Thank you @Raymond Genovese. The force sensing resistor might be what I need.

Thanks to all for providing me with great options all of which can be used for various types of projects.

@djsfantasi and @Yaakov. I originally hoped there might be one type of switch, pad, resistor, etc that I could use that could be calibrated for various uses (ex: putting something heavy on it like a mug or pushing down on it with your finger) but based on the answers I have received there might not be one magic component that will work for different pressures/weights, etc. being put upon it.

I guess at a high level I was thinking of a decorative base (possibly made out of laminate) and when you put something on it (like a mug) then the base would do something (ex: light up) and when you take the mug off it shuts off. I would somehow have to secure the component so it can receive the pressure needed to activate it but be safe so it can't give off a shock.

What's interesting is that after reading the answers people have provided I just realized I have a microswitch in my collection of components. I had no idea what it was or what it did until this post. LOL!! It was sold to me as a possible alternative to a liquid float switch but I don't think I can safely make it work for that however it might work for this project.

:-D

There may be a solution that can be calibrated so that it will work for various uses. But the solution may be mechanical rather than electrical.

I envision a base that pivots on a line connecting two points along the base perimeter. Then orthogonal to the base plane, is another piece of material which forms an L shaped lever. Thus a force on the base is amplified by the lever - which could trigger the microswitch. The microswitch could be mounted so that it is either normally closed or normally open.*

The adjustment or calibration is done with a spring whose height is set by a screw and nuts. The placement of this spring would be on the far edge opposite the midpoint of the pivot line.

Just daydreaming!


* with a SPDT microswitch, the “normal” state of the switch (without an object) is irrelevant.

 

If you were more specific about what you want to, the answers would flood in.
Some very common pressure switches as used in washing machines, operate on very low pressure by virtue of having large diaphragms to operate a micro-switch - this is the mechanical advantage mentioned earlier.
Tactile switches also come in low operation force versions.
There are magnetic switches that detect small movements without contact, plus optical.
Two thin metal plates separated by small dots of squidgy (elastic) silicone rubber might work.
The innards of a pressure gauge, gears and levers would give you a lot movement magnification.
Ask Heath Robinson.

 

In addition to what has already been offered, I would add:

Force sensing resistor. https://www.interlinkelectronics.com/standard-products

There is also: https://www.tekscan.com/force-sensors

They also have good documentations on their site, circuits and practical implementation, such as avoiding overload and shear stress on the sensor, using a metal disc to cover the sensor so that all of the sensor is mobilized (in case of larger ones), etc.

 

"I guess at a high level I was thinking of a decorative base (possibly made out of laminate) and when you put something on it (like a mug) then the base would do something (ex: light up) and when you take the mug off it shuts off. I would somehow have to secure the component so it can receive the pressure needed to activate it but be safe so it can't give off a shock".

A mug would activate any of the devices mentioned - a fly would be trickier...
Most circuits like this operate happily at less than 12-volts DC so no chance of any shocks - hopefully no dodgy power supplies.
Why don't you just use the micro-switch you have, get an LED - super bright blue? - wire it in series with a resistor, say 1k, connect to say, three AA cells in series - 4.5V DC, then press the micro-switch. If it doesn't work, swap the connections on the LED - the long leg goes to the positive.
The micro-switch might have three tabs on it - one is common, the others are either normally closed or normally open - this is a changeover switch. The basic micro-switch has a button and sometimes a lever or even a wire to operate it. You could have the base for the mug attached to a spring and the lever or whatever of the micro-switch fixed to the bottom bit. You'll learn so much just getting that to work - then start experimenting

 

As many people have already said, it's very hard to give good advice when we only have half the story!

For example, early on, you said that you were looking for something 'very flat', but then go on to consider a microswitch. Any system using a microswitch would necessarily be quite 'thick' - definitely not the thickness of a coaster!

You then talk about a mug being put on a coaster, or a finger pressing on a plate. Well, either of those could be realised by using a completely different approach - a light-sensitive resistor. When uncovered (ie lit) no action is taken; when covered by a finger, a mug, a lorry load of sand, whatever it is is switched on.

If you are looking for a universal solution to suit all circumstances, I think you'll be looking for a long time!

 

As many people have already said, it's very hard to give good advice when we only have half the story!

For example, early on, you said that you were looking for something 'very flat', but then go on to consider a microswitch. Any system using a microswitch would necessarily be quite 'thick' - definitely not the thickness of a coaster!

You then talk about a mug being put on a coaster, or a finger pressing on a plate. Well, either of those could be realised by using a completely different approach - a light-sensitive resistor. When uncovered (ie lit) no action is taken; when covered by a finger, a mug, a lorry load of sand, whatever it is is switched on.

If you are looking for a universal solution to suit all circumstances, I think you'll be looking for a long time!

@PhilTilson, thanks for this. The reason I was considering all options is because I have no idea what the components that were suggested look like so I was going to research the options to see whether they were viable for my project. I was originally thinking of a component that would be as thick as a piezo (for example) but given my housing for the circuit would be whatever I needed it to be the component could be thicker if needed (within reason). Ideally it would be thin enough/small enough to work in the thickness of a coaster but this isn't necessarily a must-have if there isn't anything that works.

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).

Thanks again!!


Karen

 

Have you taken a look at https://www.tekscan.com/sites/default/files/resources/FLX-A101-G.pdf ? Note that those sensors work poorly if the force has a shear component (parallel to the disk area). So, generally, you use a metallic disk over the sensor and a metallic plate centered over that disk to make it like a digital balance. You have then to calibrate the electronic circuit to the added weight to "zero" the sensor for that weight. The resistance offered by the sensor is inversely proportional to the total (perpendicular) weight. So, if you know the weight of the mug itself, you may be able to detect when the mug is empty, half-full or near overfull !

There are companies which prepack for you the sensor with an op-amp and make the whole thing ready for an Arduino, if that option is a preferred possibility for you.

 

The attached photos are of a control board out of a stereo with a dual cassette recorder/player. The switches are very sensitive to pressure, so a finger or a mug will definitely close the switch. You WILL need additional circuitry to control a warming pad, these tiny switches can not handle the current. However, building a coaster that can house a small switch like these is going to take some skill. Definitely doable. Especially if you have a 3D printer. Not sure how a printed coaster would hold up to the heat of a warmer, but.

In the photos - the board from left to right shows six switches. The two black tower switches on the right are what the rest of the switches looked like before I pulled their covers off. The narrow shot shows the profile of a switch. The switch is currently closed but that's because I've mis-handled them a little over the year or so they've been kicking around. I have no use for them right now, but I'm sure you can find an old stereo tape deck that has similar switches on them. Again, they can't handle the kind of current you want, but they most certainly can handle switching a MOSFET on and off. Well, ON. You'd need a pull-down resistor to shut the FET off when the switch is open.

 

@vanderghast and @Tonyr1084, thank you so much for these additional options. I just started to look at Arduino as an option but have no experience with it yet so might have to try some beginner projects first in order to get a feel for it.

 

To get some idea of pressure sensitivity, I slopped together a 3 X 3 in. sandwich of brass plate; 3.5 X 3.5 in sheet of " Velostat " topped with .001 in. 3 X 3 in brass shim stock. Resistance read with Heathkit # MM1.
No load- 10k, cup - 240 ohms; med hard thumb - 10 , 1, US $ .25 - 4.7k; 9, $ .25 stacked - 2.6k; 9, single layer
2.4k; 1, AAA alk. battery - 4.8k, 1, CR 2032 - 5.5k. Readings may be similar to Force Sensing Resistor ref.
@RaymondGenovese, # 19. You could put a ring of LEDs around the coaster which light up with pressure. AS mentioned earlier, control by comparator & LED driver.
More complicated would be an EL panel or EL wire.
What country are you in ?

 

(...) The switches are very sensitive to pressure, so a finger or a mug will definitely close the switch. (...)

View attachment 185422 View attachment 185423

Unless they are momentary switches, that won't work, I think. Indeed, for standard latching button, you press one, keep it down with your finger, and remove the pressure (remove your finger) and the button latches on. Repeat, I mean, push it again, keep it down, then remove your finger to latch it back in the off position. So, for the mug, putting the mug the first time, will be like pushing the button for the first time, and keeping the finger on it. Removing the mug, ... that would be like removing your finger. The latch will be on !!!. Even if the mug has left the spot.

If they are momentary buttons, that may work, but the displacement (the course of the button) may make the cup holder suspicious for the user, since the liquid in the mug may start to splash.

Note that two aluminium foil rectangles separated by a parchment sheet ( cooking paper sheet ) make a rudimentary capacitor. Its capacity, in nanoFarad, changes dramatically, easily measurable, if you squeeze them with simply the weight of a Canadian loony! So, should it make the weight of a mug, or its absence, even more detectable. Now, have to make a circuit detecting that change in capacity. A serious problem with this is the repeatability of such a crude device though: with time, the capacity may change so much that you will need new calibration... That is surely not an "industrial" like solution

 

Hello folks,

I'm fairly new to electronics but the more I read and the more I tinker the more I learn. I would like to build a simple battery operated circuit that uses a pressure switch/sensor/plate but I can't seem to find such a switch when searching sites like this or just general google searches so maybe I'm not searching on the right keywords.

I want it to be small like a regular push button switch and the behavior is that the circuit stays closed while there is pressure and open when there is no pressure. An example would be holding your finger on the switch keeps the light on. The minute you take your finger off the light goes off.

I've seen pressure plates but they are way too big. I wouldn't mind something like that but scaled way down. Just a tiny pad/plate is needed.

Thanks in advance for any help provided!


Karen

 

Cheap bath scales usually have a relatively small pressure sensor. To make your own carbon granules,such as were used
in telephones for many years are a possibility.
Del

 

Cheap bath scales usually have a relatively small pressure sensor. To make your own carbon granules,such as were used
in telephones for many years are a possibility.
Del

@TechGirl
https://www.google.com/search?tbm=shop&q=pressure+sensitive+switches

 

@vanderghast I believe I read the Thread Starter (TS) wants power to come on when the switch is held down by the weight of a cup. The TS also mentioned a cup warmer, so it's likely the intent is to warm a cup when present and not when there is no cup. Well, this is my take on all this.

The switches I photographed are signal switches and certainly couldn't carry the current of a cup warmer. The TS would have to build some sort of circuitry to power the warmer. I believe lights were also mentioned. Less current than a warmer, but still I wouldn't count on my switches to carry the slightest current beyond 200 mA. And that may be more than their design.

One thing concerns me with my switches: If you put them on the bottom of a cup holder - the weight of the holder itself may be sufficient to trigger the switch.

 

I do not believe TS said cup warmer, just illuminate the coaster.

 

When the mug is on the coaster it pushes down on the switch and the switch turns on and warms up the coaster.

@Bernard Does this help?

 

One thing concerning me about a coaster with a pressure sensitive switch: The switch would have to be on the top side where the cup can press down on the switch. The switches I photographed are so sensitive to pressure that the mere weight of the cup holder would close the switch. So the switch would have to be on the top side. And then the cup has to make contact with the switch. If the bottom of the cup, like mine are, has a curve to it then hitting the switch with the cup would be a hit-or-miss affair with mostly "miss". An optical sensor similar to a mouse might be more sensitive. Or even a simple light detector. If there's a cup present then the shadow can turn the warmer (or whatever you're switching) on. A second light sensor can determine when the room is dark and prevent the warmer from coming on at night when there's no cup present. The coaster will be thicker than normal; but if all your circuitry is done in SMT then it shouldn't be too thick.

Bernard and I are unsure of what you are attempting to do. So a little clarification would be useful. IF we're talking about a cup warmer then you're going to need a significant power source - be it something plugged into the wall. If a light - batteries may fill the bill. A coin cell like a 2450 (bigger than a 2032) can power an LED for a significant period of time. But again, the issue of sensing the presence of a cup. If using light to detect the cup then a lighted coaster would defeat the purpose of light sensing because the cup would then be lit. So a little more clarification on what you're doing would help us help you.

Another possibility would be a beam and detector. When something blocks the beam the unit can switch on and do whatever it is you're attempting to do.

 

Later than a heater, post #6, the TS said in Post # 26, " Put a mug on a coaster, coaster lights up." The mug would cover most of the coaster so the illuminated coaster would not be seen so a ring of LEDs or loop of EL wire was suggested.
I also re built SW with easy located aluminum foil & Velostat, added a Darlington transistor,D1326, 4.7 k & LEDs with an R; Mug on- LEDs on, mug off- LEDs off.
Power = 3 AA Alk. Other transistors like MPSA13 OK or LL N ch FET.

 

If you are into working with things instead of purchasing them, then you can capture the materials to assemble a membrane switch by dismantling something that uses a membrane switch. Quite often it is the switch that fails on a cheap appliance.
It is also quite possible that somebody of the "maker" mindset has already come up with a way to produce a pressure pad switch or a membrane-equivalent device. The big question is how much effort you want to put into the project.