Honestly, it all started with me hoping that a product like this already existed. I have now learned that it does not, so I am looking for options. The reason I am so impatient is that this is just a very small piece of my speaker project and I have spent most of my time learning to design and build BT speakers, not electronic switch design. Since the speaker has an aluminium frame, it would be so cool to avoid mechanical contacts, just touch the frame. But maybe I am just dreaming ...

For about 25 years I was one of the team that designed equipment for specific applications. In that time I experienced only one prototype build. That prototype was based on a hair-brained concept that did not work. The second effort was production and worked as intended.
Everything else was designed, built, and worked when started, and met specifications once calibrated. Prototypes and multiple iterations are rather expensive compared to getting it right the first time.
Of course, NONE of our products were consumer stuff, where the first priority is to make it as cheap as possible and the second priority is to have the greatest production yield. So probably a quite different world, as our first priority was to have the item meet all the specifications very well, and the second priority was to have the reliability and stability to continue to meet specifications.

Meeting those two goals assured us of adequate profit.e more thing, about touch-controlled devices, is that for me I always switch them, either off or on, quite by accident. I vastly prefer controls that are well defined and always do the same thing with the same action of mine.
On

 

One other thing is that It is very different to do product development than to design a personal project. And if aomething is a prototype for a product the considerations are quite different as well.

 

For about 25 years I was one of the team that designed equipment for specific applications. In that time I experienced only one prototype build. That prototype was based on a hair-brained concept that did not work. The second effort was production and worked as intended.
Everything else was designed, built, and worked when started, and met specifications once calibrated. Prototypes and multiple iterations are rather expensive compared to getting it right the first time.
Of course, NONE of our products were consumer stuff, where the first priority is to make it as cheap as possible and the second priority is to have the greatest production yield. So probably a quite different world, as our first priority was to have the item meet all the specifications very well, and the second priority was to have the reliability and stability to continue to meet specifications.

Meeting those two goals assured us of adequate profit.e more thing, about touch-controlled devices, is that for me I always switch them, either off or on, quite by accident. I vastly prefer controls that are well defined and always do the same thing with the same action of mine.
On

Thanks. Very good advice, but I am still working on improving the prototype and can afford a few iterations.

 

All I can offer for helping me with this, is one of the first limited number of wallspeakers if the project takes off

 

If you bring your product to market someone else will be able to produce the same functionality more cheaply using a cheap microcontroller Which would cost less than £1.00 in quantity. It would still need the power mosfets or relays for switching.

Les.

 

Could use P channel mosfets although relays would be simpler. I believe the biasing is correct for the mosfets at 24 volts.
The 4017 is configured to reset on count #3. Standby current is appx 200 micro amps.
View attachment 285297

Does the LMC555C have to be programmed ?

 

No. The LMC555C is a CMOS version of the the well known 555 timer IC.

Les.

 

Does the LP2950 need cooling from the voltage drop ?

 

The LP2950 should not need a heatsink as the LMC555C only takes about 250 uA at 5 volts. On thing I did notice is that I think sghioto has made a typo with the mosfet part number. He has shown a IRF9450 (Which is an N channel mosfet.) The circuit requires a P channel mosfet (As drawn on the circuit.) I think he probably meant an IRF9540.

Les.

 

Thanks. Does IFR9540 need heatsink ? And is the 4017 order correct off->audio->audio+light ?

 

I think he probably meant an IRF9540.

Absolutely correct! Schematic has been edited with the correct part numbers for Q2 and Q4.
Thank you Les for catching that detail.

 

Thanks. Does IFR9540 need heatsink ? And is the 4017 order correct off->audio->audio+light ?

I would recommend a heat sink. and yes the order is correct: OFF > Audio > Audio + Lights > OFF
According to the specs at 5 amps the dissipation on the mosfet is about 5 watts max with a loss of 1 volt.
There are other option for Q2 and Q4. I recently became aware of the BTS50085 power mosfet that would not require a heat sink at 5 amps.
Can eliminate the zener diodes and associated resistors (6 components) as well. A little pricey at 8 bucks each but no more then a relay.

 

Will you post this curcuit diagram too ? I need the smallest possible PCB.

 

sghioto has posted the circuit diagram (Which is the same thing as a schematic.) in post #9 He has now corrected the mosfet part number in that diagram. If you need to make it very small you will have to use surface mount components. The IRF9540 is only available in the TO220 package and it would require a small heat sink to dissipate 5 watts. If you search for surface mount P channel mosfets you may be able to find one that has a low enough rds(on) value so that it does not need a heatsink.
Does each mosfet have to switch 5 amps or is that the total ?

Les.

 

Will you post this curcuit diagram too ? I need the smallest possible PCB.

 

Does each mosfet have to switch 5 amps or is that the total ?

Actually the LED light only need 1a, but the BT amp need 5a.
PCB can only be 3x1.5"

 

Here's another version of the Touch Switch using a CD40106 hex inverter with certain advantages over the circuit using the LMC555C.
In the 555 version the switch activates immediately on the first touch of the plate which could be prone to erratic operation.
Another issue or not is: if your finger is held on the touch plate the output will continue to cycle through the 3 states every 2 seconds (with the component values listed) until your finger is removed.
The 40106 version provides a delay. You must hold your finger on the touch plate for appx 1 second to activate ( with the component values shown) and will not cycle again until your finger is removed and another 1 second delay has expired.
The LED when OFF indicates that the circuit is ready to activate.
This version will drop the standby current to appx 100ua.

 

Thanks. What could I use for the ligthing, if it only has to be 1a ? What about using IRF520 or D452 (I see those used a lot, but know nothing about MOSFETs) ?

 

Honestly, it all started with me hoping that a product like this already existed. I have now learned that it does not, so I am looking for options. The reason I am so impatient is that this is just a very small piece of my speaker project and I have spent most of my time learning to design and build BT speakers, not electronic switch design. Since the speaker has an aluminium frame, it would be so cool to avoid mechanical contacts, just touch the frame. But maybe I am just dreaming ...

I think you are spinning by focusing on this particular item. Better you should make the first prototype “touch switch ready“ but otherwise working, then once you’ve built and tested, and troubleshooted, and improved your first prototype, you can identify a touch switch solution and fit it.

The very simplest solution to your problem is a small MCU programmed to do what you want. Many have capacitive touch inputs built-in, and once you have an MCU in place, the other possibilities for its application are limited only by imagination.

Good luck.

 

I think you are spinning by focusing on this particular item. Better you should make the first prototype “touch switch ready“ but otherwise working, then once you’ve built and tested, and troubleshooted, and improved your first prototype, you can identify a touch switch solution and fit it.

The very simplest solution to your problem is a small MCU programmed to do what you want. Many have capacitive touch inputs built-in, and once you have an MCU in place, the other possibilities for its application are limited only by imagination.

Good luck.

Thanks, you are right of course, but there is also the "wow" factor of the prototype.