Digital thermometer circuit

admin July 21, 2011 47 Comments

Digital thermometer circuit.

CA3162, CA3161 & LM35.

A simple digital thermometer circuit with out a micro controller and having a seven segment LED read out is shown here. The circuit is based on three ICs: CA3162, CA3161 and LM35. CA3162 is a monolithic analogue to digital (A/D) converter that has BCD output. The A/D converter inside the IC is a dual slope type with differential inputs. The IC has an internal timing circuitry and hold function. When the hold function is enables, the output IC latches itself to the present state. CA3161 is a monolithic BCD to seven segment converter IC. It can directly drive a seven segment display and there is no need for current limiting resistors. LM35 is a three terminal precision temperature sensor IC from National semiconductors. The output of LM35 is highly linear and has a scale factor of 10mV/C. The IC consumes only 60uA as standby current and is calibrated directly in degree Celsius.

Circuit diagram.

digital thermometer

Digital thermometer circuit

About the circuit .

IC LM35 is used for sensing the temperatures. A voltage proportional to the temperature will be available at pin 2 of the LM35 and this voltage is coupled to the high input pin (pin11) of the CA3162. CA3162 does the job of converting this analogue voltage in to a BCD format. POT R1 connected at pin 13 of the CA3162 is used for gain adjustment while POT R2 can be used for ZERO adjustment. Capacitor C2 is the integrating capacitor of the A/D converter circuitry inside the IC. The working of the CA3162 is as follows, the voltage applied to the input pin (pin11) is converted into a current (using the built in V/I converter circuit) that charges the integrating capacitor C2 for a preset amount of them. Then the integrating is disconnected from the V/I converter circuit and a reference constant current source is connected to the integrating capacitor. The time taken for the charge to restore to its original value is noted and the number of clock cycles elapsed during this time will be a measure of the charge induced by the input voltage (voltage applied to pin 11). The point of restoration is sensed using an internal comparator which latches the counter and the count is then multiplexed into the BCD outputs and the entire cycle is repeated. The hold pin CA3162 (pin6) can be used for running the IC in different modes. When the hold pin is grounded or left open the IC runs in low speed mode (sampling rate is 4Hz). When hold pin is held at +5V, the IC runs in high speed mode i.e. a sampling rate of 96Hz. When the hold pin is held at a fixed 1.2V, the BCD output latches to the current state. C1 is the power supply bypass capacitor whose job is to bypass noise if any from the power supply line.

The next section of the circuit is the BCD to seven segment decoder plus display driver section. For that purpose CA3161 is used. The BCD output pins of the CA3162 are connected to the input pins of the CA3161. Transistors Q1, Q2, Q3 common anode terminals of the corresponding seven segments displays. Q1, Q2, Q3 are driven by the 4, 3, 5 pins (digit driver pins) of the CA3162 respectively.


  • The circuit can be assembled on a vero board or on a PCB.
  • Use 5V DC for powering the circuit.
  • POT R2 can be used for Zero adjustment.
  • IC2 and IC1 must be mounted on holders.
  • Capacitor C1 must be placed as close as possible to the power and ground pins of the CA3162.
  • Capacitor C2 could be a polyester type while C1 can be a ceramic capacitor.
  • The DC power supply used for powering this circuit must be well regulated and free from any sort of noise.
  • The type numbers of the driver transistor are not critical and you can make suitable substitutions.
  • Hold function can be enabled by providing the pin 6 with 1.2V using a voltage divider network.
  • Vijay Kumar
    October 12, 2018

    What is the use of transistors and how to connect three seven segment display together

  • sam
    October 9, 2018

    is there any kind of programming we have to do in this circuit or in the used IC’s? if any then please provide me with the programming.

  • ale73
    July 5, 2017

    Hi. How could connect the displays The output of displays are not numbered

  • Aldrin
    November 22, 2016

    Hi.. are the 7-segments really connected in parallel and are they not connected to ground? Thanks.

  • thomas
    November 20, 2016

    pin 6 connected with what

  • mostafa
    May 13, 2016 circuit show EEE.what should i do??

  • sanjeev murmu
    October 21, 2015

    i m interested in this project. what should be the current rating for 5V power supply ? plz help.

  • clemo
    March 23, 2015

    after building the circuit what should it supposed to show at first?;
    i keeping getting radom continue numbers also shows EEE,any help of tips?

  • Jerry Van Cleeff
    January 7, 2015

    On IC 1, the pin labled “17” should be “7”.

  • Abdulaziz Qwb
    November 30, 2014

    Can I use for C2 the value 0.22uF ?

    • praveen
      December 3, 2014

      I think 0.22uF will work. If it does not work then use a parallel combination of 0.22uF and 47nF.

      • Abdulaziz Qwbaiban
        December 7, 2014

        I have tried this circuit and it worked! But, there is a huge error in the datasheet, pin 3 of CA3161 should be floating not grounded.

  • Tom
    August 27, 2014

    Looks good but how do you Exactly calibrate it? I have a similar circuit using the same ic’s that I bought years ago, and it has three pots and calibration for low and high. Thanks!

  • Vicky
    July 31, 2014

    Hi …..

    I have a doubt on this circuit……

    How to connect the all seven segment display….??
    If we connect the all seven segment display in parallel with IC 3161 means all display showing the same value or what???????????

    • ale73
      July 7, 2017

      I have understood. In parallel commanded by ca3162 by putting ON OFF the BJTs. Put all display pins in touch a with a, b with b, c with c and so on

  • Naveed
    July 24, 2014

    i made this project on project board but when i put 0.27uf capacitor the circuit shows EEE. What is it mean and how can i solve this issue.

    March 16, 2014

    CA3161 & CA3162 are out of stock in all the electronic shops of my area.Can U please suggest the alternative devices for CA3161 & CA3162 with same functionality & not much modifications in the existing circuit?

    • Seetharaman
      April 14, 2014

      For CA3162 you can use CD7447. CA3161 available from at a cost of around INR1000/=, but it has also an equivalent NTE2054 you can try for one , which is pin compatible.

  • rahul
    March 10, 2014

    plz send me the componenet list of digital thermometer

  • MAsaood
    October 1, 2013

    is there ready made pcb layout??? plzz help

  • July 12, 2013

    Using projet

  • Abdullah Zohaib Kiani
    May 17, 2013

    Yes, i have tried this circuit it works very nice. You have to do a little bit calibration after assembling the circuit using two (POT).

    • Divakar.R
      April 1, 2014

      Hi I m just a beginner. Do we need a software to be burnt in the IC to make the circuit work.

      • Seetharaman
        April 2, 2014

        No they are A to D converters no need for any software.

    • Atchyut
      April 13, 2014

      i want the IC ca3162 where we will get that IC…please help us.

  • Abdullah Zohaib Kiani
    May 4, 2013

    Can we use any other BCD decoder other than Ca3161??

  • Abdullah Zohaib Kiani
    April 22, 2013

    Is there any substitute for ca3162????.
    Actually I could not find it.
    I am making this circuit so any one who knows substitute of ca3162 plz help………….

  • spharow
    April 18, 2013

    what kind of seven segment is this?common cathode or common anode?

  • AZ
    July 25, 2012

    if i want to get alarm to be send to remote desktop computer were i can tap to it to connect at least contact relay

  • Keith
    February 5, 2012

    The two ICs appear to be discontinued/very expensive. Are there substitutes?

  • Keith
    February 4, 2012

    Sirs, The LM35 operates from -55 to +150 Celsius. Does this circuit indicate temperatures below 0 Celsius?

  • haitham hashim
    September 13, 2011

    hi to all,this site is very usefull ,i like this sit very much,thanks

  • August 16, 2011

    i want to make it, is it tested circuit can you plz give me a conformation

  • July 28, 2011

    i like this type of circuit

  • haitham hashim
    July 25, 2011

    Scientific topics in a simple beautiful

  • beh
    July 21, 2011

    it is good to see an assembeld of this thermometr

    • otonye
      March 10, 2014

      if common cathode is used what will be the challenge ?

      • otonye
        March 10, 2014

        the common anode used is it single or triple ?

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