DS1621 pc thermometer

Thread Starter


Joined Apr 10, 2007
i made the project DS1621 pc thermometer From electronics-lab.com
http://www.electronics-lab.com/projects/pc/010/index.html or From

i'm a newbie for electronics so There is something i want to ask?

1. about voltage limiters (R1, DZ2, R2, DZ1) on the SCL clock input and SDA data input.
how to look for the value for resistor and dioda zener for that project, i mean 4700 ohm and 5 volt dioda zener, what formula used in searching the value for that resistor and dioda zener. ? how to count it ? until we get the value for resistor and dioda zener?.

2.i'm learning the source code, but there is something i don't understand.

temperature = (temperature_int * 256 + temperature_frac) / 128 * 5 / 10
If temperature_int >= 128 Then
temperature = temperature - 256

i don't understand about the code, does anyone can help me to explain this code and explain the formula code? Please help.

i'm sorry about the english sentence
i'm doing my best to write it.
Thank You

Please Help me explain this project!!!!


Joined Apr 20, 2004
1. You read the data sheet for the temperature chip to determine the current it/they need. The zener linits the voltage and the resistor limits the current.
2. The data sheet will give you the information you need to interpret the code fragment. It has to do with the way the chip outputs it temperature conversion.


Joined Jan 22, 2004
have you tried presenting your question to electronic-lab forum where you got those schems. i'm sure they are more than willing to assist you since they are the ones presenting this project. "audioguru" is there to answer your query.

as to why the author chose the 4k7 & 5v zener only he can exactly tell you why. but i presume he based his decision on the datasheet of that device. use ohms law ( I = E/R ) to derive the current.


Thread Starter


Joined Apr 10, 2007

that is being my problem !! because i don't really understand how to read the datasheet DS1621 cause is to many symbol i don't understand.??


1. which current must i use in data sheet to count mathematics ( I don't Understand how to read the data sheet " and My english not very well"), which to find why the resistor and the zener 4k7 & 5v.? please tell me how to count mathematic?

2. why the pin SDA must connect to CTS and DTR line and SCL connect to RTS Line? why does not connect to Tx or Rx in serial Port.

3. how much the voltage output from DTR and RTS which the voltage for DS1621 take from that line.


Joined May 16, 2005
The 5.1V zener is used because the DS1621 cannot handle signal levels greater than VDD+0.3 and VDD is 5 Volts. Any zener between .7VDD (3.5V) and VDD+0.3 (5.3V) could be used. See page thirteen of the datasheet.

The 4700 ohm resistors limit current drawn through the zeners to about 1.5mA if the RS232 is running at 12V.

How this thing works without connection to Rx is beyond my ken. Perhaps he uses software to monitor CTS. I suggest asking Mr. Riccibitti himself: http://www.riccibitti.com/e-mail/e-mail.htm He desinged it, who better to explain its workings?:)

Thread Starter


Joined Apr 10, 2007
The LM2936Z5 convert the voltage from PC to 5 Volt about project in Wesite http://www.riccibitti.com/pc_therm.htm

But how the LM2936Z5 have enough Power to powered the Two Sensor DS1621.
The First Sensor DS1621 need 5 Volt, The Second One Need 5 Volt To
So Total Need 10 Volt
So What I'm Ask in Here, How The Regulator Voltage LM2936z5 Power the sensor if the output voltage lm2936z5 just 5 volt.? That mean he cannot powered the other sensor.

So does anyone can Explain This To me.?

Sorry About The English
I try my best To write in English Thank You

Thread Starter


Joined Apr 10, 2007
I want to ask about the project http://www.riccibitti.com/pc_therm.htm

In that schematic using two Dioda 1N4148, rs232 uses between -12V and +12V.
If it's true that this dioda eliminate the negatif voltage and only allow the positif voltage through dioda.!
In that case the output from the dioda must be 12V, and the one should be 12V to. so total output voltage from both of dioda must be 24V, and then this Voltage filter by capasitor47uF/16V.
I'm Checking the output voltage from dioda using multimeter.
I measure the output two dioda betwen the output from the two dioda and capasitor, but the result when i use multimeter to measure dioda it only 10V.
why the output 10V, and why not the 24V.
please I need an explain why it diffrent.

I'm very sorry about the lack of my english.
i'm trying my best to write it in english.
Thank You.


Joined Jan 28, 2005
Most likely the explanation is that the RS-232 driver in your computer is not outputting standard RS-232 levels. This is true of many of the +5V logic to RS-232 signal levels converter chips. Some of the MAXIM chips I have used only output +8 and -8 RS-232 levels.


Thread Starter


Joined Apr 10, 2007
So My Computer output standart RS232 level is between -5V and +5V. it's okay if i have this output standart in my computer.?
It's It true that many computer have diffrent output rs232 level.?
the Dioda 1N4148 eliminate the negative voltage -5V and only allow the positive voltage +5V through Dioda.It's it true with this statement.?