# help with d-type bistable

Discussion in 'Homework Help' started by killer6008, Feb 4, 2010.

1. ### killer6008 Thread Starter New Member

Jan 26, 2010
20
0
I have a truth table with sw1 as input and two outputs LED 1 and LED2.

The outputs i am getting are

SW1
H 1 0
L 0 0
H 1 1
L 0 1

After this the sequence starts repeating itself.

Can someone explain what is happening and what will this type of circuit be used for??

Thanks

2. ### JDT Well-Known Member

Feb 12, 2009
658
85
Looks like a 2-bit binary down counter:-
SW1 decimal
H 1 0 1
L 0 0 0
H 1 1 3
L 0 1 2

And then it rolls back around as you say.

You don't say but I assume that you have 2 D-types.

The only thing I am not sure of is that your counter decrements on both transitions of the switch. Some extra logic is normally required for this.

3. ### killer6008 Thread Starter New Member

Jan 26, 2010
20
0
Sw1 is connect to Clk
LED 1 is through SW1 and LED2 is connected to Q.
D is connected to Q-

Its in a logic box 74HCT74.

4. ### JDT Well-Known Member

Feb 12, 2009
658
85
Ah! I see. The D-type changes state (toggles) on the low to high (0 to 1) transition of the switch.

This is a divide-by-two circuit. Used in frequency dividers and binary counters.

Bit more explanation:-
On the low to high transition of the clock input, the state of the D input gets transferred to the Q output.
Because the /Q output (the opposite of Q) is connected to the D input, on every clock the Q output changes state.

Last edited: Feb 5, 2010
5. ### killer6008 Thread Starter New Member

Jan 26, 2010
20
0
thanks for that