On 5/1/2013 9:28 AM, Richard (Rick) Karlquist wrote:
Do you want a 1 out of 8 output or binary coded output?
If the former, you could use my beverage selector circuit, which uses
a single IC. One of the tricks for doing this is to use SPDT
pushbuttons. It makes this much easier.
Rick N6RK
Additional clarification:
Each pushbutton is associated with a D flip flop.
Each NC contact is connected to the D input of its
corresponding flip flop. Each NC contact also
has its own pull down resistor. All of the NO contacts
are wired in parallel and connected to all of the
clock inputs of the flip flops. There is also a
pulldown resistor on this node. The moving contacts
of the pushbuttons are all connected to the +5V
supply.
When a button is pushed, it initially causes the
D input of its flip flop to go down while in transit.
When the button reaches the NO contact, it pulls up
on the clock inputs of all flip flops. The output
of the selected flip flop goes low and all the others
go high. When the button is released, the clock input
goes low and the flip flops are locked in the new
state. It does not matter if the NO contact bounces
and produces spurious clock pulses.
Now you can see why using SPDT pushbuttons is the
magic bullet.
The 8 flip flops can be implemented by using an
octal register, hence only 1 IC needed.
Rick N6RK
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