--- In Electronics_101@..., "davidglaister"
<david.glaister@v...> wrote:
Thanks again for the help, can I just run this past you to
make sure I understand things.
I connect pin 3 of the 555 to the pin 15 of the '157
Then on the '157 connect:
Pin 8 to ground
Pin 16 to +5v
Pin 1 to 0v or +5v to change the direction of the count.
Pins 5 and 3 to ground
Pins 2 and 16 to +5v thru a resistor
pin 4 to the up wire of the counter
pin 7 to the down wire of the counter
That's correct. Note, however, that you might have to switch the UP
and DN connections if the counter doesn't move the direction that you
have in mind.
If pins 2 and 16 are connected to +5, what should the value of the
resistor shown in the diagram be?
Pullup resistors are not that critical. Anything from 1K to 10K
should work just fine. However, the more inputs that you tie high
the lower value you should choose. I generally use 2.2K for up to 6
or so inputs.
One other thing. I mentioned the problem of glitches on the outputs
of the '138. The way that you have the counter stages wired, this
problem will be exacerbated because you have the output of the first
stage clocking the second stage. Because of that, you'll have an
invalid transitory state for the duration of the propagation delay of
the second stage.
You can reduce the glitch width by using a synchronous counter - one
where all of the stages are clocked simultaneously. When all of the
stages are on the same chip (i.e. the propagation delays are
virtually the same) you minimize the false transitory states.
An example of a two stage synchronous counter using J-K/ flip flops
can be seen here:
Note that this circuit only counts when the STEP input is high. When
STEP is low it holds its state.
Don
