Hakko FX-888 Power LED modification

        I have had my FX-888 in use for about 6 months now. And in about that time span, I forgot to turn it off and have left it on several times because the power LED of the FX-888 just blinks when the heater element is on. I could unplug it when not in use but it is too bothersome (and I would forget that too )

        While watching around in Youtube, I came across Dave Jones' EEVBlog about the FX-888 and he had the same dilemma as I did. A circuit was shown in the video using a transistor, a few resistors and a bi-color LED that blinks red/green when the unit is on.

        I thought to myself, I might be able to simplify his circuit a bit more. This is what I came up with:

        T1 is the open collector output of the Hakko IC as mentioned in the vBlog. My change was using a bi-color SMD LED which has two electrically independent LED's so that I can wire it in common anode. The choice of red/blue is for brightness so that it can easily be seen even during daytime. I wanted a red/true-green combo but that wasn't available.

        The circuit works this way: When the open collector output is off, the red LED is basically out of the circuit as its cathode is left floating. The supply flows through R1, LED2 and D1 so the LED glows blue. A blue LED needs more than 3V to start to conduct, adding the series diode increases by about 0.7V so the anode terminals sit at about 3.7V. Once the open collector output turns on, The red LED is inserted into the circuit and its cathode is basically connected to ground. A red LED's voltage drop is around 1.8V so the anode terminals now sit at about 1.8V above ground and because this is much too low to make the blue LED and diode to conduct, they turn off. The desired operation is now achieved with even less components.


I started with cutting the dome top of a normal 5mm clear LED using a cutting disc. (You'll see why, later) I wanted a milky translucent lens but I didn't have any in my parts bin.

Here are the SMD led's that I used beside the chopped 5mm LED. (I used only one)

The resistor and diode soldered to the LED and leads bent to reach the specific points in the PCB.

I was originally thinking of drilling another spot on the PCB to tap a ground connection for the 1N4148 cathode but an unpopulated resistor provided a convenient negative connection.

The LED dome that I cut earlier is then superglued to the top of the SMD LED to make it look like a normal 5mm LED from the outside since the SMD LED is rectangular.

The original 0603 SMD resistor (marked R2 in the PCB) that is between the original LED cathode and the IC output is removed and replaced with a 0 ohm resistor to connect it directly to the new red LED cathode.

Before I put it back together, here are some pictures of the insides of my FX-888.

The LED is supported by the resistor and diode leads.

This is behind the front DIN iron connector.

Now for the fun part.... Once the switch was turned on, I was greeted with a bright red light.

When it reached the desired temp, it turned blue. I like it when it works the way I hoped it would.

Here is a shot of it turned off. Still looks original with the LED dome just flush with the front panel.

I wanted the new power LED to be bright. It turned out to be too bright even with the 2.2K resistor. After a few minutes of thinking about it, I decided to keep it that way. When placed on the side of the workbench, and maybe after some getting used to, It looks like it's going to be just fine and hopefully not too annoying.

I also realized that my FX-888 (120V US version) can survive 240V when I accidentally plugged it into the wrong outlet with nothing more than a blown 2A fuse.

Page updated and copyright R.Quan © 11 Feb 2012.