I’m looking for a component that I can integrate into an LED straw so that when the user starts sucking on it the light turns on. What should I be looking at? I disassembled a Juul pod and see that there are two leads that connect when you suck on them, although if that is inside of the straw while liquid is moving over it might not be ideal.
Any ideas?
My understanding is you're designing a novelty LED straw which lights up when a drink is sucked through it
The length of the straw is 100 mm
Thus there is a pressure drop of approx 1 kPa (ref)
Sucking time is about 5 seconds
Desired response time < 0.1 sec
The drink is likely to be about 3 °C (Recommended by drinks company)
Drinks are likely to vary and might be transparent, orange, brown, white and might have particulates or be filtered (water, milk, juice, iced coffee, sodas, cocktails)
Drinks may be carbonated, acidic, alkaline, salty, sweet
Drinks might be viscous (milkshake) or not (water)
Device must be cleanable
I'll assume you have something like this and can put together T-junctions to measure at.
Use a device such as MPL3115A2 which is designed for use as an altimeter in drones, with accuracy of approx 1 m in altitude.
Our 1 Kpa drop is equivalent to about 100 metres or altitude, so the range is about right.(ref)
Consider a device such as Honeywell Micropressure Sensor which is for measuring pressures in liquids!
A hole in the straw can be covered with a flexible membrane. This moves when the internal pressure changes, which is then detected by any photosensing method. Any flexible tube will move under pressure change, or a pipette or dropper head.
Many drinks absorb ultraviolet light. So a sinple UV LED and photodiode pair might well detect drinks.
If the drinks are cold, a thermosensor will detect it. There are endless, from analogue to digital, but one with direct threshold outputs is the AS6212
I would suggest having two metal plates on both sides. Water has a much higher dielectric constant than air. By using 1 Darlington pair you could most certainly detect this change in capacitance and use it to power an LED.
On a non- conducting tube print conducting pads along its length on one side. On the opposite print a solid conducting pad along the entire length. The linear array of capacitors will allow detection the height of water in the straw. LEDs can then be turned on in a sequence driven by the height.
The best solution in my opinion is one which does not have moving parts or sensors that depend on what the user does with the straw. Even if they drink room temperature vodka with it, it should work correctly. This rules out temperature, color, pressure, and many other methods. Two thoughts:
Fully encased microphone (Diffifculty: parsing signal)
If the microphone is fully encased in the straw material alongside the electronics, it should be fully washable, and would only really "hear" or focus on conducted sound waves. I imagine that there would be a sound signature for the user's lips touching the tip of the straw, and then sucking would produce a slight vibratio/sound which may be picked up. To try this method, tape the headphones of any earbuds to a straw, connect to your computer and using software like Audacity record and try to analyze any interesting trends in sound. I dont think this problem is fundimentally different to the clap on/off switches, you just need to detect some predefined pattern.
Capacitive (Difficulty: liquid viscocity)
You can get really creative with a capacitive solution. Lets look at two things you can do. 1- Capacitive slider, as seen below (I think geometry one works better for this usecase)
Source: http://ww1.microchip.com/downloads/en/AppNotes/Atmel-42479-Capacitive-Touch-Long-Slider-Design-with-PTC_AT11805_ApplicationNote.pdf
Source: https://www.matrixtsl.com/webshop/capacitive-touch-slider-sensor-module.html
This slider type sensor allows for two things: When the straw is inserted into the drink, some animation can be played. As soon as the liquid level moves up, the capacitance changes (many microcontrollers have ready libraries for this kind of touch sensor which make this easier to implement). Problem: what if the user adds liquid while straw is in the drink? it may be confused with them sucking. If that's something you care about, think about:
2- A capacitive ring on the top of the straw (or both ends if straw is reversible). This allows there to be an "interlock", which only plays the sucking animation if user's lips touch the straw and liquid level rises. Alternatively, a "lossy" approximation of the user sucking is just them placing their lip onto the staw, so you wouldnt need the sliding sensor.
The problem with this method is that the liquid may just stay on the straw's edges after the first use, which may happen for example if they drink a thick milkshake. Even if the resulting layer is thin, I think it will be sufficient for the capacitance sensor to staturate.
What I would probably do is use the microphone alongside with a capacitive sensor for the lips at either end as an interlock. The user will need both capacitive sensors activated before the straw turns on (the bottom one by the drink, the top one by their lips. This allows protection from the straw being needlessly "woken up" by a spoon getting near one sensor while in storage). Even if something like a milkshake keeps the capacitive sensor engaged, the mic will have the final say in turning on the LEDs, and the power expended by keeping the mic for a little while isnt horrible. The capacitive sensor will allow you to keep the straw's MCU in a low power sleep mode, and when activated you can run the mic to process whether the user is sucking.
To keep it food-safe a capacitive or inductive sensor might be a good idea. There are industrial-type I2C connected sensors available if this is a one-off otherwise most modern microcontrollers (esp32 f.i.) support touch input which might be utilizable for this matter.
Also copper plates on both sides could be a method as the capacity will change when the "dielectric" changes from air to liquid.
Temperature change is also an option: Use a sensor that heats up over ambient temp which is thermic coupled to the straw.
A barometric pressure sensor could do this. Baro sensors measure absolute pressure, so you could measure the vacuum and thus estimate the height of the liquid in the straw from it if you wanted to do a fancy animation of some sort.
Related: Low cost barometric pressure sensor
Difficulty: keeping the sensor port clean.
