Talkback for article: 365, February2005

A digital thermometer or talk I2C to your atmel microcontroller

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From: Rodrigo Fagundes <darktopera(at)> [ date: 2005-02-04 ]
Very interesting this article, a very good work. But I did not understand yet why to use I2C? I┤m used to build a simple inverter circuit (with transistor, resistors and two clamp diodes), and then I can invert and protect the atmega8 serial output, and the computers I tested worked very well even with 0 to 5V signal inverted (without the -10 negative signal).
Thanks :)
From: Mark <montafon(at)lycos(dot)com> [ date: 2005-02-04 ]
I have to agree with Rodrigo. Two transistors and three (or better four) resistors is enough to communicate over rxd/txd.
From: Guido Socher [ date: 2005-02-04 ]
Hi Rodrigo and Mark,
I know that but those transistors and resistors are still much
more than the parts used here.
The software is more complex here but software can be copied once
it is written.

From: Troy [ date: 2005-02-07 ]
There are external I2C bus connection on most PC, The mouse and Keyboard coneecters, they us a bus called the SMbus but from what I understatnd there is very little difference between them, and one could use the mouse connector to talk I2C to a slave device.
From: Jay [ date: 2005-02-22 ]
Thanks for this article-- its great. I've been planning to try and find a way to interface a mac to a device that has I2C as its only interface (a devantech SRF10 sonar unit.) I think I can work up a solution using the code you've provided and a USB to RS232 adapter.

If you do a followup article, I'd appreciate a little bit more detail about how the software works, and how the RS232 connection to the AVR circuit emulates I2C. It wasn't clear to me why the diodes were there and why the resisters were inline instead of tied to Vcc.

But much thanks! I won't be building the temperature circuit, but I will be building some sort of an RS232 to I2C cicuit to interface with these I2C devices.
From: Bob [ date: 2005-02-22 ]
Thanks a lot for this wonderful project !

But is there a way to add a fan monitoring ??
From: Edouardo <edouardo.manza(at)> [ date: 2005-02-22 ]
I agree with Bob, the best would be
but with an avr for the rs232 interface

From: essadi <essa842000(at)> [ date: 2005-06-25 ]
je ss tres satisfe de tout se qui est ecrite

From: Joeri <joeri(at)> [ date: 2005-07-04 ]
Hi Guido, it looks great and I just bought the package from the Tux shop...
One thing is not clear to me: to program the Atmel, you need to connect to the "load" plug, and issue the commands "make, make load" and after that has been done you can connect the RS232 to the "regular" plug (with the SDA/ SCL signals).

Thanks, Joeri
From: Guido Socher [ date: 2005-07-04 ]
Hello Joeri,
yes, that is if you have a rs232 based programer. If your programer
uses the parallel port then you do not re-connect anything.

From: Joeri [ date: 2005-07-07 ]
Thanks Guido, but I actually meant "in circuit" progranmming the device. So I dont have a separate programmer... just the "I2C digital thermometer" circuit.
From: David.fang <david_fang_cn(at)> [ date: 2005-10-23 ]

If you are sourcing NTC sensor. me! it your right contact for the cost and quality reason.

David Fang

Sensor advanced market Manager
Hengshinki Elec. Co.,

From: success chinwe <success4real2007(at)> [ date: 2005-10-26 ]
oh i'm very grateful to get to this site. realy i 'm presently writing a project on design , construction and testing of digital thermometer... i have gotten enough information here and as well hoping to get more
From: Rolf <rolf.freitag(at)> [ date: 2005-11-01 ]
Interesting, but i do prefer a DMM like the VC 820 at a serial port and with a PT1000 resistor for temperature measurement because the PTs are calibrated and long time stable. I'm using the PTs from 2 up to 1000 Kelvin.
And you don't need a microcontroller for I2C; you can use bit banging via the parallel port but you need hard realtime e. g. with the RTAI patch.

From: Stefan Groi▀meier <punkundercover (at) gmx .de> [ date: 2006-06-06 ]
It┤s a great Article!

I want to build an pc interface via USB, that is very easy to build and that can be used on different operating systems.

I have found some I/O expander for the I2C Bus and analog digital converters. The new interface should be a basis for communication for external hardware.

Has anyone tried to simulate the I2C Bus with an RS-232 over USB adapter (/chip) ?


From: chicolesky <chicolesky(at)> [ date: 2006-10-12 ]
muy bueno, lo implementaremos a unos sensores de polvo, pero ahora tengo que convertir mi se˝al del sensor a voltaje, ya que el sensor que ocupare entrega de 4-20 mA
pero el esquema me sirve
From: Wolfgang Tomaschitz <wolfgang(at)> [ date: 2006-10-30 ]
Thanks for this very nice Produkt and the Software. I am a absolutly beginner
to build electronic circuits, and have bought the package from the Tux shop.
I dont have problems to build it, and now I can put the output into a postgres databaseserver. The little weatherstation can be fond at
so everybody can see the functions.The Themperature will be actual every 30 min.
It stays in the northern part of Austria.
Greetings from Austria!.

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