Need a easy hack to monitor 1.8V

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#1
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I ran into a problem for an extention on a existing product.

 

The problem is that this needs to work even when the AVR is in reset :(

(It's some times hold in reset and that can't change).

 

I need at signal to a PLC (dry set of contacts) to tell when a voltage get's below about 1.8V (and release above 2-3V not critical ).

 

I plan on using a reed relay for the signal.

The system is powered with 12V.

 

I have not done this kind of work for many years, so I would try to get it working with a LM339 or perhaps a NE555

 

But perhaps there is a better way ?

 

 

 

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The LM339 would probably be a good idea.  You could run it at +12V and ground.  A resistor divider (connected to +12V) would bring +1.8V to one of the comparator's pins, and the voltage under test could cause the output of the LM339 comparator to be either at 0 or +12V.  This signal could turn off/on the relay, which, I assume, is going to switch the dry set of contacts.   I assume that "dry set of contacts" means that the switching mechanism is not controlled by a device that needs an external power source.

 

 

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I'm no electronic engineer but wouldn't the "reset output" of a standlone BOD do pretty much what you want? Assuming the BOD threshold is set to 1.8V then it should assert the signal at that level and because of the hysteresis the things have it presumably won't release it until above 2.x ?

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#2 yes that was what I had in mind.

 

#3 and it was something like that I was looking for. But I can't find them , what is the official name? (BOD and "reset chip"  don't give anything I can use!)

 

 

Add to #3

ok voltage monitor worked :)

Last Edited: Mon. Feb 4, 2019 - 12:55 PM
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Not specifically looking for AVR solutions I googled things like "brownout detector chip" and "micro reset controller" and things and by a curious coincidence found myself back at:

 

http://ww1.microchip.com/downloa...

 

That presents various transistor based solutions. Also mentions these:

Last Edited: Mon. Feb 4, 2019 - 01:01 PM
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I've used these before for a similar "valid voltage" detection. Not driving relay contacts though - that may be too much current for these little things.

https://www.torexsemi.com/products/single-type-voltage-detectors/series/?name=xc61c

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The problem is that they don't like 12V power :(

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Most 431 shunt regulators can reach to nearly 40V.

http://www.ti.com/document-viewer/ATL431LI/datasheet/comparator-with-integrated-reference-slvs5433090#SLVS5433090

combine that with Cliff's mention of AVR180 to obtain current gain and either default low (AVR180 alternative 2) or default high (AVR180 alternative 1) (hysteresis from the transistor's collector, not from the 431's cathode)

NCP431 is an alternative to ATL431 and ATL431LI that has some better, and some worse, characteristics :

NCP431: Voltage Reference, Low Cathode Current, Programmable, Shunt Regulator

This form of comparator is not as low quiescent current as a slow comparator (microseconds of propagation delay versus 100ns)

 

"Dare to be naïve." - Buckminster Fuller

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You could search on "supervisor and reset IC". The Fairchild FM809 is one such device but only covers down to 2.63volts but is limited to a 5 volt supply.

 

edit: spelling of reset (bouncy keyboard)

Ross McKenzie ValuSoft Melbourne Australia

Last Edited: Tue. Feb 5, 2019 - 01:16 AM
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This form of comparator is not as low quiescent current as a slow comparator (microseconds of propagation delay versus 100ns)

An order of magnitude less current and two orders of magnitude greater delay :

TLV6713 36-V Comparator with Integrated Reference | TI.com

 

"Dare to be naïve." - Buckminster Fuller

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sparrow2 wrote:
I would try to get it working with a LM339 or perhaps a NE555

Well LM339 itself need at least 2v as power supply,
.
Maybe you would change requirement, so that instead of LOW-VOLTAGE signal to PLC,
Send POWER-GOOD signal to PLC,
Very simple to implement using an optocoupler such as PC817
İnput of optocoupler is drived by MCU [via 82 ohm res] , in your code. output of optocoupler connocted to PLC.
At reset of MCU, I/O ports of MCU goes to high impedance and release the optocoupler so PLC lose POWER-GOOD signal.
.
.
Edit:[via 82 ohm res]

Majid

Last Edited: Mon. Feb 4, 2019 - 06:19 PM
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Is the signal that drops to 1.8 V the 12V power supply, or is the 12V power supply always good, and it is another signal whose voltage is being monitored?

 

JC

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did a quick check in "reset supervisor"  and that gave a number of links.

there is a TI chip that should be able to do the trick: TPS3808

the only problem is that it has a max voltage of 6,5V but that would mean all you need to to is put a series resistor + zener diode into the design to max out the voltage.

the chip itself only seems to be doing 6uA so if you set the resistor such that 100uA causes 6V with a 6V zener you should be just fine I think (no calculations done yet)

As long as the voltage is below 4V ( for sure) the zener will not do anything so the supply voltage is the voltage to be supervised. above that the zener slowly starts to kick in and limit the supply to let say 6V even when the actual voltage goes to 12V.

But there might be other ICs that have a wider voltage range, this was the first I encountered.

 

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For #12

There is always 12V

 

It's a sensor we feed with 12V and then have a 0-5V output. 

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ncv33161 - window comparator with 1.27V reference.

 

- S

 

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Thanks that is more like it :)

 

I'm actually just now started on a test, using a LM311 because it also has a real transistor as the output.