Modifying Air Purifier Buttons for Automatic Start - Page 1
Forum section: electronics / beginners

author: HomeHacker
author_rank: Newbie
timestamp: January 25, 2021, 07:46:46 am
content: Hi everyone,I have a Levoit air purifier here. Upon plugging it in, the device needs a button press to start. I would like to modify it so it starts immediately upon plugging it in.The device has no-contact switches - I assume they work by detecting the magnetic field of my finger via the spring. I have attached two pictures of the switch circuit.The device behaves as follows: When I plug it in while my finger hovers over the spring, nothing happens. When I lift my finger, the device turns on. So the device reacts to the moment when I lift my finger.How can I modify this so that it turns on automatically? The act of lifting my finger must induce some kind of signal, and I'd need to build something to induce that signal. However, I don't know how these contactless switches work.I have two specific question:1. What kind of signal do these contactless switches produce, or how can I find out? (I have a multimeter at hand)2. How can I replicate that signal so the device turns on automatically?Thanks!

author: Gyro
author_rank: Super Contributor
timestamp: January 25, 2021, 10:11:21 am
content: Those touch buttons are capacitive. They either put a high impedance waveform on the springs and sense the capacitance of your finger (body), or detect mains frequency hum field if it is a non-grounded device. Either way, at switch-on they need to determine a background 'reference' level so that they will be able to detect when your finger approaches. Just holding your finger on the button won't let then establish this (until you remove your finger and the microprocessor detects the drop and is able to properly initialise the state).Bypassing this function isn't going to be simple - probably impossible without things getting complex. The only immediate possibilities I can think of are...1. Check the instruction manual carefully to see if there is a button sequence to make the purifier turn on by default.2. Look around the microprocessor to see if there is maybe a configuration jumper to modify its default operation.Sometimes having fancy touch controls comes at a cost.

author: HomeHacker
author_rank: Newbie
timestamp: January 25, 2021, 06:45:30 pm
content: Thanks! I was suspecting something like that.The device certainly does not support autostart. I will check for a jumper setting on the microprocessor, but I doubt it exists. So the way to simulate a button press would require simulating a certain capacitance and then changing that over time. Which probably can't be done with any simple modification. Thanks for the explanation though!

author: ledtester
author_rank: Super Contributor
timestamp: January 25, 2021, 07:21:33 pm
content: There are a lot of hits for "trigger capacitive touch".Check the comments of this SE question. It contains anecdotes of how people have had success with electronically triggering touch switches:-https://electronics.stackexchange.com/questions/19435/triggering-a-capacitive-sensor-electronicallyAnd there is a lot on the web for simulating presses on a capacitive touch screen which might be helpful, such as:https://youtu.be/JDgDMBquBw0Update:I experimented with taping some aluminum foil against a TTP223 touch sensor module. If I earth ground the foil (plugged it into the earth ground on a power supply, for instance) it triggered the sensor. The TTP223 performs some calibration when power is first applied, so it's better to ground the foil only after power up. Indeed, the entire cable acts as an extension of the sensor pad, so it gets triggered if you put your hand anywhere (or near) along the length of it.

author: thomwo
author_rank: Newbie
timestamp: April 19, 2021, 09:07:08 am
content: Why dont you buy a new one which support autorestart option, try Honewell or Levoit they have autostart feature.

author: Simon
author_rank: Unknown
timestamp: April 19, 2021, 11:38:55 am
content: Because people like to repair things.

author: Gyro
author_rank: Super Contributor
timestamp: April 19, 2021, 12:50:14 pm
content: Quote from: thomwo on April 19, 2021, 09:07:08 amBecause they break?https://www.eevblog.com/forum/beginners/air-purifier-stops-working-after-cleaning/msg3552250/#msg3552250Sorry, couldn't resist.

author: amyk
author_rank: Super Contributor
timestamp: April 19, 2021, 01:41:56 pm
content: By "air purifier" do you mean "fan in a box with a filter"? It might be easy to bypass the microcontroller and controls completely, depending on how it's wired.