Digital clock date problem

[deniah]

I build clock with 6 common cathode seven segment displays. Display drivers are 6 shift registers 74HC595 and 6 x 6 NPN BC337. From PIC I use two ports to drive 4 leds for time separator and two leds for date separator, all via BC337.
BC337 collector is connected to +12V, emitters connected to 280R and then to one segment, or time-date separator led.
Positive logic is used in software.
Problem is that separator leds are working ok, but displays are not. Logic levels on HC595 are ok, but there is no output on BC337.
Can someone help me with this?

Here is schematic.

 

[karesz]

Hi,
What kind of resistor values do you have between BC337 Emitters & SA23 LEDs?
K.

 

[deniah]

between BC337 and displays 280ohm, between BC337 and separator leds 1K.

 

[karesz]

Hi deniah,
Your only problem is, that these displays are common anode version & you MUST HAVE common Cathode if your switches are to +12V working...:-(
Here, as simplest "resque the system", you need insteed SA23-11 than SC23-11 version!
Good luck!
K.

 

[waseem]

I think that the 10K resistors at the base of the transistor is too much. Moreover measure the voltage drop between the emitter of the transistor and the Ground (Common Cathode of the 7-segments). If the measured voltage is more than 4.2V then the segments will not turn ON. Here i have assumed that the 74HC595 is being supplied with +5V.

Regards.

 

[karesz]

Hi,
NO. Its not to believe, then here is another problem to solute!
Im sorry, Waseem, but a current of Base is not a problem with serial 10K Ohm, but a reverse biased LED is a problem if you wish to light with them!
You need for lighting forward biased LED! I hope; you can agree?
You can check pls the attached datasheet too...
K.

 

[deniah]

First, thank you all for help.

@ karesz

Actual I use CC displays. Common anode is only in schematic. That is because I did not have common cathode display in Eagle library. In my first post I wrote I use common cathode displays.

@ waseem

I try your suggestion and report result

Added after 1 minutes:

Voltage supply for HC595 is 5V. Voltage drop between emitter and GND is 4.61V and between base and emitter is 0.29V.
Tried with 10K base resistor, 4.7K and 680ohm.
Any suggestion?

Regards

 

[karesz]

Hallo Deniah,
Sorry, but I forget your first mails contest, especially the Cc LED types...
I checked repeatedly & in detail your circuit:
If you have i.e. 10mA than it needs on the BC337 base more as 5V...:-(
(ca. 2V on LED + 2.8V on the 280 Ohm & 0.7V transistor)
Change the 280 Ohms pls i.e. to ca. 27 Ohms-> I think; after that it must work.
K.

 

[deniah]

Still not working. Looks like I have to modify schematic and buy CA displays. But I don't understand why MCU->BC337->4leds are working, and HC595->BC337->one segment not?

 

[karesz]

Is the GND on the LEDs surly?
Can you try external one display + 1 transistor pls?
Check it pls, how much control voltage you needs for sufficient lighting(max current at 20mA)...
K.

 

[waseem]

karesz, you are right about the polarity of the displays but i had assumed that it is the mistake in the schematic.

Now as the voltage drop across the base and emitter junction is only 0.29 volts which means that the transistor cannot TURN ON. It should be at least 0.6 volts to turn the transistor ON.

So my suggestion is to remove the resistor at the emitter and instead connect it between the supply and the collector of the transistor.

I mean the arrangement should be like the one shown in the attached document.



Implement this for one of the segments and then check.

Regards.
[/img]

 

[karesz]

Hi,

For me it seems not needed to use the second supply of +12V too...
What do you mean waseem?
K

 

[waseem]

Hi Karesz!
I agree with you because the displays that are being used have optimum performance at 2.5V and 20mA. So 5V is ok for the entire circuit.

Regards.

 

[deniah]

I'm sorry. It was my mistake I did not put in circuit right values for displays. I use FYS-230011AS-21 displays. Digit is 57mm high and 46mm width. Minimum voltage to light up is 6.6V

I try all yours suggestions but it didn't work. Finally I put 10k resistor between base of bc337 and +12V and displays light up!!
In attachment is schematic.
It's really nice to see it working after three days of work.

Thank you all for help

Regards

 

[karesz]

HI,
thats life deniah....
Important is for you that it does_I ask only;
1,
do you have enough voltage reserve for your display pls?
2,
Is FYS-230011AS-21 surly correct?, I can not find some info over that...
K.

Added after 14 minutes:

So; yet have I:
correct is ="PartNO.: FYS-23011E/FXX"
BUT;
these isnt correct with 6.6V version of your china display, than it needs the same max. 2.5V as the other, original SA23-11, type from your circuit...
K.