Thursday, 4 August 2016

Hacking my Hakko

Hakko 936 LED Mod

So I've come home once again, to find, yet again, that after tinkering with some new toys I've left my soldering iron on ALL DAY yet again...

And this time I'm taking care of business!  (I'll talk about the toys later).

Shown above is my trusty HAKKO 936 soldering iron station.  I was lucky enough to snag this from the bin many years ago - the handle was broken and work had upgraded to a bunch of Metcals, so I took it home.

For many years I lived with dodgy epoxy fix on the handle, and finally swapped it out with a cheapie from eBay.  All the HAKKO bits (tip, barrel, heating element) got transfered to the  plastic handle and all was good in the world.

Except for the heater LED.  Like all Hakko's the LED only illuminates when heating.  With a quick glance you can't tell if your station is on or not.

So, I did the first rational thing - I googled it.  Daniel had the same issue and his mod was quick and easy.  I like quick and easy so  I went down that route.   With a twist, but I'll get to that in a bit.

First thing first, it was time to crack this puppy open.

Score!  This is a genuine Hakko and not a clone.

Note the barrels for the high power connections to the soldering iron plug.  Quality plus, and a real pain in the ass to desolder.  

But I got there in the end.

I too had some 5mm RGB leds lying around ... 

.. and I also turned it into a RG LED.  

I took a notch out of the stand-off so I could pass the Green LED Cathode out the side while installing the Red LED Cathode and the Common Annode in the position that the RED led used to occupy.  

Referring to the schematic you can see that I also removed R2 and put the green Cathode through one of it's vacant holes.

I then took to the board with my trusty Stanely knife and isolated all the pads.  Why?

Remember I said 'Common Anode' above.  Daniel's mod used a common CATHODE so I could not do what he did.  I also like my hack better and this is why:

With the 24V AC supply feeding the RGB led through a single dropping resistor, and by only switching on / off the Red LED, I can exploit the differences in forward voltage of the Red and Green LEDs to get a Green on / Red Heating display, rather than the Yellow on / Red Heating that Daniel achieved.


With the Red Led off (switching controlled by IC1) the Green LED (being hard wired across the 24V AC supply) will light (at 50% duty as it's AC).

The forward voltage of a Green LED is about 2 and a bit volts.

When the Red LED is turned on (it's anode pulled low by IC1) it will conduct.  As it's forward voltage is around 1.8 Volts, which is more importantly LESS that that of the Green LED, there's no longer enough voltage across the Green LED to light it and only the Red LED is illuminated.

Wired up.

Modern day LEDs are much brighter than older counterparts and I knew that the 1k2 orignally used by Hakko for the Red LED would lead to a very annoying LED brightness.  So I simply added another 1k in series.  It's hard to see in the pic above, but the LED Common Anode is now connected to 24V AC through the resistor string, the Red LED Cathode is connected to Pin 2 on IC1 and the Green LED Cathode is connected to the other side of the 24V AC supply.

I just happened to have a random EEVBlog clip playing on youtube when I tested this, and Dave's timing was spot on.  

So overall, success - at a glance I can see if my Hakko is on or not. Win.


  1. Left my 926 on several times, need to try this hack out sometime.

  2. Nice simple solution but was hoping for a reduce temperature then power off after X time Mod as my kids (I am blaming them, but it could be me) sometimes leave the iron on.