Remember that $4 multimeter that I took apart? Well, I still have it. And its not very reliable nor useful. So, I have decided to make it useful! How you may ask? Well, I am going to make it display temperature so I can estimate the temperatures of different components and heatsinks. This is based on a tutorial by Embedded Lab, so I take no credit for the idea of this build. Here is the link to the project: http://embedded-lab.com/blog/?p=4951. I was thinking about doing a tutorial, but it’s already on Embedded Lab and I don’t want to plagiarise so I am just going to go through how I built this and a few modifications that I have made to the original circuit.
In the original tutorial it talks about how there are two different voltage points in the cheapie multimeter, there is a 9v voltage point based on the battery, and there are 3v present from the battery with reference to the common terminal of the multimeter which is where the black negative lead goes when you measure something. The first temperature sensor that is listed in the tutorial is the LM35 centigrade temperature sensor, however it is then disregarded since we have to use the common point on the multimeter in order to get a correct reading on the multimeter which meant that the sensor had to be powered from 3v, which it doesn’t accept. Then the next sensor that they used was a TMP35. I however was unable to find the TMP35 and decided to take a bit of a risk and order an LM35 sensor. I also want to add that they are really expensive! About $4 each on eBay, which is the same price as the cheapie multimeter that I am using for this project. Still I decided to get two because I thought it would be a great learning experience since I have never worked with temperature sensors before. Luckily enough the sensor worked perfectly on 3v and even better I measured the voltage between common and the battery on my multimeter and it was slightly higher than 3v, so just perfect!
First off I started by measuring if it worked., it sort of did… I guess mine was a faulty copy resold by some eBay seller (thaishop) because when I put the correct voltage in it got really hot and then my power supply got into current protection mode. However when I swapped the pins around it worked perfectly. The dangers of eBay!
As seen above you can probably work out that the temperature is a bit incorrect, well the location of the dot is at least. To solve this problem we have to use a resistor divider so that only 1/10th of the voltage would be measured and not the whole voltage. This effectively makes the multimeter display a proper temperature. I tested the circuit on a breadboard and here is the result:
As seen in the image above the divider moved the dot over, however it made the reading a bit off by about 4 degrees. This was unacceptable and I had to play around with the values a bit. I used this online resistor divider calculator. At first I used a 39K and a 330K, but that mucked everything up even more since the resistance through the sensor was even less than the actual resistors and the readings got all wrong. I decided to go for a lower value of resistors… A lot lower. I tested a ratio of 91 to 10, which had a better result, however I had to test all of the resistors that I had to make the 91 go to 90 (which was a bit challenging with 1% resistors) and the 10 to be very close to 10 or else my readings would be wrong again. After I finished picking out the values and the resistors I made a resistor divider similar to this one:
The next thing was to find a switch so that I would be able to turn the sensor off and use the multimeter as a multimeter when I needed to, just in case. My requirements were that it had to fit in the housing of the multimeter, it took me a bit to find a small switch in my mixed parts box, but I found it. I also had to make an incision on the casing of the multimeter in order to be able to have the switch there in the first place and to be able to close the back lid.
Then there was the final bit of soldering everything together, minimising the wiring and glueing everything down. Here is the result:
There was the option of keeping everything inside the multimeter and hidden away like it was shown in the tutorial, but I decided it would be best if I were to make a small hole at the side and get the sensor outside of the housing of the multimeter in order for me to be able to measure the temperature of anything just by simply pressing the sensor onto the surface. Unfortunately my switch did not all fit into the multimeter so I had to remove the actual big button that was on it and just leave it with the bare surface, which was alright, since it made it a bit more compact, but it also made it a little less prettier.
In the end this was a fantastic project and it is great for beginners and others alike. I learnt a fair bit from this and I also have a working temperature meter now! The downsides are that the meter is not as pretty anymore and also that it takes a bit for the sensor to heat up and start raising its output voltage. I also am not sure if this will last long, but so far it works a treat! The cost is probably only just the sensor, which I am sure you can find for cheaper than the $4 that I paid. Everything else an electronics hobbyist would most likely already have.
Links for both LM35 and TMP35: