 (bcd to 7seg)for it. the bcd code is fed from 7490 ic(decade counter). The problem is, sometimes, the value shown by the display goes erratic..changing quickly to values like 521 etc from low values like 6 or 7 etc.

What could be the problem? could there be something wrong with the counter IC? or some loose connections in the display ckt?

thanks
 

Gene

New Member
I would look for loose connections for a problem display that goes erratic for no apparent reason; especially in the display driver area. If this circuit is not soldered up (on a solderless board, wire wrap, or alligator clips), this looseness is somewhat normal and the slightest loss of a good connection could easily drive the display driver (or even the display) nuts.
 

McGuinn

New Member
Ensure that you have a smoothing capacitor on each of the chips. Place this as close to the chip as physically possible, connecting it from pins 7 to 14 (Vpp to Vcc). The size should be .01 or .001 uF.
 

kinjalgp

Active Member
Also use a good quality ripple free power supply. It is also good to use ferrite bead inductors along with the de-coupling capaitors in the +5V rail to reduce noise in the systen.
 

mechie

New Member
Switching speed

Also worth checking is the pulse train your circuit is counting.
If you have a slow rising or falling edge to the pulse (triangular ?) then the circuit will spend time in an undefined state.

If you are using a push button for the pulses then it may be 'noisy', try a small (0.1uF or less) capacitor straight across the switch contacts.

TTL can count pretty fast but it is fussy about the pulses - another possibility is a Schmitt trigger on the input to clean the pulses up (maybe one gate of a 7414 Hex Schmitt inverter).

TTL takes lots of power when switching, as McGuinn says use power line smoothing caps.
 

bluegirl

New Member
i'm using an astable mutlivibrator(using 555 timer) to generate pulses.

also i have just one RPS 5v for all 12 ic's. is that ok? i'm using a large capacitor 470 microfd. will the 7805 turn off automatically if it gets too hot or someting?

the display also sometimes turns off completely. could it be becoause i've not connected the LT, Rippple blank etc
 

Sebi

Active Member
Yes, 7805 have internal thermal protection and switch down if overheated. Mount it to heatsink. Don`t forget the decoupling caps. as McGuinn wrote, it`s very important.
 

jakethepython

New Member
crazieness in 7 segment display

there could also be problems because of the propogation delay. if you set it up to a D flip flop too to controll your times you may solve this problem.
 
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