A 230V power strip triggered by my PC 5V/USB

One of my small but very useful DIY projects was modifying a standard 230V power strip so it automatically turns on and off together with my PC. The goal was simple: power my 2.1 audio system only when the computer is actually on, without having to manually flip a switch every time.

I started with a normal 230V power strip equipped with a mechanical on/off switch. After disassembling it, I removed the original switch entirely. Instead of replacing it with a relay PCB board.

In place of the original switch, I installed a custom PCB with a 5V relay. The relay is wired so it switches the mains line exactly like the original power switch did, but now it’s controlled by a low-voltage signal.

To power the relay, I connected it to a 5V USB cable. This way, when the PC provides 5V, the relay closes and the power strip turns on. Simple and effective.

I realized a limitation of my PC, unfortunately, the USB ports on my motherboard remain powered even when the PC is turned off. This meant the power strip stayed on all the time, defeating the whole purpose. Some BIOS setups allow disabling USB power when the PC is off, but in my case, that option simply wasn’t available.

To fix this, I redesigned the activation method. Instead of relying on USB power, I created a custom adapter from an internal PC power supply connector to a 2.5mm DC power jack mounted on the back of the power strip.

Now the relay gets its 5V directly from the PC’s internal power supply, meaning it only receives power when the PC is actually on. When the PC shuts down, the relay releases and the entire power strip turns off automatically.

Notes

• read risk disclaimer
• excuse my bad english

A MC2100ELS motor control board repair adventure

The MC2100ELS is a well-known motor control board used in many treadmills. This repair story started when I bought a used treadmill. It worked fine at first, but after a few months it suddenly stopped working.

After opening the treadmill and inspecting the MC2100ELS board, I focused on the switching regulation section. The root cause of the first failure turned out to be a capacitor on the output line of the switching regulator. The capacitor had degraded and was no longer doing its job properly.

While working on that section, I also noticed a ceramic capacitor on the same output line that was emitting an audible buzzing noise. Even if it was still technically working, that noise was a clear sign of stress, so I replaced it as well.

Using a thermal camera during testing, I found another weak point: a resistor on that same line was running extremely hot. The original resistor was rated at 15k/2W, which seemed undersized for the actual power dissipation. To improve reliability, I replaced it with a 15k/5W resistor.

After these fixes, the board was reinstalled and the treadmill worked perfectly for several months.

After some time, the treadmill stopped working again. This time the symptoms were different and much more frustrating. The LED on the MC2100ELS board, which normally blinks to indicate the operating frequency, was now solid on instead of flashing.

The worst part was that the problem was intermittent. Sometimes the treadmill would start, sometimes it wouldn’t. I ran many tests, checked solder joints, remeasured voltages, and tried to reproduce the failure consistently, but it appeared randomly.

After many attempts, I focused my attention on the microcontroller and the control signals. I suspected that either power instability or noise on the PWM control line could be causing the microcontroller to lock up.

Two changes finally solved the issue: replacing a capacitor on the microcontroller power supply line, adding a 100 pF capacitor on the trace from the optocoupler output to the PWM input pin down to ground. It helps filter high-frequency noise and clean up the PWM signal reaching the MC2100ELS.

After these modifications, the LED behavior returned to normal (blinking as expected), and the treadmill has been working reliably since.

It has now been running for several months without any issues, so things are looking good, fingers crossed.

I would also like to thank the users of the allaboutcircuits.com forum (https://forum.allaboutcircuits.com/), and especially MaxHeadRoom, for their valuable help and insights during this repair. Their suggestions were instrumental in tracking down and finally solving this tricky and intermittent problem.

Find below a couple of thermal images from my treadmill.

Notes

• read risk disclaimer
• excuse my bad english