How To Measure Temperature with a Soldering Iron

Forget those expensive temperature sensors. Now you can use an ordinary heating element like a soldering iron to measure temperature. Daniel Maliks’s upgraded soldering iron will be a great addition to your workbench.

In Circuit Cellar 191, Malik writes:

There are many applications that involve the conversion of electric current into heat by means of a heating element with some degree of temperature control. Hot water boilers, kettles, and irons are typical examples. In this article, I’ll explain how you can eliminate the need for a temperature sensor by using the heating element itself to accurately measure temperature.


The finished soldering station. Think of it as a heating element turned temperature sensor.

I appreciate that electronics enthusiasts don’t necessarily want to read about cooking and ironing. So, I’ll describe a device you might be more comfortable thinking about: a soldering iron. All of the aforementioned appliances and tools have one important thing in common that makes them different from, say, a hair dryer. Any thoughts?

The important common factor is that the thermal resistance between the heating element and the heated medium is much lower than the thermal resistance between the medium and the ambient world. Thus, if electric current stops flowing through the heating element, the temperatures of the element and the medium will equalize long before the medium loses much of its temperature via heat radiation and conduction.

Because the resistance of all the conductors used for constructing heating elements has some temperature coefficient, you can measure the temperature of the heating element by measuring its resistance and comparing it to its resistance at, say, 25°C. This brings us neatly to the basic idea behind this project.

First, you turn on the heating element for a while. Then, switch it off and wait for the temperatures to equalize. After that, you must measure the resistance and calculate the temperature. And then do it again: switch on, switch off, measure, and so on. It’s easy to see why this approach wouldn’t work with a hair dryer. The air forced through a hair dryer moves quickly and has poor heat conductivity.

I can hear you asking the obvious questions. How difficult is it to measure the element resistance? How much does it change with temperature? Wouldn’t a simple sensor be cheaper and easier to use? It depends on the application. I will address these concerns as I describe the soldering iron example.

The schematic below shows Malik’s complete application. The power supply is the only part not shown in the schematic of the soldering station. The current source is powered
by 8 V to reduce power dissipation.

In the measurement circuit, the current source is built from an adjustable linear regulator. The current is set to a little higher than 200 mA. The main power switch Q1 is connected to an additional transistor circuit to translate the control voltages down to the 0- to 5-V range the Freescale MC68HC908QT4 microcontroller is capable of generating.

The complete application, without the power supply

The complete application, without the power supply

The MC68HC908QT4 microcontroller is an inexpensive 8-bit HC08 device housed in an eight-pin DIL package that’s easy to work with. The microcontroller is connected to an LED that indicates whether the soldering tip is below, equal to, or above the desired temperature. A potentiometer regulates the desired temperature. The remaining pieces of the circuit are two trimmers that are used to calibrate the offset and gain of the temperature regulation.

A schematic of the power supply I used isn’t shown here. I used a small, lightweight custom switch-mode power supply. However, the circuit will work equally well with a mains transformer-based power supply.

This temperature measurement application is very simple. As a result, the small amount of code for the ’HC908QT4 is little more than 700 bytes.

Download the entire CC191 article.

Electrical Engineering Tools & Preparation (CC 25th Anniversary Issue Preview)

Electrical engineering is frequently about solving problems. Success requires a smart plan of action and the proper tools. But as all designers know, getting started can be difficult. We’re here to help.

You don’t have to procrastinate or spend a fortune on tools to start building your own electronic circuits. As engineer/columnist Jeff Bachiochi has proved countless times during the past 25 years,  there are hardware and software tools that fit any budget. In Circuit Cellar‘s 25th Anniversary issue, he offers some handy tips on building a tool set for successful electrical engineering. Bachiochi writes:

In this essay, I’ll cover the “build” portion of the design process. For instance, I’ll detail various tips for prototyping, circuit wiring, enclosure preparation, and more. I’ll also describe several of the most useful parts and tools (e.g., protoboards, scopes, and design software) for working on successful electronic design projects. When you’re finished with this essay, you’ll be well on your way to completing a successful electronic design project.

The Prototyping Process

Prototyping is an essential part of engineering. Whether you’re working on a complicated embedded system or a simple blinking LED project, building a prototype can save you a lot of time, money, and hassle in the long run. You can choose one of three basic styles of prototyping: solderless breadboard, perfboard, and manufactured PCB. Your project goals, your schedule, and your circuit’s complexity are variables that will influence your choice. (I am not including styles like flying leads and wire-wrapping.)

Prototyping Tools

The building phase of a design might include wiring up your circuit design and altering an enclosure to provide access to any I/O on the PCB. Let’s begin with some tools that you will need for circuit prototyping.

The nearby photo shows a variety of small tools that I use when wiring a perfboard or assembling a manufactured PCB. The needle-nose pliers/cutter is the most useful.

These are my smallest hand tools. With them I can poke, pinch, bend, cut, smooth, clean, and trim parts, boards, and enclosures. I can use the set of special driver tips to open almost any product that uses security screws.

Don’t skimp on this; a good pair will last many years. …

Once everything seems to be in order, you can fill up the sockets. You might need to provide some stimulus if you are building something like a filter. A small waveform generator is great for this. There are even a few hand probes that will provide outputs that can stimulate your circuitry. An oscilloscope might be the first “big ticket” item in which you invest. There are some inexpensive digital scope front ends that use an app running on a PC for display and control, but I suggest a basic analog scope (20 MHz) if you can swing it (starting at less than $500).

If the circuit doesn’t perform the expected task, you should give the wiring job a quick once over. Look to see if something is missing, such as an unconnected or misconnected wire. If you don’t find something obvious, perform a complete continuity check of all the components and their connections using an ohmmeter.

I use a few different meters. One has a transistor checker. Another has a high-current probe. For years I used a small battery-powered hand drill before purchasing the Dremel and drill press. The tweezers are actually an SMT parts measurer. Many are unmarked and impossible to identify without using this device (and the magnifier).

It usually will be a stupid mistake. To do a complete troubleshooting job, you’ll need to know how the circuit is supposed to work. Without that knowledge, you can’t be expected to know where to look and what to look for.

Make a Label

You’ll likely want to label your design… Once printed, you can protect a label by carefully covering it with a single strip of packing tape.

The label for this project came straight off a printer. Using circuit-mount parts made assembling the design a breeze.

A more expensive alternative is to use a laminating machine that puts your label between two thin plastic sheets. There are a number of ways to attach your label to an enclosure. Double-sided tape and spray adhesive (available at craft stores) are viable options.”

Ready to start innovating? There’s no time like now to begin your adventure.

Check out the upcoming anniversary issue for Bachiochi’s complete essay.

Elektor RF & Microwave App for Android

Elektor has an iPhone/iPad app for several months. And now Android users can have an Elektor app of their own. The Elektor RF & Microwave Toolbox app is perfect for engineers and RF technicians who need easy, reliable access to essential equations, converters, calculators, and tools.

A screenshot of the Elektor RF & Microwave app for Android

The app includes the following handy tools:

1.Noise floor (Kelvin,dBm)
2.Amplifier cascade (NF, Gain, P1db, OIP2, OIP3)
3.Radar equation (2-way path loss)
4.Radio equation (1-way path loss)
5.Power and voltage converter (W,dBm,V,dBµV)
6.Field intensity and power density converter (W/m2, V/m, A/m, Tesla, Gauss,dBm, W)
7.Mismatch error limits (VSWR, Return loss)
8.Reflectometer (VSWR, Return loss)
9.Mitered Bend
10.Divider and Couplers (Wilkinson, Rat race, Branchline , microstrip and lumped)
11.Balanced and und balanced PI and T attenuator
12.Skin depth (DC and AC resistance)
13.PCB Trace calculator (impedance/dimensions)
14.Image rejection (amplitude and phase imbalance)
15.Mixer harmonics (up and down conversion)
16.Helical antenna
17.Peak to RMS (peak, RMS, average, CF)
18.Air Core Inductor Inductance
19.Parallel plate Capacitor
20.PI and T attenuator
21.Ohm’s Law
22.Parallel LCR impedance/resonance
23.Series LCR impedance/resonance
24.Inductor impedance
25.Capacitance impedance
26.Antenna temperature (Kelvin)
27.Radar Cross Section (RCS) calculator (Sphere,Cylinder, flat plate, corners, dBsm)
28.Noise Figure Y-Factor Method
29.EMC (EIRP, ERP, dBµV/m)
30.Noise figure converter (dB, linear, Kelvin)
31.Frequency Band Designations
32.Resistor color code (reverse lookup, 3 to 6 band)
33.Filter Design (Butterworth, Chebyshev, prototype):
34.µ-Filter Design (microstrip, stripline)
35.PCB Trace Width and Clearance Calculator

Visit the Android Market for more information about the Elektor app.

Circuit Cellar does not yet have an app for Android. The Circuit Cellar iPhone/iPad app is available on iTunes.

Screenshots of the Circuit Cellar app

Elektor International Media is the parent company of Circuit Cellar.