• Skip to main content
  • Skip to primary sidebar

Homemade Circuit Projects

Need circuit help? Post them in the comments! I've answered over 50,000!

Blog | Categories | About | Contact | Calculators-online
You are here: Home / Health related Projects / Digital Thermometer Circuit – Uses a Solar Cell for Power

Circuit Simulator: Assemble and Simulate

Digital Thermometer Circuit – Uses a Solar Cell for Power

Last Updated on June 9, 2020 by Swagatam Leave a Comment

The articles explains a digital thermometer circuit project that works without a battery. Instead of a battery the circuit utilizes a small solar cell and operates by deriving power from the ambient light available from the surrounding light sources.

Table of Contents
  • Basic Working concept
  • How the Circuit Works
  • Working with Low Ambient Light
  • Construction and Setting up

This allows the circuit to be extremely compact, light weight, versatile and hassle free while measuring temperatures from a given source.

The thermometer can be used for measuring temperature of a human body, temperature of a room, heatsink, for weather analysis, or any other suitable application which requires critical temperature measurements between 0 degree and 100 degree Celsius.

Basic Working concept

Referring to the circuit diagram below, IC1 works like a temperature sensor device. This IC is a popular LM35 chip which produce a linearly increasing DC output in response to a proportionately increasing ambient temperature around it. To be precise it generates an output DC at a rate of 10 mV per degree Celsius rise in its case temperature.

The LM35 has an in-built calibrated circuitry, which enable to produce 0 V at 0 °C.

Apart from this IC, the other main element of this light powered thermometer is the integrated circuit ICL7136 (ICI) which internally is made up of a digital voltmeter stage, a decimal shifter and an LCD output interface which operates a 3 and 1/2 digit LCD panel for the temperature readout.

IC7135 voltmeter
ICL7136 voltmeter

This IC also has an internal oscillator which works with a minimal clock frequency which ensures that the the entire module is able to function using minimum power, yet without any flicker on the display.

The temperature readout calibration of the circuit is carried by adjusting the preset P1 appropriately.

How the Circuit Works

Diode D1, and resistor R11 ensure that the LM35 turns negative voltage in response to an ambient lower than 0 °C.

The LEDs D1, and D2 here do not function as normal indicator LEDs rather as a reference voltage generators for getting a reasonably accurate 1.6 V constant reference, which require only as low as a few uAmps for this functionality. Although standard zener diodes are more accurate with their reference potential, zener diodes require much higher forward current compared to LEDs, and therefore zeners have been avoided for this application.

light thermometer 1

The IC3 along with this associated components work like a voltage monitor stage for the solar cell supply.

The op amp switches OFF the main thermometer circuit stage through the transistor T2 whenever the solar cell output voltage drops below 0.7 V.

This feature makes sure that the IC1, IC2 stages do not malfunction during such low voltage and produce temperature readouts with errors.

In order to work correctly the LM35 requires a minimum supply voltage of 5.5 V, while for IC2 the minimum reference potential required is 7 V for its normal functioning.

Working with Low Ambient Light

Op amp IC3 is rigged as a Schmitt -trigger such that it works with a hysteresis level of 1V. Meaning, the IC output will switch ON when the solar cell voltage is 8 V, and switch OFF when it drops below 7 V.

The 7 V switch ON threshold is precisely adjusted using the preset P2.

The circuit comprising IC1, IC2 is able to work normally within a current range of 10 to 200 micro amps. When light source on the solar cell is insufficient, and its current drops, the IC3 switches OFF power to IC1/IC2, which removes loading on the solar cell and its voltage rise to 8 V. This 8 V gets stored in the capacitor C6. IC3 detects this and switches ON power to the circuit so that the thermometer now works using this stored energy. When C6 discharges below the 7 V threshold, IC3 yet again cuts off power to the circuit through T2.

The above functioning of IC3 is actually very useful in situations when the ambient light is low or drops to a level where the solar cell is unable to generate sufficient power for the thermometer for its normal functioning. In such conditions, the IC3 toggles the power from the solar cell ON/OFF such that the user is able to check the temperature in a switched ON/OFF mode, but definitely without an error. This allows the thermometer to keep working perfectly even in low ambient light conditions, instead of completely shutting down.

The hysteresis level (1 V) can be changed according to user preference by changing the value of the resistor R7

The capacitor C6 value determine how fast the ON/OFF happens for IC3/T2 in low light conditions. Decreasing C6 value will cause faster ON/OFF of the display and vice versa.

Construction and Setting up

The PB design for the light powered thermometer can be visualized in the following image.

pcb for solar thermometer

Assembling the PCB is easy, but the LCD module must be handled with precautions while inserting into the PCB, since the device is quite delicate and vulnerable to breaking.

3 and half digit lcd module

Make sure you do not forget the A couple of wire connections on the PCB. Do not initially fit IC2 LM35 on the PCB to allow the introduction of +1.000 V across Vout and GND terminals of the LM35. Before this make sure to adjust P1 so that the display reads 100 °C. Once this is done, remove the solar cell or the external supply if any used, and now fix IC2 on the PCB.

Solar Cell

The solar cell can be any mini or micro solar cell combined to produce 9 V, at 10 mA.

If you do not wish to use a solar cell or light power, rather a normal battery, you can replace the power source with an ordinary 9 V PP3 battery which would probably last for ages due to extreme low consumption of the design.

WARNING: The proposed light powered digital thermometer shouldn't be used as a clinical thermometer, unless the circuit is verified and confirmed from an authorized laboratory.

You'll also like:

  • 1.  Alcohol Detector Meter Circuit using MQ-3 Sensor Module
  • 2.  RF Signal Meter Circuit
  • 3.  How to Make a Digital Voltmeter, Ammeter Module Circuits
  • 4.  Simple Frequency Meter Circuits – Analogue Designs
  • 5.  High Voltage Meter Circuit [for Measuring 10kV]
  • 6.  Capacitor Leakage Tester Circuit – Find Leaky Capacitors Quickly

Filed Under: Health related Projects, Meters and Testers Tagged With: Cell, Digital, Power, Solar, Thermometer, Uses

About Swagatam

I am an electronics engineer and doing practical hands-on work from more than 15 years now. Building real circuits, testing them and also making PCB layouts by myself. I really love doing all these things like inventing something new, designing electronics and also helping other people like hobby guys who want to make their own cool circuits at home.

And that is the main reason why I started this website homemade-circuits.com, to share different types of circuit ideas..

If you are having any kind of doubt or question related to circuits then just write down your question in the comment box below, I am like always checking, so I guarantee I will reply you for sure!

Previous Post: « Arduino 2-Step Programmable Timer Circuit
Next Post: Adjustable Switching Power Supply Circuit – 50 V, 2.5 Amps »

Reader Interactions

Need Help? Please Leave a Comment! We value your input—Kindly keep it relevant to the above topic! Cancel reply

Your email address will not be published. Required fields are marked *

Primary Sidebar




Subscribe to New Posts

Categories

  • Arduino Projects (89)
  • Audio and Amplifier Projects (132)
  • Automation Projects (17)
  • Automobile Electronics (101)
  • Battery Charger Circuits (83)
  • Datasheets and Components (104)
  • Electronics Theory (143)
  • Free Energy (37)
  • Games and Sports Projects (11)
  • Grid and 3-Phase (19)
  • Health related Projects (25)
  • Home Electrical Circuits (12)
  • Indicator Circuits (14)
  • Inverter Circuits (88)
  • Lamps and Lights (142)
  • Meters and Testers (69)
  • Mini Projects (46)
  • Motor Controller (64)
  • Oscillator Circuits (27)
  • Pets and Pests (15)
  • Power Supply Circuits (108)
  • Remote Control Circuits (50)
  • Security and Alarm (64)
  • Sensors and Detectors (101)
  • Solar Controller Circuits (59)
  • Temperature Controllers (42)
  • Timer and Delay Relay (49)
  • Transmitter Circuits (29)
  • Voltage Control and Protection (39)
  • Water Controller (36)

Tags

AC Alarm Amplifier Application Arduino Automatic Battery Charger Circuits Control Controlled Controller Current Datasheet DC Detector Digital Driver Electronic Explained Explored Generator High Indicator Inverter Lamp LED Light Meter Motor Power Regulator Remote Sensor Simple Single Solar Supply Switch Timer Transistor Voltage Water Watt Working




Other Links

  • Privacy Policy
  • Cookie Policy
  • Disclaimer
  • Copyright
  • Videos
  • Sitemap




People also Search

555 Circuits | 741 Circuits | LM324 Circuits | LM338 Circuits | 4017 Circuits | Ultrasonic Projects | SMPS Projects | Christmas Projects | MOSFETs | Radio Circuits | Laser Circuits | PIR Projects |

Social Profiles

  • Twitter
  • YouTube
  • Instagram
  • Pinterest
  • My Facebook-Page
  • Quora
  • Stack Exchange
  • Linkedin



  • Recent Comments

    • Swagatam on How to Make HHO Fuel Cell Circuit in Automobiles for better Fuel Efficiency
    • Swagatam on How to Make HHO Fuel Cell Circuit in Automobiles for better Fuel Efficiency
    • eq on How to Make HHO Fuel Cell Circuit in Automobiles for better Fuel Efficiency
    • Swagatam on How to Make HHO Fuel Cell Circuit in Automobiles for better Fuel Efficiency
    • eq on How to Make HHO Fuel Cell Circuit in Automobiles for better Fuel Efficiency

    © 2025 · Swagatam Innovations