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'12 Volt Logic to 5V Logic'
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A PNP transistor + 1 resistor!

Andrew

On Thu, 9 Mar 2000 15:57:11 -0000 Nigel Porter
<nigelporterKELVEDON99.FREESERVE.CO.UK> writes:
{Quote hidden}

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Hi

You might want to use two resistors. This is not a completely fail-safe way
implementing this, but I''ve used it previously with varying results.
Connect as follow:

+12V  (Vin)
|
|
|
=
|   |  R1
=
|
|-------  +5V   (Vout)
|
=
|   |  R2
=
|
|
|
GND

The formula works as follow:

Rt=R1+R2

Lets say we want Rt (total resistance in the series) to be 10K
Then Rt=R1+R2=10K
Vout=5v  (wanted)
Vin=12V (given)

The relationship is:
R2/Rt=Vout/Vin
Therefor,
R2=Vout/Vin*Rt
R2=5/12*10K
R2=4166.66 Ohm

Then, because Rt=R1+R2, then
R1=Rt-R2
R1=10Kohm - 4.16666Kohm
R1=5.833.34 Kohm

Hope it helps!

Werner Soekoe
wsoekoeglobal.co.za

----- Original Message -----
From: Nigel Porter
To: PICLISTMITVMA.MIT.EDU
Sent: Thursday, March 09, 2000 5:57 PM
Subject: 12 Volt Logic to 5V Logic

I have 12 Volt logic signals (0 Volt or 12 Volt signal) which I wish to
convert to 5 Volt logic (0 Volts or 5 Volts signal) levels. I am considering
the use of:-

Opto-coupler

analog switch (4066 or similar)

resistor and 5V1 zener

Can anyone suggest a good method?

I intend on interfacing automotive system to a PIC16F84

Thanks

Nigel

or use diode and variable resistor:

12v
!
!
variable res
!
!------------5.1volts
diode
!
!
ground

then calculate res for adecuate current needed.
Lucky,
Ismael.

Werner Soekoe wrote:

{Quote hidden}

> {Original Message removed}
> I have 12 Volt logic signals (0 Volt or 12 Volt signal) which I wish to
> convert to 5 Volt logic (0 Volts or 5 Volts signal) levels. I am
considering
> the use of:-

I would probably use a current limiting resistor (about 3.3-10K) and a
pair of clipping diodes.

It would be safe and work with anything from 5V up. Well, not exactly,
it could burn something if you plug it to 5,000V ;)

Best regards,

Brusque

<x-rich>The safest bet is to use optocouplers. Voltage spikes and noise tend to
bite you in the rear end when you least expect it. Unless it's going to
be produced in high volume, it's worth the money to get optocouplers.

Cheers

Terry

At 03:57 PM 3/9/00 -0000, you wrote:

>>>>

<excerpt>I have 12 Volt logic signals (0 Volt or 12 Volt signal) which  I
wish to convert to 5 Volt logic (0 Volts or 5 Volts signal) levels. I am
considering the use of:-  Opto-coupler  analog switch (4066 or similar)
resistor and 5V1 zener  Can anyone suggest a good method?  I intend on
interfacing automotive system to a  PIC16F84  Thanks  Nigel

</excerpt>

</x-rich>
part 0 16 bytes
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Nigel;

You must judge the method by the quality and speed of the signal.  If
you have fast signals, you should avoid the plain old resistor dviders
or transistor solutions.  If you have noisy signals, and cannot
withstand brief false logic states, then you also must avoid the above.

If these cases are true, try using quad comparators and cover each input
with a comparator.  This would be one proper way to do it.  Then you can
add a threshold and hysteresis if you desire.  The output can be pulled
up to 5V, or use a totem pole output comparator and no pullup.  You can
invert the state by reversing + and - on the comparator.

I have done the transistor method before, and they tend to start to turn
on at 0.7V.  This is not much, and a bit of noise can whack you over the
head.  You can add a zener in series, but leakage currents can sneak up
on you and damn near turn it on anyhow.  I have found myself sprinkling
capacitors to cover up noise problems.

G'luck.
Chris Eddy

Nigel Porter wrote:

> I have 12 Volt logic signals (0 Volt or 12 Volt signal) which I wish
> to convert to 5 Volt logic (0 Volts or 5 Volts signal) levels. I am
> considering the use of:- Opto-coupler analog switch (4066 or
> similar) resistor and 5V1 zener Can anyone suggest a good method? I
> intend on interfacing automotive system to a PIC16F84 Thanks Nigel

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