Introduction: Simple LED Digital Temperature Sensor

Picture of Simple LED Digital Temperature Sensor

A Simple, Low-cost, Digital Electronic Temperature Sensor

H. William James, August, 2015

Abstract Blinking LEDS contain a small IC chip that cause them to continuously blink on and off when a voltage is applied. This study shows that the blink rate is dependent upon temperature if the applied voltage across the LED remains constant. Thus, the blinking LED can be used to measure temperature and provides a digital output.

Introduction

Light Emiting Diodes (LED) come in many shapes and emit a variety of colors. Another type of LED is the blinking or flashing LED. These are LEDs with a tiny IC multivibrator chip embedded inside that cause the LED to start blinking when connected to a power source. Blinking LEDs can be purchased for less than a dollar each and come in a variety of colors.

The number of LED flashes per minute or the blink rate of the LED is not constant. It will vary with significant changes in the applied voltage (lower voltage = faster flash rate and vice versa). However, studies by the author, starting in 2010, found that the flash rate per minute varies linearly and accurately with changing temperature. As the temperature decreases (increases) the blink rate of the LED increases (decreases). Red LEDS blink the fastest, while yellow ones blink slower and green ones even slower over a given time range.

Using a blinking LED to measure temperature

To measure temperature accurately with a blinking LED, a constant voltage source is required. A 2 to 6V DC power supply from an AC wall outlet supply can provide stable voltage across a blinking LED placed in series with a 10 to 30 Ohm resistor. If a battery used, the voltage can be stabilized by using a voltage regulator IC chip across the battery.

As the LED blinks the voltage drop across it varies. To record the blink rate of the LED it can be built into a circuit that counts and even displays and transmits the number of blinks (and the temperature) that occurred over a time period such as one minute. In this study, a blinking LED was incorporated into a simple, audio-oscillator circuit. As the LED blinks on and off, the oscillator emits audible “beeps” to a speaker. The software application or App, “LiveBPM”, which displays the beats-per-minute of a song, picks up these beeps and counts and displays them as beats per minute (BPM). See Figure 1. A calibration chart or table showing beep rate versus temperature allows for the temperature to be determined from the display.

LED blink rate vs. temperature change

Figure 2 shows a plot of the blink rate per temperature change for two yellow blinking LEDs. The LED was compared to an accurate electronic digital thermometer placed near-by. Note in the figure that the calibration is linear from at least +16 to near -20C. Over this range, the rate of temperature change is about 0.95C/blink for a yellow LED.

Figure 3 shows the blink rate per minute for a yellow blinking LED from +35.2 to -18.5C. A best fit logarithmic curve was added (thin line). The overall rate of change is about 1C/blink.

The LEDs have been tested for months and the calibration remains stable. Using LiveBPM, one can detect temperature changes near 0.1C. The accuracy of the blinking LED is around +/- 0.5C from at least +35 to -20C. The temperature response time of the sensor is not slow. After removal from a freezer where it was colder than -15C, the sensor recovered to +17C in just a couple of minutes. Shaving away the LED plastic cover helps speed the response time. Further testing the LEDS over a wider temperature range will be done and posted on this website.

What causes the LED blink rate to change with temperature is not clear. Temperature changes do affect the performance of diodes, resistors, and capacitors. These components are inside the LED and IC chip. Another possibility is that the LED components are physically changing (e.g., expanding and contracting) with temperature change and this changes the IC circuit, causing a change in blink rate.

Conclusions

The blinking LED can be used to easily measure temperature. The temperature response in this study shows that it is generally linear from about +35 to -20C. Further testing will be done over a wider temperature range and the results posted on this website. The blinking LED sensor allows for simpler, lower cost electronic circuit designs to measure and display temperature.

Figures

Figure 1. LiveBPM App display of "beats per minute". However, here it is displaying temperature changes over a 30 minutes period from a blinking red LED inserted into an audio oscillator circuit. The rate of change for a red LED is about 0.84C/blink

Figure 2.Temperature calibration plot for two blinking yellow LEDs. The x-axis is temperature (degrees C) and the Y-axis is the blink rate of the LED during 1 minute. LiveBPM software was used to determine the blink rate of the LEDs.

Figure 3. Calibration plot for one yellow blinking LED. The x-axis is the blinks per minute and the y-axis is temperature (C) and each data point shows the temperature measured. Thin black line is a best fit logarithmic curve.

References:

Light Emitting Diode: https://en.wikipedia.org/wiki/Light-emitting_diod...

Temperature effect on diodes:

https://en.wikipedia.org/wiki/Diode#Temperature_measurements

LiveBPM: https://itunes.apple.com/us/app/livebpm-beat-dete...

My other webpages,

Homemade Weather Instruments http://sites.google.com/site/weathermake2000/

Homemade Large Telescope http://sites.google.com/site/telescope1999/

Homemade Hot Pepper sauce https://sites.google.com/site/pepperhotsauce/

Copyright 2016: H.W.James

Step 1: Figure 1

Picture of Figure 1

Step 2: Figure 2

Picture of Figure 2

Step 3: Figure 3

Picture of Figure 3

Comments

russ_hensel (author)2016-03-21

The forward voltage drop of any semiconductor diode varies with temp. when driven with a constant current. That said the LM35 is cheap, easy to use ( 3 wires analog ) and gives a bigger linear response over a larger temp. range. Still this is a nice application and explanation.

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