From: Spehro Pefhany
Subject: Re: Digital circuit help needed
X-Newsreader: Forte Agent 1.92/32.572
Date: Sun, 15 Dec 2002 18:49:20 GMT
NNTP-Posting-Date: Sun, 15 Dec 2002 13:49:20 EST
On Sun, 15 Dec 2002 19:08:22 GMT, the renowned "Bob Pierfelice"
>I have a 8 bit input that I need to use to control 12 relays. The relays in
>use will all be turned on and off at the same time. They may be randomly
>selected like 1,4,6,12.
>I looked at the 4 to 16 latching decoders but they only select one output
>line. I'm thinking I may need some sort of storage (flip-flop?) at the
>output of the decoder and "walk" through the desired 1..12 channels, storing
>the active ones. The relays are controlled with Darlington transisitors.
A few ways come to mind immediately:
Two 8 bit parallel latches, use 6 bits out of each, 6 data lines + 2
clocks.Eg. 74HC374 (not expandable)
Two 8 bit serial in parallel out SRs with output latches. 1 data
line, 2 clocks (expandable indefinitely, and you get 16 lines for the
same complexity as 12 with this setup) Eg. 74HC595. This methiod is
nice if you have hardware SPI available, and is easily opto-isolated.
Two 8 bit addressable latches- two data lines, one clock (or one data
and two clocks, or one data and one clock and one additional address
line plus an inverter) and 3 address lines. Eg. 74HC259 Expandable.
Random access. You get 16 lines with just the two 74HC259s.
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