Monday, July 13, 2015

1. Starting my first AVR project: Poorman's soldering station

In this post I'd like to discuss a bit about the various modules of the station, my choices for the components as well as help you make an idea of how it will all merge together into the final product.

To keep things simple, I've split the project in the following pieces:

Power supply:

This is what powers the electronics as well as the iron itself. In its most primitive form it comes as a transformer, but it can also come as a switching-mode power supply. As with everything, there are advantages and disadvantages between the different power supply types. Here are some of them:

Linear power supply (transformer) advantages: low cost, mechanically simple, reliable, low leakage current, low ripple and noise

Transformer disadvantages: big and heavy, not very efficient

Switch-mode power supply advantages: smaller, lighter, up to 80% efficiency at 24 volts output

SMPS  disadvantages: not very reliable (especially if poor quality components are used), high ripple / noise. One problem (as you probably know) is that heat greatly minimizes the lifespan of electrolytic capacitors. Combine this with the myriad of counterfeit, bulging <crapacitors> on the market and you have the recipe for disaster and a guaranteed fireworks show.

Personally, I went with the transformer since I had one lying around. Mine can only furnish about 25 watts of power but that's enough for the handle I bought.
So, a 24 volts, center tapped transformer (12 - 0 - 12 volts) delivering 1 A (3A or more recommended for use with 48 watt irons) will be a good starting point. Even if your soldering iron will only draw 2 amps, it's a good idea to always choose a more powerful transformer, otherwise it will heat up very quickly (valid especially for no-name units). The center tap is necessary since the operational amplifier used as thermocouple amplifier needs a symmetrical power supply.

A transformer (not the one I'll use though, mine needs rewinding first)

The power board:

The power board contains the capacitors for smoothing out the ripples from the transformer, some linear regulators to step down the voltage for the different components as well as the optocoupler and the triac that starts and stops the soldering iron (it will be driven by AC current to keep things simple).

A selection of electrolytic capacitors, most of which won't work more than 1 year judging by their looks and "brand"

Instead of the optocoupler / triac combination, a transistor and a relay could be used, but in my tests the relay's coil wasted a lot of power. The internal contacts got stuck together after some time, most probably because of the high current involved. In addition, with low hysteresis the repeated clicking noise from the relay became very annoying.

The logic board

This board hosts the microcontroller, the thermocouple amplifier and a few passives. For this project an AVR from the MEGA family will be used (Atmega 168 with additional support for Atmega 328P). The Atmega 168 seems like a good choice since it has plenty of flash space (16384 bytes), enough RAM (1024 bytes) as well as 512 bytes of EEPROM and it enjoys great support from a multitude of languages. Programming them is easy too, either via a dedicated USB programmer (like USBASP) or by the means of a simple serial port programmer (if your computer has one).

 To get the signal from the K-type thermocouple up to a usable level, an OP07C instrumentation operational amplifier from Texas Instruments will be employed. This op-amp was recommended in many discussions about thermocouple amplification and it looks perfect for the task. It should also be easy to find and very affordable compared to other dedicated solutions like MAX6675 or LT1025. And after all, the temperature level is informative, a hobbyist soldering iron driver is most certainly isn't a thermometer. According to the datasheet, OP07C offers low offset and long-term stability and wide input-voltage range which makes it ideal for the task.


For showing information, a 16 characters x 2 lines, Hitachi HD44780 compatible LCD will be used. This should be more than enough for a soldering station. In addition, they're very popular, supported by many programming languages and easy to reuse in other projects.

The 16x2 lines LCD, OPO7C op-amp and an ATMEGA168 MCU used for development

This is it regarding the components, next up is the schematic diagram for the soldering station. Stay tuned!