[TriEmbed] Protection from Dual 12V inputs

Shane Trent shanedtrent at gmail.com
Sat Mar 12 12:48:39 CST 2016


Robert,

You can reconfigure the NAND gates to get non-inverted outputs. I had to
use an on-line simulator to convince myself it would work. I really liked
the logic.ly demo site. The image below shows the circuit locking out the
second output because the first is already on.

http://logic.ly/demo/

[image: Quad NAND Lockout .jpg]

You could also just add a single NPN transistor and a couple of resistors
and allow one input to override the other input (pulling it down on the far
side of that pin's current limiting resistor), forcing it's output low. The
single transistor approach has one drawback/benefit by allowing you to
select which input would have priority and predefine the output that would
turn on anytime both inputs we driven high.

Shane


On Sat, Mar 12, 2016 at 8:53 AM Robert Gasiorowski via TriEmbed <
triembed at triembed.org> wrote:

> Here's another idea which uses 7400/4011. Keep in mind that outputs are
> inverted.
>
> [image: 7400.png]
>
> On Sat, Mar 12, 2016 at 8:42 AM, Robert Gasiorowski <rgresume at gmail.com>
> wrote:
>
>> Below is a better drawing of my circuit.
>>
>> If you want to get fancy, you could use configurable gate, like 74LVC1G58
>> (you would need one for each channel.)
>> Use A as input, B permanently high, C opposite input.
>>
>>
>>
>> [image: 2ch circuit.png]
>>
>> On Fri, Mar 11, 2016 at 10:16 PM, Pete Soper via TriEmbed <
>> triembed at triembed.org> wrote:
>>
>>> Hi Brian,
>>>
>>> (Here's your original msg
>>> <http://mail.triembed.org/pipermail/triembed_triembed.org/2016-February/002402.html>
>>> for others to catch up on if they're interested. I should also point out
>>> that folks can set their mail to "digest mode" and get a single msg a day
>>> if this spurt of traffic gets too bad. One person unsubscribed yesterday.)
>>>
>>>   As one relative beginner to another, let me ask what's between the
>>> level translator and your relays? You sort of imply below you might be
>>> expecting the level translator to deliver a ton of current despite it going
>>> through the 470 ohm resistor (limit with 5v source is about 10ma). It's
>>> hard to picture the level translator operating even the most puny relay.
>>>   But Robert's circuit is predicated on the MOSFET gate needing very
>>> little current (i.e. with 5v out of the level translator about 10ma). The
>>> other transistor has to be able to yank the gate on the other side of one
>>> of these resistors to ground, so making them too small defeats the
>>> either/or aspect.
>>>   But it really needs resistors in the path from each level translator
>>> to the transistor base Rob accidently connected it directly to. Without
>>> those it's a survival duel between too high current through the level
>>> translator or the transistor base to emitter path and one or both parts may
>>> be destroyed instantly or at some random, inconvenient time. The base
>>> resistors could be a few k ohms and even the crummiest transistors would
>>> sink the 10ma to prevent the MOSFET connected to its collector from
>>> switching on (the 470 ohm resistor is kind of like a bus pullup in this
>>> situation).
>>>   Now that we're up to two transistors and four resistors, a single
>>> 74LV04 (Mouser part 771-LV04N112) seems attractive.  One of it's
>>> inverters could feed a complement of a single MCP23008 line to one
>>> relay(driver?) and the original signal would drive the other one.  But this
>>> LV chip is wimpy. For more drive the 74AHC04 would work as below. Or you
>>> could use the LV04 in between the MCP and the level shifter (i.e. sticking
>>> with something you know is working now).
>>>
>>> -Pete
>>>
>>>
>>> On 03/11/2016 02:24 PM, Grawburg via TriEmbed wrote:
>>>
>>> I've tried this and it doesn't work.  I did change the resistors to 220
>>> just to see if that made a difference.
>>> My actual set-up does not use MOSFETs as I originally asked about but
>>> instead uses two DPDT relays.
>>> The input voltage goes through a level shifter since I determined that
>>> the 3V3 through the resistor was too close to
>>> the 2V5 that produces an unknown state in the relay modules. So the
>>> output from the shifter is 5V which does work fine. I still need to
>>> be able to make sure a bad code doesn't allow both relays to activate.
>>>
>>>
>>> Brian Grawburg
>>>
>>> ------------------------------
>>> -----Original Message-----
>>> From: "Robert Gasiorowski" <rgresume at gmail.com> <rgresume at gmail.com>
>>> To: "Grawburg" <grawburg at myglnc.com> <grawburg at myglnc.com>
>>> Cc: "Triangle Embedded Devices" <TriEmbed at triembed.org>
>>> <TriEmbed at triembed.org>
>>> Date: 02/24/16 05:34 PM
>>> Subject: Re: [TriEmbed] Protection from Dual 12V inputs
>>>
>>> Ooops, forgot to add base resistors.
>>>
>>> On Wed, Feb 24, 2016 at 5:23 PM, Robert Gasiorowski <rgresume at gmail.com>
>>> wrote:
>>>
>>>> How about something simple like this:
>>>>
>>>> [image: Inline image 1]
>>>>
>>>>
>>>>
>>>>
>>>
>>> --
>>>
>>> *[This was sent from a PC running Debian 7, 64-bit Linux. No Microsoft
>>> products were used.] *
>>>
>>>
>>> _______________________________________________
>>> Triangle, NC Embedded Computing mailing listTriEmbed at triembed.orghttp://mail.triembed.org/mailman/listinfo/triembed_triembed.org
>>> TriEmbed web site: http://TriEmbed.org
>>>
>>>
>>>
>>> _______________________________________________
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>>>
>>
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