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multiplexing 2 seven segment display with one port

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dawuzi

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hi everyone
i intend multiplexing two 7 common anode segment displays with a port of a micro controller by making 7 pins common to both of them and then the last one for the multiplexing. i dont know if there is a transistor that can switch as fast as a micro controller . some one suggested to me FETs but i have not used them before. i have simulated it and it works well when the delay for the multiplexing is quite large and noticeable. at shorter delays it throws a simulation error
 

alexan_e

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multiplexing would be in a frequency about 100-200 times/sec (enough to fool the eye that both displays are constantly on), any transistor can do that and even 1000 times that and much more.
You can use a mosfet but there is no need for that, this is usually done with one transistors for each display in common cathode but you have common anode and you may need two so that you can turn off the transistor that drives the display.
Any transistor will do the job as long as it can work with the needed current which should probably be about 100mA when all segments are on.

Alex
 
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    dawuzi

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dawuzi

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In your circuit diagram, you used two not gates, any reasons for this as the output of the second not gate will be the same as the output from the micro controller. Would it be possible to an npn transistor like 2n2222 in place of the bc557. if so i would appreciate a circuit diagram for that as well

---------- Post added at 14:06 ---------- Previous post was at 14:03 ----------

Thank you very much. i will dump the FET idea
 

alexan_e

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I don't see a way to use a NPN to turn on/off the common anode of you display unless you use the emitter as output but the problem in that case is that the emitter voltage will be Vbase-0.7 (assuming 5v to the base you will get 4.3v to the emitter, it it suits you then it can be done), that is why I said that it is easier with a common cathode display in which a NPN provides the gnd (collector as output).

Alex
 

dawuzi

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so am i better off with kak's idea?
 

alexan_e

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It the emitter voltage is not enough then you have to use a PNP transistor but note that your circuit has to drive the PNP base almost to the emitter voltage so that it tuns off,
what I mean is that if you have 5v connected to the emitter of the PNP then it will not turn off with 0 or 3v3 to the base, you will need 5v.
If the voltage of the emitter is more then you will also need a NPN to control the PNP and turn it on/off.

Alex
 

Bheemsen

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please help me ... i want to multiplex two 7 segment displays on single port....so i need a technique of multiplexing program ...m not aware of it...plz help me...any code can do...c or assembly language...
 

Bheemsen

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thank u alex :) will try it and let u knw if any problem..

Bheemsen
 

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