Help needed for pressure switch/sensor/plate

Bernard

Joined Aug 7, 2008
5,784
What is the effect of a coaster on pressure plate? I guess a coaster is to contain or absorb moisture.
'tried 3 different coasters, Cardboard, .3 oz. = 10k;, with mug = 1k; rosewood, 1.3 oz. = 3k, with mug = 1.2k;
thirsty stone, 6 oz.= 1.7k, with mug = 1k. Measurements are some what un steady, around +- 10% so the greater the difference between loaded & unloaded the better.
 

Reloadron

Joined Jan 15, 2015
7,501
I thought I had described what I was looking for in a later post. Basically it's "Put a mug on a coaster and the coaster lights up". "Take the mug off and the coaster turns off". The pressure switch activates the light and the force comes from the weight of the mug. After a few months of not thinking of this I thought of a digital scale as being similar. You stand of the scale and it gives you your weight. You hop off the scale and the display goes dark. I looked this up and I think they're called load sensors (or maybe it's force sensors). I have an old digital scale that I was about to pull apart to see what the component looks like because it's hard to tell from pictures on the internet as to it's actual size (as is my problem in researching all of the suggestions that have been given).
Pretty slick idea. My initial thought was just place a tactile switch under a coaster pad. I measured a coaster I had lying around and it was a 3.8" diameter. When I say Tactile switch here is what I am getting at. If all you want is On/Off about all you need is a simple tactile switch and a few LEDs mounted into or on a maybe Plexiglas ring around your coaster. You would have to maybe play around to see how much force is required for the switch to operate. Figure the weight of the platen (disk) placed on the switch plus the weight of a mug or cup. Pretty sure if you Google Tactile Switch Pressure you can get some real world numbers to work with. There are literally hundreds of versions of these little switches out there. Now if all you want to do is turn On/Off a LED illuminated ring all you need is a simple switch. Now if you wish to get fancy then you start looking at pressure sensing schemes and things get a little more complex. Just for example if you want the LED light scheme to change based on weight of the cup and amount of content in the cup.

I would make a small round PCB and actually for prototype work it need not even be round, mount a switch, place a platen (round disk) on it and work from there.

Edit:
Plexiglass was a poor choice of words. What you likely want is an acrylic outer ring of the coaster because when illuminated with a LED you can get some nice effects.

Also, here are additional switch examples and data.

Ron
 
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MisterBill2

Joined Jan 23, 2018
18,179
If the goal is really to have a coaster light up when mug is placed on it, then a very simple approach is to have a radial array of perhaps 8 thin stainless steel wires, alternating so that alternate wires are connected as opposite sides of the circuit, serving as a target for a conductive material on the bottom of the mug. And that could be a neat feature because if somebody else placed a non-prepared mug on the coaster it would not light up. With some variations it could produce different colors with different mugs, if the switching arrangement got more complex. And the best part is it would require no additional electronics except for the battery and current limiting resistor. All you need is some #30 wire of some material that does not corrode or otherwise develop an insulating layer.
 

Thread Starter

TechGirl

Joined May 23, 2019
10
@Tonyr1084. I should clarify. My original post was asking to light up the coaster only. Further down I mentioned warming the mug but that was just an example of what the switch might do. What I was trying to explain is that I needed a pressure sensitive switch that when pressure is placed on it the circuit is closed and then something happens. I wouldn't mind an automatic mug warmer that lights up but that's probably beyond my capacity. Sorry for the confusion. I shouldn't have used that as an example. At this point I only want to have LEDs light up. A more advanced circuit would have the LEDs begin to fade as the pressure on the switch decreases (less liquid in the mug). Like a visual indicator of when the mug is almost empty or is empty but at this point if I can just get it to turn on when the mug is placed on it and turns off when the mug is removed I would be thrilled. :)
 

Tonyr1084

Joined Sep 24, 2015
7,853
A small switch like the kind I posted can drive a MOSFET which in turn drive an LED. The only thing you need to be sure of is that the switch gets triggered when any size or shape mug is placed on the coaster. You could even encorporate several switches so that it's likely one of them will be triggered. Use an 8 input OR gate so that when ANY of the switches are depressed the circuit is activated. When any switch is activated the LED can come on. When the switches are all open you can have the LED light level decay via an RC circuit to keep the MOSFET driven then face out. There will be some small heat generated during the fade out - but it shouldn't be a problem.
 

Reloadron

Joined Jan 15, 2015
7,501
At this point I only want to have LEDs light up. A more advanced circuit would have the LEDs begin to fade as the pressure on the switch decreases (less liquid in the mug). Like a visual indicator of when the mug is almost empty or is empty but at this point if I can just get it to turn on when the mug is placed on it and turns off when the mug is removed I would be thrilled.
With that in mind you may wish to think about what I suggested. All you need is a pressure sensitive On/Off switch, a power source like a battery and a few LEDs.

Ron.
 

Bernard

Joined Aug 7, 2008
5,784
Re post # 45, fading LED. Just as proof of concept used Velcro sandwich, PS, & 10 k pot. as voltage divider
with + 4.5 V ( 3 AAs ) to top of PS, bottom to pot. wiper, one led of pot to neg. supply. 2N3904 as emitter follower
; base to wiper, emitter to 68 ohm & white LED & _, collector to+ V. With mug on PS adjust pot for dim. Adding liquid should brighten LED. Clear as mud but it worked for me.
 
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