89S52 , ULN2803 & 7 segment display- Weird behavior !!

[kunaltg]

Heyy i am driving 4inch 7 segment common anode display using 89S52 & ULN2803 !!

normally my circuit works perfectly ,,but whenever i touch any pin of ULN2803(which is used for driving 7 segment display) with my multimeter's probe, the respective segment switches off !! ??? & this happen with every other segment when i touch respective pin of 2803 or 89s52 !!

I checked circuit without ULN2803 connected ,, checked voltage at port pin of 89s52 ,,it remains 5V which is right !! but after connecting uln ,,i cant read voltage at its pin !! as it automatically falls & switches segment off !

Can anybody please tell me why is this happening ??

 

[jayanth.devarayanadurga]

Are you using ULN to enable SSDs? Are you using CC type SSDs? Post your Circuit.

 

[kunaltg]

Yess ,, i guess ,i already mentioned that all !! my SSD is Common Anode & i am using ULN2803 for driving them !! Srry but i didn't draw any circuit ,, directly implemented it on 0 PCB !!

My circuit is functioning Properly !! but only this weird issue !! Circuit is very simple ,,,89S52 -> Port P2 -> ULN2803 -> Output to CA SSD ,, SSD is rated for 9V ,,I am operating it at 8V.

So pin9 of 2803 is GND & pin10 is connected to 8V !!

 

[jayanth.devarayanadurga]

What is the purpose of ULN if it is connected to a,b,c,d,e,f,g,dp pins of SSD? It is not needed. Each uC pin can source or sink 25 mA. Only one SSD will be ON at a time as it is multiplexed and each segment is connected to one pin of SSD and with a current limiting resistor each segment of one SSD will draw onlu 10 to 15 mA current. ULN can only sink current. You need transistors to enable/disable the SSDs. As you are using CA type SSD the enable pins should be connected to Collector of PNP transistor and emitter to +Ve supply. This way enable pin is made high to turn ON the particular SSD and a,p,c,d,e,f,g,dp pins are made low to turn on the segments of a SSD.


Edit: You are driving 4 in SSDs, so you need ULN. Try pulling-up the outputs of ULN2803.

 

[kunaltg]

1) I am Using 89S52 !
2) According to datasheet Max sink current per port pin is 10 mA ! BUT maximum sink current per PORT is 15 mA for port 1,2,3 !!
3) My display's single segment takes 5 mA , so total it will be 35 mA Max !! Thats why i am using ULN for sinking current !!
4) I am using 2 CA SSDs ! But not multiplexing them, one is connected to port 2 & another to port 1 !!(so obviously using 2 ULN2803 for 2 displays)

 

[jayanth.devarayanadurga]

What is the reason for not multiplexing. You can connect ULN between SSD and uC. Pull-up the ULN outputs.

Edit: You can also use 74HC541 instead of ULN2803.

 

[milind.a.kulkarni]

I think it's better that you put your circuit diagram(schematic) on the forum..... Then the reviews will be much clear...

With regards,

Milind

 

[Venkadesh_M]

when you are touching in input or output pins of ULN????

If it is input pins the pulling up is surely needed (because ULN pins are very sensitive)..
If you touch it, it takes it as low inputs and turns the Segment OFF...

 

[kunaltg]

Thnx a lot Venkadesh_M !!! i was touching those pins ,just for testing purpose using multimeter !! ,,i guess now i got what i wanted !!! I will pull up input pins of 2803 !!

 

[Venkadesh_M]

and also another thing is you have to give 8V (Vcc of ULN) in the pull ups don forget...!

 

[BradtheRad]

Your body is able to pick up (1) static charge, and (2) electrical hum from mains wiring nearby.

When your finger touches the sensitive input, you may change its volt level.

Even a short length of wire, attached to the input, can pick up static charge, or mains hum. Results are unpredictable.

That is why the advice says not to leave input terminals disconnected (floating).

 

[kunaltg]

^ thnx but they are not disconnected !!