Re: Exposing my ignorance...

On Thu, 13 Dec 2007 18:14:03 -0800 (PST), MooseFET
<kensmith@xxxxxxxxx> wrote:

On Dec 13, 6:21 am, John Fields <jfie...@xxxxxxxxxxxxxxxxxxxxx> wrote:
[.....]
---
Here's a SP18T switch:

View in Courier, all LEDs are high-efficiency 2mA If, ground all
unused HC04 inputs.

I am going to suggest a change to the circuit. Look for the "*"s.
I'll explain below:


Modified:
.+5V----+-----------+------+-------+--------+-----------+
. | | |K | |8 |
. [10K] [100K] [1N4148] [220K] +---+---+ |
. | | | | 2|_ Vcc | |
. +--[0.1µF]--+------+-------|---O|T | |
. | | 6| | [0.1µF]
. | +----|TH 555| |C1
. | O | 7|_ |3 |
.S1-->| +---O|D OUT|---+ |
. | O | | GND | | |
. | |+ +---+---+ | |
. | [0.47µF] |1 | |
. | | | | |
.GND>---+--------------------------+--------+--+----|---+
. | |
. +-------------------------------------|----+
. | / |HCT04*A |
. | +------------------O< |----+ |
. | | HC164 \ | | [2K] *B
. | | +--------+ | |
. +V | +--|SINA QA|----[LED]------+-------+
. | | | | | |
.[100K] | +--|SINB | !
. | | |__ | *C |
. +---+-|---O|MR QB|----[LED]--------------+
. | | | | | |
.[0.1] | +----|>CP | !

... etc ...

*A the input of a HCT04 is considered high if it is over 2V. For most
types of LEDs, the forward drop is much less than 3V.

---
I doesn't work that way.

The 17 diodes and the inverter make a 17 input NOR gate that decodes
when the first 17 outputs are low and then forces the first serial
data input high. Then the next clock makes the first Q high which
kills the decode and forces the serial data input low. Next clock
propagates the high to QB and the low to QA. That way, as the high
propagates through the register the first serial data input will
always be low until the 18th Q goes high, at which time the 17 lows
from the first 17 outputs will make the input to the inverter low,
making its output high, starting the cycle again with the next
clock.
---


*B Since only one LED is on at a time, the 2K ressitor can be shared
among all the LEDs.

---
Yes, good one!
---

*C LEDs can withstand at least 5V backwards so the one that is not lit
isn't harmed by having the backwards voltage on it. (It doesn't light
up in the process)

---
It's even better than that, since the reverse voltage will be
Vcc - Vf, and for a 5V supply and high efficiency LEDs with a Vf of
1.2V that'll be 3.8V.


--
JF
.

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