Positive and negative supply

[FvM]

Apparently the photo shows a sensor unit, not the WS-2300 weather station.

 

[Audioguru]

I forgot to say that the output of the first transistor charges its output coupling capacitor then this capacitor drives the base of the second transistor way below ground and causes the emitter-base of the second transistor to have avalanche breakdown that damages the transistor.
If the horrible transistor circuit was designed properly then its huge 12Vp-p output swing would damage the microcontroller.

 

[KlausST]

Hi,

the description is not clear: 200mV pp.
Does it mean: High = +100mV; Low = -100mV?

In the link to the weather station you gave i find no such voltage level. It says RS232...
But for RS232: High = -3V or below; Low = +3V or above.

Klaus

 

[d123]

Hi!

I had a quick look at comparators on TI (I know nothing about ATMega or PIC,s or comparators by device), the LMP7300 is a flexible looking comparator with hysteresis and at 5V has under 1mV noise/input offset voltage, no doubt no use to your needs, but perhaps worth a look, or at any manufacturers web for product comparison charts.

This is TI's comparators page, if you want:

https://www.ti.com/lsds/ti/amplifiers-linear/comparator-products.page#

I think you just want to know how to connect one to the weather station signal. At a guess, the high signal from the weather station would go into the inverting or the non-inverting input (depending on how your circuit works).

That phone connector is an RJ11, Tx and Rx, +V and 0V.

 

[I14R10]

No, it's low 0V and high 200mV.
Thanks d123, I found out those two https://www.ti.com/lit/ds/symlink/lm392-n.pdf **broken link removed**. They have both op amp and comparator in one package.

 

[KlausST]

Hi,

it's low 0V and high 200mV.

This is good. Then almost any logic level output comparator will work.

No need for extra parts (except from VCC capacitor and eventually a pull up resistor), no amplifier, no transistor.

Klaus

 

[I14R10]

How should I set the reference voltage for comparator? With potentiometer?

 

[KlausST]

Hi,

Use any reliable/stable voltage, even VCC could do, then use two resistors as voltage divider to generate 100mV and add a capacitor to GND as low pass filter.

Klaus

 

[I14R10]

Would this comparator be good https://www.ti.com/lit/ds/symlink/lm392-n.pdf ? They have not written what the slew rate is, but they have response time: 1.3us. Whatever that means, it's probably fast enough. The shortest signal that weather station sends is about 500us wide.

 

[d123]

Hi, sparse datasheet that one . Not sure, in principle, so long as it's as fast as - and preferably faster than - your signal. If you know the off time between weather station signals, and it's longer than 1.3uS then it should be fine.
In my ignorance, not sure if you actually need something that includes both devices, a comparator alone should be enough, shouldn't it? As long as it's output voltage is high enough/low enough to speak with the ATMega. Output voltage swing (from rail to rail) or gain (e.g. 100V/V) will tell you that. This one says the comparator gain is 50 - 200V/mV with supply V+ at 15V, and then the amplifier says 25 - 100V/mV at +15V supply.

- - - Updated - - -

Hi again! Hope all is going well.

I looked at the ATMega1284, and basing this comparator on its 5V supply voltage, assuming a lot of things like the input voltage to the ICP pin has to be roughly speaking above 4V for high or below 2V for low*. Trawling a few comparators to see (selecting devices is a bit of a nightmare at times...), this one looks okay.

Page 11 has a graph showing input voltage to output voltage; it looks "simple" to wire up as an interface between your devices (following KlausST's recommendations above, I'd imagine); low level output is well below 2V; input offset voltage is only max. 5 - 10mV; worst case speed 5uS; it's cheap (I'd guess approx. $0.60 retail). Disadvantage is that it's a dual package - you only need one, I think. It's a SOIC package, could be worse soldering-wise.

https://www.ti.com/lit/ds/symlink/tlc393-q1.pdf

*Also, page 4 of this document is quite useful to compare logic levels between device some types (mainly TTL and CMOS), should you have any need to compare:

**broken link removed**

 

[KlausST]

Hi,

Would this comparator be good

as said before:

No need for extra parts (except from VCC capacitor and eventually a pull up resistor), no amplifier, no transistor.

almost any logic level output comparator will work.

Klaus

 

[I14R10]

I don't understand what you wanted to say, can you clarify?

 

[KlausST]

Hi,

The device you refer to is a combination of comparator and amplifier.
But you don't need the amplifier.

Your device is just more expensive, more supply current, more connections to make....But you can use the device if you want.
(SInce datasheet says the comparator output is logic compatible)

My advice is to keep it simple.

Klaus

 

[I14R10]

I understand. What comparator would you recommend?