problem with 7 segment 4 digit multiplex

sir i made a clock with 4 digit 7 segment multiplex with a big digit display.it works but problem is in the display .bkz the digit segment is not turned off properly to display a digit. as example to display 3 we need just 5 segment to turned on.but rest of the 2 also turned on dimly. i used 4 bc557 for 4 digit only with 12 volts for digits. and 7 segments are connected with at89c2051 with 330 ohms. the bc557 is connected with at89c205 with 10k resistance. each segment contain green 4 leds. plz help me out from that problem to display digit clearly.

without seeing the circuit it will be difficult
to tell.

srizbf
22ndmay2010

Hi rohansinha2000,

You have used BC557 which is a PNP transistor, so I infer you have used common anode displays.

Assume you need to display '3' . What you provide at the common anode through the transistor is 12V. You supply 0V at the cathode pins of the segments you need to turn ON, here the pins are: A,B,C,D & E. Fine upto here.

Now think about the segments you tried to turn OFF. Since you are connecting the cathodes to a microcontroller, you put a logic high on those pins which corresponds to 5V. Now what you have done is that the common anode of the leds is connected to 12V and the cathodes(of leds which you wish to be OFF) to 5V. Effectively, the leds turn on at the difference voltage of 7V (12-5=7V). So they turn ON dim.

Again, another problem you may face is that, you won't be able to turn off the transistor with the microcontroller. The base-emitter junction would always be forward biased whatever you put at the microcontroller pin, bcoz of same reason stated above.

To solve the problem:

1) Use an NPN transistor and common cathode displays.
2) Provide 5V supply to the displays if possible (preferred solution)
3) Go for a two transistor circuit involving an NPN transistor as first stage switching the PNP in the second stage
4) Use ULN2803 in between display and microcontoller, with supply voltage of 12V for ULN2803.

thanks for reply today i tried with uln2003. just i do add uln2003 directly with at89c2051 as 7 segment negative volt.and the other side i add another uln2003 directly. with this uln2003 i add bc557 with 4k7 and 12 volt as 4 digits positive supply.
bkz my display drives with 12 volt dc.each segment contain 4 leds. but i connected both uln 2003 with 5 volt regular supply as at89c2051.
i also added 470 ohms in each of 7 segment connected with uln 2003. it do not solve my problem. with digits rest of segment also turned on dimly.

hi rohan, check the schematic attached.


regards
kj

As per your discussions above, you are mentioning about BC557 in all your querries and repiles. But wheras in your posted schematic, BC557 is not to be seen.
Anyways...in reference to the present circuit posted, the pull up provided to T3's Base (680E ) in reference for 12 volts is too high. You may comfortably use 10K or even 22K in the place. Too i strongly suggest a resistor to ground the base of T3. May be with 33K for better performance. Cheers

You don't mention if you are alredy doing this, if not try:

Before changing the digit value, try to turn on all the segments, this is a common problem when multiplexing.

So you will need to:

- Turn off all the segments
- Drive the next transistor
- Set the new value.

Let us know.