Replace Capacitance ON/OFF With Manual Switch - Page 1
Forum section: electronics / projects

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: August 06, 2025, 07:17:26 pm
content: Good Afternoon,I am seeking help on a method to bypass a dehumidifier capacitive on/off switch with a manual switch. Mainly because it's in an RV and every time there's an AC power blip it turns off, requiring another touch to turn it back on. I'd like it to just stay on all the time regardless. That way I can just put an appliance timer on it if I want if off and on a specific times.If anyone has any suggestions after looking at the circuit where a jumper might work or a modification to the board might accomplish this I would be grateful.ThanksThe circuit is simple but uses a SC92F84A3M, High-speed 1T 8051-based Flash MCU.Datasheet.....https://socmcu.com/upfile/SC92F84A3_84A2_84A1v0.1en.pdfHere is a snapshot of the front and back of the board...MicroController PinOut

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: August 07, 2025, 03:02:38 pm
content: So far I've been able to determine that the default state for the ON/OFF bit register is 0 or Off.PIN 18 is connected to the ON/OFF touch sensor at R16Ideally, I could reprogram the register to default the on/off state to the 1 or ON state.This requires a JAG diagnostic interface. Since this would be useful for MANY modern appliances, maybe I'll look into getting one.The other way would be to bypass the MCU but I'd have to analyze the entire circuit to determine how that would be done.I don't need the fancy LED's, just the basics to function which are......The FanThe Peltier modulesThe full tank sensorI may end up designing and making my own PCB to handle these basic tasks and just replace the OEM board completely.I've used KiCAD extensively and had dozens of PCBs fabricatged at JCLPCB. So I think I can pull it off.Now to create a Block diagram.

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: August 07, 2025, 03:08:45 pm
content: The large Fan is 12V and there are two Peltier modules and each uses 6V/6A

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 07, 2025, 07:31:57 pm
content: Does the power blip reset everything, or is one button alone much more sensitive than the others?If the whole MCU resets, you may be better to focus on hold-up improvements/ filtering to the power side.

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: August 07, 2025, 08:03:52 pm
content: Good question.I was focused on the ON/OFF only and didn't pay attention to other settings and whether they all reset if power was dropped.

author: ledtester
author_rank: Super Contributor
timestamp: August 07, 2025, 08:10:35 pm
content: There was a recent thread discussing triggering capacitive touch switches:https://www.eevblog.com/forum/beginners/first-project-automating-an-old-washing-machine/It seems the OP found this thread in another forum useful:https://www.mikrocontroller.net/topic/554673It's in German, but Google Translate does a good job of translating it.It seems you can trigger it by making a capacitive connection to ground. There should be no need to make a direct connection.

author: Gyro
author_rank: Super Contributor
timestamp: August 07, 2025, 08:23:10 pm
content: You might find it easier to make the logic supply a bit more resilient, eg. increasing the value of the reservoir capacitors (within reason) to give it a bit more ride-through ability on supply dips.Edit: Already mentioned by PCB.Wiz

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: August 07, 2025, 08:52:01 pm
content: The Problem with that approach (as mentioned in the OP) is that I want to be able to put the unit on a timer so that it turns on at 8PM every evening then back off at 8AM every morning.It's outdoors in an RV that can get quite warm during the hot day and the Dehumidifiers utilizes Peltier modules which would not do well in such heat (I think).

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: August 07, 2025, 08:54:59 pm
content: Thanks.My thought on that was that the touch sensor for ON/OFF simply flips a bit in one of the MCU registers. Default is 0 or Off.Touching it flips that register to 1 or ON.So I was thinking that grounding would not work.But after reading the messages at the link you posted I may see it differently.Thanks

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 07, 2025, 11:54:52 pm
content: Quote from: ThermallyFrigid on August 07, 2025, 08:54:59 pmConnecting a modest cap thru a low capacitance switch like a reed relay should look just like a button change.A short pulse may be better to avoid any auto-zero averaging effects that the MCU might do.If it is a toggle button, you may need some method to feed back the actual state (another reed relay?) so your timer can keep in phase, giving a double pulse if needed.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 08, 2025, 12:23:31 pm
content: Or use a latching relay. From about €3 on mouser or under €1 on Aliexpress.

author: MLXXXp
author_rank: Frequent Contributor
timestamp: August 08, 2025, 08:00:48 pm
content: I'm assuming you can come up with a way of simulating "pressing" the capacitive on/off switch, as has already been discussed. If you're somewhat savvy with microprocessors, I'd use one to control the switch. A simple module, such as an Arduino Pro Mini, would suffice.Is there a "power on" LED, or some other signal that you could use to detect if the dehumidifier is in the "powered on" state, as controlled by its on/off button? For this to work, you need that to determine if you need to "press" the on/off switch to turn the dehumidifier on (or possibly off).For timing, if you powered both the dehumidifier and the controller through an AC timer, the controller would just have to monitor the "power on" signal and "press" the on/off switch as necessary to keep the dehumidifier on. You could add an on/off toggle switch to one the controller's I/O pins if you wanted a physical power switch separate from the timer. Essentially, the microcontroller is just doing whatever is necessary to keep the dehumidifier powered on (or off) whenever the microcontroller is powered on.Getting fancy, instead of an AC timer, you could add a real time clock module with a backup battery to the microcontroller, along with a display and buttons, or some other way of setting it. Or, instead of an RTC module, you could use the microcontroller itself for timing and figure out a way to battery backup it (assuming the controller's clock is accurate enough for your requirements).If necessary, you could use optocouplers to isolate the power switch and power detect signals, but if the power supply for the microcontroller is isolated from mains, you may not need them. Or, you could probably just tap off of the dehumidifier's logic power itself.

author: MLXXXp
author_rank: Frequent Contributor
timestamp: August 08, 2025, 08:26:42 pm
content: Quote from: MLXXXp on August 08, 2025, 08:00:48 pmGetting somewhat elaborate; if there's room in the dehumidifier itself to house and power my proposed microcontroller, and you were to isolate the capacitive on/off switch from the dehumidifier's circuitry, you could feed the on/off signal to the controller's microprocessor. Assuming a capacitive switch could be detected, you could use it instead of an added-on toggle switch. You could save the on/off state in EEPROM so that it would be remembered and restored if power was lost.

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: August 09, 2025, 01:00:02 pm
content: Thanks guys.For the moment, until I can grasp your suggestions, I've begun work in KiCAD to duplicate the board with the Microprocessor eliminated so that all there will be on the replacement board is a physical ON/OFF switch, a voltage regulator and direct power supply to the components necessary only for dehumidification, eliminating all the fancy color changing LEDs and other capacitive option selections.It seems small vehicle (RV) sized dehumidifiers are all going to this capacitive touch system (and I really don't like it).Any time the power drops even for a second, you have to go out to the RV and turn it back on.I know I could use a battery backup, but that's getting excessive imo.

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: August 10, 2025, 03:35:05 pm
content: The little solder dots marked with a red arrow....I thought they were vias and the board was multi layer but using a VOM I cannot find any continuity between them and any other point on the PCB?What are they?My Best WAG.....Bed of Nails Test Points

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: August 11, 2025, 04:38:12 pm
content: For anyone interested....This is a BREEZOME 60 OZ Dehumidifier, found on Amazon.https://a.co/d/iIUSsvcHere is the schematic of the Original board before I make changes (has not been proofed yet...so could still contain a few errors)If you care to make suggestions on eliminating all components except the essentials for power to the FAN and PELTIER Chips it would be grand.

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: August 12, 2025, 02:52:05 pm
content: And here is the schematic for the PCB above the Water Bucket that controls the FILL OFF SWITCH and LEDS for the water light show

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: August 14, 2025, 02:04:55 pm
content: This is the Main Board Schematic After I stripped away all the LED lights and Touch Sensitive Controls.What I have to do now is figure out how to make this stripped version work with manual controls.I'm not sure yet if I can do this with the MCU IC still in circuit.