# From 5-24VDC ---> TTL.. HELP PLEASE!!!!

Discussion in 'Electronic Basics' started by Henry, Apr 11, 2004.

1. ### HenryGuest

Hi, I'm working on an interface board that have to get state inputs
from sensors of many types with signals ranging from 5VDC to 24VDC
(i.e. signals from 5 to 24V mean logic "1"), the problem is that this
interface should be able to work with direct TTL outputs from sensors,
too. The problem is that I was thinking about using optoisolators to
isolate the target system but I know that a TTL signal won't be able
to drive the LED from the opto, so, how do I do this???
I can use a 5VDC and 12VDC power supply.
Any help with ideas and/or schematics would be appreciated.
Regards

Henry

2. ### John FieldsGuest

OUT goes high when TTL input is high:

+5V +V
| |
E |
TTL>---[R]---B PNP [R]
C |
| +---->OUT
[R] |
+--|------|--+
| |A C |
| [CR]-> B |
| | E |
+--|------|--+
| OPTO |
GND>----------+------+---->GND

OUT goes high high when TTL input is low:

+5 +V
| |
[R] [R]
| |
| +----->OUT
+--|------|--+
| |A C |
| [CR]-> B |
| | E |
+--|------|--+
| OPTO |
| |
___ C |
TTL>---[R]---B NPN |
E |
| |
GND>----------+------+----->GND

3. ### John PopelishGuest

I think I would make a universal input with comparators, like the quad
LM339. Connect the inputs through a high resistance voltage divider
with overvoltage clamping to protect against spikes. If the LM339 was
powered from 12 volts, it responds to inputs up to 10.5 volts.
Connect one side of all comparators to a forward biased diode (.6
volts) and divide all inputs by about 3 (say, a 200k to a 100k
resistor to ground) (8 volts max. from a 24 volt input, .8 volts max.
from a loaded TTL output of 2.4 volts. If you don't need microsecond
response, load each of the resistive dividers with some capacitance to
ground to improve the noise rejection.

4. ### Spehro PefhanyGuest

If you can guarantee that the low level input voltage is <= 500mV for
all signal types you can use something like this (at least up to a few
hundred kHz):

1/6 74HC4050

+5
|
in | \
o--[10K]--x-------x------------| ----> 0/5V
| | | /
[100K] /---/ |
| / \ 12V 0V
| --- 500mW
| | 1N5242
0V 0V

Best regards,
Spehro Pefhany

5. ### Fritz SchlunderGuest

Greetings,

Hmmm... IIRC TTL only guarantees an output high of 2.4V. The datasheet for
the 74HC4050 suggests it has a minimum input high requirement of 3.15V when
operated from 4.5V supply. After feeding through that resistive divider you
are only guaranteed 2.18V highs. Assuming the sensors wouldn't mind this
could be remedied by putting a pull up resistor on the input line to +5V.
Of course if the sensor does output 24V, then some current would flow into
the +5V rail and try to pull it up, so make sure it is properly loaded down
to stay at 5V.

Datasheet for the 74HC4050 here:

http://www.fairchildsemi.com/ds/MM/MM74HC4050.pdf

If the speed requirements were really slow and other requirements were
reasonable I think I might consider just making an inverter using a bipolar
junction transistor with pull up resistor and large base resistor. I might
throw in an extra diode in series with the base to raise the turn on
threshold somewhat. Alternatively one might consider just buffering the
signal with some device like the 74HCT14 fed from a large value current
limiting resistor. Of course this isn't fully orthodox practice since it
makes use of the input protection diodes, but it will more likely than not
work just fine (provided the input current limiting resistance is large
enough).

6. ### Jan PanteltjeGuest

That will work, if you can find the right kind of junction FET you
can save 2 resistors.
Did this once with a mpfxxx , in such a case the current is limited.
Not very reproducable, so different for different FETS.
JP

7. ### mindspringnewsGuest

Look at data sheets for 2N7000 fetlingtons and the older 2N2222. These were
designed for TTL level translation circuits and are available in quad DIP
packages. There are also TTL input or output bus driver type IC's, for
multiple lines. Also look at line drivers.

http://home.btconnect.com/woodgreen/tech_tips/techtip/art8.htm 2N7000
examples

You could also use a quad open collector output comparator IC to drive a

http://www.national.com/ds/LM/LM139.pdf
http://www.nteinc.com/specs/3200to3299/NTE3220.html

Or one of these http://www.linear.com/pdf/lt1161.pdf LT1161, for more
demanding needs / POWER.

8. ### budgieGuest

Use any 74HCT buffer or inverter, with input via a suitable resistor and with a
diode clamp on the HCT input.

9. ### j.b. millerGuest

Easiest way would be to use an IDC5 optocoupler...as they were designed for
this application...
jay

10. ### Roger GtGuest

X-No-Archive: yes
: Hi, I'm working on an interface board that have to get state
inputs
: from sensors of many types with signals ranging from 5VDC to
24VDC
: (i.e. signals from 5 to 24V mean logic "1"), the problem is that
this
: interface should be able to work with direct TTL outputs from
sensors,
: too. The problem is that I was thinking about using
optoisolators to
: isolate the target system but I know that a TTL signal won't be
able
: to drive the LED from the opto, so, how do I do this???
: I can use a 5VDC and 12VDC power supply.
: Any help with ideas and/or schematics would be appreciated.
: Regards
: Henry

For a universal logic input a 74C914 is useful, providing the
grounds are common.

For an output a 74LS07 with a pull up will allow voltages to 40
VDC. So 24VDC is really not a problem.

Do you also need to generate the voltage (24VDC) to provide the
output?

There are many converters for this purpose, 12V in 24V out.

11. ### HenryGuest

Hi Jay, do you know where I can get them? (I tried mouser, jameco and
newarkinone and I couldnt find them) Are they easy to implement?
Regards

Henry

12. ### Spehro PefhanyGuest

These are industrial opto-isolated input modules, cost about
\$10-\$20/each. Digikey and many other distributors should have an
equivalent if that's what you are looking for- okay, try this number:
Grayhill 70M-IDC5

Best regards,
Spehro Pefhany

13. ### HenryGuest

Hi, the idea is that I have to design the circuitry because a
constraint on the hight of the board because it is supposed to be
packed on a low profile case (no more than 15mm). I looked at the
module and it looks perfect for my case but it is 1" tall... so there
is no way I can fit it on the case.
So I have to use discrete components to create the interface.
Thanks anyway for your suggestion... I wish I could use one of those..
I saw they even work with a mixed VAC/VDC input... how can they do
that?
Thanks to everybody who's helping me out in my quest.
Regards
Henry

14. ### mindspringnewsGuest

Do you need optical isolation? I kind of suspect you do as most likely
interfacing with a uPC and mixed 5 and 24V signals in. If that's the case,
best to isolate and not even tie the commons together, which rules out some
of the level shifting digital IC's. Some Opto isolators have parallel
opposite LED's on the input and are used to do the AC signals. I have two
circuit designs that will work for your 5 - 24 Vdc input signals, but will
take a couple hours of my time to draw up, if you have not found a solution
yet. Need more info on your needs too, as to PCB space and # of I/O points.

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Hi, the idea is that I have to design the circuitry because a
constraint on the hight of the board because it is supposed to be
packed on a low profile case (no more than 15mm). I looked at the
module and it looks perfect for my case but it is 1" tall... so there
is no way I can fit it on the case.
So I have to use discrete components to create the interface.
Thanks anyway for your suggestion... I wish I could use one of those..
I saw they even work with a mixed VAC/VDC input... how can they do
that?
Thanks to everybody who's helping me out in my quest.
Regards
Henry[/QUOTE]

15. ### Ken SmithGuest

+5V
!
V LED of low current opto

16. ### mindspringnewsGuest

Diagram needs work?