Timer Switch

Switches off the soldering iron* when the timer counts down

* or anything that uses 5A or less of mains power

The user interface on my soldering iron is a model of simplicity, it's a physical on/off button on the front which is super but you can forget to switch it off. There is a power LED but that's easily missed.

Soldering Iron

After getting back from holiday and noticing that I'd left the soldering iron switched on the whole time I looked around for something to turn it off with a timer.

There are lots of mains timer switches available, but nothing which quite works for my use case.

Some are just for turning the power on and off at certain times during the day

Timer 1

Some don't allow you to set the timer with enough resolution

Timer 2

Some have horrible little displays with overly complicated user interfaces

Timer 3

This one comes close but doesn't show the timer

Timer 4

The most common problems with the existing solutions were:

So I came up with some requirements:

Electrical Mains powered
Handle 1.2KW power (5A @ 240V)
Replaceable fuse
Compliant with safety regulations
Physical Wall mounted 'breaker' style
80 x 80 x 40mm form factor
Interface As simple as possible, one click on/off
On-time can be increased/decreased on the fly
Large timer display (visible from across the room)
Beep to warn of impending shut off
Configurable default on-time
Power status LED indicator


It's a very simple gadget using the crappiest STM32 MCU available, a push-button rotary encoder, a relay, and a MAX7219 to drive the display. Electronics are powered by a generic HLK-PM01 5V PCB mounted PSU. Separation/creepage distances between the 'hot' mains side and the rest of the board are maintained.


The schematic is very simple and fits on a single sheet.



This was all done before the great component shortage so little thought was given to availability, but as of September 2021 all components are currently available somewhere, although pricing is a bit silly in some places. Note this BOM is just for prototyping, a real BOM has many more columns than this. Note that everything except the MCU, the MAX7219, the PSU and the relay can be substituted with any old generic part you can find.

Show the BOM...
Designator Quantity Value Footprint Comment Description
BZ1 1 - PIEZO_BUZZER BUZZER Piezo buzzer
C1, C3, C4 3 100nF CAPC1608X07L 100nF Ceramic capacitor
C2 1 100uF CAPAE9585X105L 100uF Electrolytic cap
C5, C7 2 1uF CAPC1608X07L 1uF Ceramic capacitor
C9 1 4.7uF CAPC3216X07L 4.7uF Ceramic capacitor
C10, C11 2 3.3nF CAPC1608X07L 3.3nF Ceramic capacitor
D1 1 20mA LED LED 5mm through hole
D2 1 20V 500mA SODFL250X100-2L DSF05S30U Schottky Diode
F1 1 - 5x20 FUSE 520_102701 Fuse speed F
R1 1 620 RESC1608X05L 620 Resistor 1%
R3 1 28k RESC1608X05L 28k Resistor 1%
R5 1 2K RESC1608X05L 2K Resistor 1%
R6, R7 2 10k RESC1608X05L 10k Resistor 1%
R8, R9 2 120 RESC1608X05L 120 Resistor 1%
Q1 1 - SOT95P230X110-3L SSM3K36MFV n-channel mosfet
PSU 1 5V 600mA HLK-PM01 HLK-PM01 5V PSU 0.6A
RELAY1 1 240VAC/5A G5RL-1A-E-HR G5RL-1A-E-HR Relay 5V
ENC 1 - PEC12R-4XXXF-SXXXX_1 ENCODER Rotary encoder
LDO1 1 3.5v 150mA TSOP95P280X110-6L NCV8560SN350T1G 3.5V 150mA LDO
SEG 1 - 5641AH 5641AH 4x7 Segment display
U2 1 - SOIC127P600X175-14L 74HC125 4 way level shifter
U1 1 - TSOP65P630X120-20L STM32F030F4P6 Microcontroller
U3 1 - SOIC127P1030X265-24L MAX7219 LED Driver


For such a simple board, 2 layers is fine and in fact the 2nd side of the board has only a ground plane, earth surround and some duplicated higher current traces on it. Note that the 'top' of the PCB faces the wall and the display and rotary encoder are on the 'bottom'. 2nd revision of PCBs from JLCPCB was bodge free after bringup.





Note the PCB can carry up to 10A (and the relay can do 16A) but using a 5A fuse leaves a good margin. Gerbers are here, the zip file can be uploaded directly to JLCPCB order page to get bare boards.

Glamour shots




The firmware is also quite straightforward, the relay is controlled with a GPIO via a mosfet, the MAX7219 is a simple SPI device and reading the rotary encoder is just a handful of lines of code. To save the settings through a power cycle I used some STM32 flash code from another project.


I used Autodesk Fusion 360 (when it was still free) to design the enclosure which consists of three parts: the back, the front and the fuse holder. It looks like this


There's also a small slab of 3mm red perspex in front of the digits to make them more visible. Any old plasterboard screws are used for wall mounting, they attach through the base and then four other screws attach the front to the back. The fuse holder uses some tiny machine screws which go into some threaded inserts on the front. Sadly Autodesk are holding the files hostage since they changed the licensing model for Fusion 360 so step/stl files can't be extracted without paying them for a license.


It works, I use it all the time, I haven't changed it since I mounted it on the wall and don't expect to need to. Making rather than buying something like this means I get exactly what I want and have some fun along the way so this goes in the success pile. And here it is in action (it's green because I was trying to use up a roll of filament)



Item Description
set set the default on-time. This is what the timer will be set to when the power is switched on
brt set display brightness
beep set when the warning beeps start
flsh set when the display starts to flash
done exit menu