Precise Thermometer with 3½ Digits LCD Display ICL7106 AD590

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Go to the project BP003 if you want to add a bi-colour LED backlight and a power supply board on the thermometer. In addition, an useful video clip is available there to watch the thermometer in action.

Preface

Fig.1 – The digital thermometer's LCD display in action

Fig.1 – The digital thermometer in action

I wanted to build this project because I needed a thermometer in my house, but also, because I wanted to have the occasion to use a LCD display for the first time.

I have always been fascinated by LCDs. Their capability to make numbers, letters and drawings on a glass plate is something magic. Thanks to this project I learnt a bit more about them and I discovered the tricks of the wizardry. Despite this, and even if the LCD displays have become obsolete, I still think they are the result of an amazing technology.

I chose to use the ICL7106 as the driver for the LCD display. This specific IC by Intersil, has also inside an A/D converter necessary to “transform” the analogue signal generated by the temperature transducer into a digital one. For practical reasons, I decided to split the project in two different sections, and then, in two different boards: one for the LCD display and another one for the ICL7106 IC.

 

Two core components, two sections, two boards

Fig.2 – The LCD display and the ICL7106 boards' placement

Fig.2 – LCD display and ICL7106 boards’ placement

The main reason I decided to split the PCB in two sections, is due to the different circuit complexity required by the two core components. I designed a single layer board for the LCD display section and a dual layer board for the ICL7106 one.

The scheme on the left shows the placement of the two boards and how to connect each other by a 26pin connector.

The low power dissipation of the two core components make the project suitable to build a portable device. For that reason you can take the advantage of the free space under the LCD display board to put a 9V battery. Alternatively, you could use this space to add an ad-hoc 9V 100mA stabilized DC power supply PCB.

 

The LCD display section

The purpose of this circuit it is to connect the display to the 26dip connector that puts in communication this section with the other one.

Fig.3 – The LCD display's schematic

Fig.3 – The LCD display’s schematic

 

Make the LCD display PCB

Fig.5 – The LCD display's PCB mask

Fig.5 – The PCB mask

Fig.4 – The LCD display's PCB layout

Fig.4 – The PCB layout

The PCB is very easy to make. Pay attention only to solder the 26dip connector faced on the bottom layer of the board.

If you notice, in the LCD display shown in figure 1 I removed the reflective surface for a future purpose. For the same reason I designed the PCB leaving the display area empty of tracks. I’ll publish this project later on the “boxed projects” section.

The ICL7106 section

To be honest the ICL7106 is not the only star of this section, indeed, the AD590 is the most important component of the whole project. As defined in the data sheet, the AD590 is a two-terminal IC temperature transducer. In the schematic it is drawn as a current generator, which it actually is.

Fig.6 – The ICL7106's schematic

Fig.6 – The ICL7106’s schematic

 

Make the ICL7106 PCB

Fig.7 – The ICL7106 PCB's layout

Fig.7 – The ICL7106 PCB’s layout

The higher circuit complexity, compared to the other section, is enormous.

Start soldering all the vias in order to connect electrically the two layers, and then all the components. All of them have to be placed on the top side of the board.

A very important warning about the 26pin connector. Check accurately, using a multimeter, to have soldered properly the connector to the LCD display board and the vias in the ICL7106 one. As you can notice in the scheme in figure 2, once the boards are soldered together, the overlapped part of them will become hardly accessible.

Fig.9 – The ICL7106 PCB's top-mask

Fig.9 – The top-mask

Fig.8 – The ICL7106 PCB's bottom-mask

Fig.8 – The bottom-mask

Now you are ready to put in communication the two circuits soldering the 26pin connector, and then the LCD display board, to the ICL7106 one.

Power on the PCB. You should see some values on the display. If so, follow the next instructions; otherwise check better to have soldered everything properly.

 

Calibration

Power off the circuit and adjust the AD590 compensation circuit by setting the trimmer R4 to obtain:

R4 + R5 = 8,895 Ω

The trimmer R3, instead, changes the sensitivity. Power on the PCB and wait a while until the values displayed become stable. Measure the voltage between the ICL7106’s pin31 and GND, then, find T using the formula:

T = 8.9 / VPIN31

That is the temperature in Celsius you are measuring. Turn the trimmer R3 until you can read that value on the display.

 

Download the project

Pressing the button below you can download a compressed file with all the necessary material to build this project:

  • the AD590 and the ICL7106 data sheets;
  • the pictures of the final project, the schematic, the PCB masks and the PCB layouts as shown in this post;
  • the schematics and the boards files in Eagle format.

If you need some help please do not hesitate to contact me or leave your comments below. Enjoy it!

SP0003 (4.3 MiB, 336 downloads)

 

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Grazie di cuore, Andrea Dal Maso

 


 

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