another version for this circuit pls

Thread Starter

whitearrow

Joined Jan 6, 2009
13
hi, can anyone help me to design another circuit which has the same outputs as the one in the attachment? the problem is that it works perfectly in simulation but in reality it is not working. the problem is that pin 12 of U2:B is an input which is determined by its own output. pls help!!!
 

Attachments

beenthere

Joined Apr 20, 2004
15,808
Your truth table is disorganized. In two places, you indicate the output true for A*B* C^ (C not). It also has two listings for a true output for A*B^*C^, which contradicts the two false conditions for the same states.

The circuit has 4 inputs. Perhaps a table based on inputs A, B, C, & D might help. Notice that the U2B gate is a latch function - that is why the output of the OR gate is one of the inputs. The output will be maintained after the top input (A?) has gone low, but the others are maintained.
 

Thread Starter

whitearrow

Joined Jan 6, 2009
13
no i need the two of them. ie A*B*C^ and A*B^*C^ will be a true output. this is becauce a bulb should switch on at point B and switches off at points A and C where the sequence starts from A ie A starts with a true value than B and than C. that is why i have an output serving an input. can you help with this problem

thanks
 

SgtWookie

Joined Jul 17, 2007
22,201
As Beenthere mentioned, you have four inputs.
Or, is the topmost input (to U2:A pin 2) always a 1?
If U2:A pin 2 is always a logic 1, then I'll assume that your inputs are A, B, and C, from the top down on the far left.
If A=1, B=1, C=0 (leaving U2:A pin 2 high) then the output is 1.

If B is then toggled low, there's a problem; the output is indeterminate - how long does it take B to toggle low in relationship to the propagation delays in the gates? If the toggle is instantaneous, it might work - but only in simulation. It won't be instantaneous in the real world.

You need to simplify the circuit, and eliminate the feedback.
 

beenthere

Joined Apr 20, 2004
15,808
You will have to set up conditions where A*B*C^ is true, and A*B^*C^ is true, and use an OR gate to pass either condition. Otherwise, the states are mutually exclusive.
 
Top