I'm currently building a countdown timer with 4 digits of 7 segment display that are connected to a 8051.
To minimize the cost, instead of using a decoder , is there any other ways to implement this? Can a transistor be used to replace the decoder? If so, what kind of transistor should be used? Any circuits diagram available?

 

im afraid there is no other way. there might be expensive alternatives (program a PAL or GAL). but a single transistor cant do that job.

a decoder is a combination of logic gates which are made from transistors. if it had been possible then the decoder wouldnt have been there.

just like the fact that u need a hockey stick and a ball to play hockey, u need a decoder for the 7 seg displays

 

It's easily done, I can't help with 8051 code, but one of my PIC tutorials does this for 2 seven segment displays, the diagram is at http://www.winpicprog.co.uk/pic_tutorial_7seg_board.htm - you could easily extend this to four or more, simply use a PNP transistor for each display, each fed from it's own output pin (my two display version only uses one output pin, but you can't do that with more than two).

Depending on your displays (common anode, or common cathode), you may need to use NPN transistors down to ground, rather than PNP fed from +ve.

The technique is called 'multiplexing'.

 

thanks guys!