how does this circuit work?

Thread Starter


Joined Dec 10, 2013

Can someone please explain to me how this thing works? Like if I gave the A input as 0100 and B as 0011 (for example), how does 4 and 3 get displayed on the A and B displays respectively? I'm confused at how the multiplexer works, and what the purpose of the decoder at the bottom of the circuit is.

Help would be appreciated.


Joined Oct 18, 2012
It works by quickly switching two 4-bit values, having been decoded by the 74LS47, to drive the two seven segment displays.

The 74LS157 switches between A and B as the output.

Is there a particular portion you are having trouble with? Have you looked at the datasheets for each IC mentioned and understand how it works?


Joined Nov 9, 2007
OK so, as is often the case, it is easiest to work back from the end (output ) to the beginning.

You have two common cathode displays.
This means that the anodes in the displays are accessible, and will light a particular segment if supplied with power and the single cathode is grounded.
So to show a number on one display all we have to do is to select the appropriate anodes and keep the cathode permanantly grounded.
However we have two displays and one input decoder (the 74LS47).
So if we send the decoded information to both displays (note they are wired in parallel) at the same time but only ground one of them then only that one will light.
If we now switch to the other display it will light instead.
The purpose of the 74LS139 is to achieve this switching between the cathode grounds, in synchronisation with the input IC (multiplexer 74LS157) selecting the A input or the B input.
Hence its connection to the data select output of the MUX.

So the purpose of the multiplexer is not only to select (swicth between) inputs, but also to select outputs, in sync with its selection of inputs.

There is no clock circuit shown, although the pulses are shown in blue, so this circuit would need some additional circuitry to function as it is not capable of generating its own clock pulses.


Joined Jan 2, 2010

Well Yes, The mechanics for multiplexed systems is that you mentioned.
But in the displays will be no illuminated number. unless we see the figures of the numbers as segments off.

This is because the 74LS47 IC requires a 7-segment display common anode.
and you look at the attached drawing by OP is common cathode.


Joined Nov 9, 2007
Good catch Mr Carlos, I should have picked that up!

This circuit does indeed require common anode displays.

Further the 7447 is an open collector device and requires a pull up resistor to + to complete its action.