[SOLVED] latch d output of decoder

[cgchayan]

Hi,
I am using 74154 decoder to operate 16 different appliances. But since only 1 o/p of decoder is active at any point, I cannot keep multiple appliances on simultaneously.. somehow I need to latch d outputs of the decoder, so that any o/p once made active will remain so until made explicitly inactive..

I guess it's a naive question.. but i really need your help guys..need to implement it in h/w, so preferably suggest me a complete circuit..

Thanks

 

[majestic.eda]

You can feed each of the 16 outputs from decoder to the Clock input of 16 individual T-FlipFlops (positive edge-triggered). Connect all T-inputs to '1' (Vdd) permanently.

When any of the decoder outputs goes 0 -> 1, the bit in the T flip-flop will toggle (assume it is '0' initially).

Now, for eg., when the decoder o/p becomes 0001_0000_0000_0000, device number 4 is switched ON. It will remain ON unless the same decoder output arrives again (because T flip-flop will toggle only for a 0 to 1 transition at its clock input).
So if the decoder output goes to (say) 0000_0010_0000_0000 in next cycle, device number 4 remains unaffected (ON) and simultaneously device 7 also turns ON.

The limitation here will be that you will be applying same 4-bit input code to turn it ON/OFF.. so you'll need some mechanism to keep track which device is ON and which one is OFF. I think you may put some LEDs in parallel at each flip-flop output (for user convenience).