Which End is The Hot End?

Which End is The Hot End?

Who says applications engineers never do any practical electronics? Well, it has been a while I admit. An annual ten thousand Inbox messages and six thousand Sent Items means I don’t get much time to power up my soldering iron.

But since lockdown, when the emails have slowed down slightly and I am not dashing off to another customer visit, I have found time to do some practical work and really get my head around some of the Analog Devices product range. After all, the only way to fully understand any subject is to actually do it.

I had some spare components lying around and I wanted to cobble them together to make something of some use. These included a radio module that receives the MSF Rugby clock, two 16-character, 2-line LCD displays, a few evaluation kits of various ADI power supplies, an evaluation kit for ADI’s latest highly accurate temperature sensors (ADT7320 and ADT7422), a whole ton of PIC microcontrollers, old bits of copper clad board and an unregulated wall cube power supply, 15V/0.8A.

It was like an episode of Can’t Cook, Won’t Cook. What could I make...?

I was inspired by the digital thermometer that Jim Williams had on his living room wall, also shown on the front cover of Analog Circuit Design Volume 1.

The photo above shows the beginnings of my highly accurate desktop clock and temperature sensor. A few hundred lines of C code were written to decode the MSF signal and output the reading on one of the LCD displays. An SPI routine was ‘bit bashed’ to enable me to interrogate the ADT7320 and after yet more C code, it was proudly displaying temperature on the second LCD display to an accuracy of 0.2 degC. A prototype was built on my plugboard (remember those, back in the days of DIP?). It all works; I now just need to build it.

I will let you know how I get on…

I also have an evaluation board for the LT4363 (surge stopper), the LTM4650 (50A buck converter) and countless high current LED driver evaluation kits. The right hand side of the photo also shows me playing with ADI’s latest Automotive Audio Bus, A2B.

So if anyone wants a design that can indicate time to an accuracy of 1ms, give an ambient temperature readout accurate to a tenth of a degree, weld two pieces of metal together, can withstand an automotive load dump, plays music and has an LED backlight that can be seen from space, this could be your lucky day.

However, before I get going… These soldering iron things… Which end is the hot end?

It is now February 2021, eight months since since I originally wrote this blog and the circuit has been sitting on the windowsill in my office. The clock is accurate to 3 milliseconds and needs no adjustment. The temperature reading is accurate at about 0.2 degC.

Below is a photo of the final circuit:

The final circuit

Behind the circuit is an LT8614 evaluation kit and the white feet of the evaluation kit act as the feet of the main circuit – you can see them in the photo. The evaluation kit is powered from the 15V wall cube and its output voltage is 5V that powers the microcontroller (on the left of the photo) as well as the 2 displays and the temperature sensor.