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12 volts 7-segment display

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vicky29

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hi friends,
i've to make a 4 digit counter. i'm trying to run a "1-inch 7-segment display" through microcontroller. the shopkeeper told me the display would run with 12 volts along with 1k ohm resistor. i propose to make the circit diagram as shown in attachment. since the microcontroller runs with 5 volts supply, will the 12 volts supply of 7-segment display affect the microcontroller?

thanks,
 

I don't think so mate. It should work fine

Maverickmax
 

It can be dangerous for the 89C51.
First, the voltage is to high, and second, microcontroller pins are not designed to source that much current, maybe per pin, but not per whole port.
Supply this 7-segment of 5V or, using reverse logic use 7 npn transistors to drive the 7-segment display of 12V (with 1kΩ resistors between each transistor and display, and ≈4.7kΩ resistors between microcontroller pins and bases of transistors.
Other thing is that that you may overload this "key-transistor" that in your case will drop voltage from 12V to 4.something if driven by weak pull-up from microcontroller pin. Much better solution, if you decide to change the supply voltage to +5V, would be again revers the logic and use pnp transistor.
Regards,
IanP
 

hi Ianp
thanks for replying . as suggested by you i made a new ckt diagram. i have made two changes, first i have added seven transistors with 1k resistors and secondly have changed the "key transistor" from npn to pnp. please check if it is correct.

thanks once again.
 

These 7-segment drivers are OK, but the PNP has to be driven by another NPN (4.7kΩ between µC pin and the base), as if it is connected to microcontroller as it is now, it may not only damage the "pin" but it will be permanently ON.
In fact I don't see any reason for this transistor; it can be easily removed from this circuit and 7-segment connected directly to (+)12V..
Regards,
IanP
 

Thanks for replying, actually i have to design 4 digit counter (i've drawn only one digit), so that's why i need to put this transistor. I've futher changed the circuit as u suggested, please check it.

Thanks once again.
 

In this case you should have 4 npn transistors driving 4 pnp transistors for 4 7-segment units. Don't forget about resistor (can be another 4.7kΩ between the collector of NPN and the base of the PNP!!!
Are you designing sequential (dynamic) display?
In this case you will have to reduce the value of resistor connected to segments so the average current will be similar to static display.
Regards,
IanP
 

thanks for replying. yes i'm making sequential (dynamic) display (all the 4 "a" to "g" of 7-segment displays are connected in parallel. please suggest the value for resistors and is the npn&pnp combination correct?

i have made the changes again in circuit diagram, please check it.

Thanks once again.
 

That looks all right.
For sequential display change the values of 1kΩ resistors connected to segments to, say, 240Ω.
Regards,
IanP
 

Thanks a lot. I've made the full diagram again with 4-displays, re-attached it. i hope my continous questions didn't disturbed you much.

Thanks once again.
 

This should work.
Here is a handy link to a real time clock with sequential display (code in C). You may try adopt part of this code to your project:
**broken link removed**
Regards,
IanP
 

hi Ianp
thanks for replying . as suggested by you i made the ckt .but some of the segments glow bright and some glow dull...segemnts which glow dull ...glow bright for different digits and vice-versa.....i tried using different value of the resistors like 100 ohm 220 ohm 330 ohm 680ohm 1kilo-ohm (with 1k ohm the segments brightness is equall but is dull)

what should i do?

thanks once again.

Added after 4 hours 45 minutes:

Anyway thanks Ianp. I pulled up the port P1, with 10k and the brightness got ok.

Thanks once again.
 

you must place 7407 between out of cpu and driver, out of 7407 should use 10k Ohm pull up.
 

I'd add a high value resistor eg 100k between +12v and the bases of each pnp transistor to make sure they turn off fully.
 

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