Asking for help on a simple liquid sensor project - Page 1
Forum section: electronics / projects

author: mistercris
author_rank: Contributor
timestamp: April 30, 2018, 12:32:36 am
content: So I've got an aunt who's gone blind and I'm trying to build her a liquid level sensor probe (sort of likethis one by the RNIB).Can't use this cause she's not really using it on mugs. She's from Argentina and they drink a tea called Mate (pronounced mah - teh) that uses some weird sized and shaped containers (in many cases it's actually served in hollowed out gourds).Naturally, being blind causes issues since you're pouring very hot water into a small receptacle, usually very full of hot tea leaves.This is a very strong culturally rooted social activity in argentina, you get together with friends and or family and a single gourd or cup is served and passed to each participant as you sit and chat. So I really want her to be able to go back to doing so.That's the background to explain why I'm doing it this way and I'm hoping some kind and knowledgable people might be able to offer some suggestions and advice.I was thinking to keep it as simple as possible and just came up with this simple circuit (in the included image).Basically a power source (battery), a small speaker and three resistors in parallel.I had figured using a long probe (a stick basically) with lead from power at the lowest point and a lead from each resistor attached up the probe at varying levels. Every time the water reached a new lead the volume in the speaker increased, till max at the highest lead (corresponding to the highest level of water).Did not work. Don't know enough to figure out why.... Works if I connect the leads in circuit but can't figure out why it won't make a circuit in the water.Help please?

author: sokoloff
author_rank: Super Contributor
timestamp: April 30, 2018, 11:39:41 pm
content: If Mate is non-ionic, it's not really going to work.(Try your solution again with salt water. It will  work. Try it with plain tap water. It  won't.)

author: sokoloff
author_rank: Super Contributor
timestamp: May 01, 2018, 12:58:59 am
content: As for other approaches, could you try weight sensing?Could you try a thermistor or thermocouple (or several) on a stick?And I don't mean to be insensitive, but if this is a social gathering thing, surely the family/friends would be more than wiling to let the blind woman skip out on her turn to make the next cup of Mate, no?

author: mistercris
author_rank: Contributor
timestamp: May 01, 2018, 01:39:51 am
content: Quote from: sokoloff on May 01, 2018, 12:58:59 amOh everyone tries, and there's always someone willing but it's a thing for her. Hard to explain. She wants to be the one doing it. Right now she's just sticking a finger near the lip of the cup till it gets hot. Needless to say, she spills and burns herself a lot. Still wants to do it. They just put the thing on a tray so at least it contains the mess.With regards to the suggestions you gave, how small can these be made? I have a sensor that came with an arduino kit but it's much too big.

author: sokoloff
author_rank: Super Contributor
timestamp: May 01, 2018, 01:53:21 am
content: A thermocouple is just two pieces of different wire fused together. They can be tiny as a result. Being tiny has the other effect of being fast reacting due to low thermal mass.You might have a DS1820 type sensor, which is much bigger than a bare thermocouple.You might also be able to do mass estimation by combining a piezo or cell phone buzzer motor and seeing how that changes as the cup/gourd gets heavier. This might not even need sensing electronics. Just attach the vibrating motor to the tray or cup/gourd and see if you can tell the difference in vibrations as the vessel fills. I’d bet you can and I bet you aunt might be even more capable of sensing the change.

author: Blinkenlights
author_rank: Contributor
timestamp: May 02, 2018, 06:24:47 am
content: Yes, you will be able to make this work by sensing a resistance across the two terminals when the liquid reaches that level.But your suggested circuit is not good enough - even when replacing the speaker by a very sensitive Piezo sounder you are unlikely to get sufficient current to hear a sound.You need to sense the resistance with a voltage comparator or transistor.That needs a little care to keep power supply consumption low.Normally when making these circuits it is also important to sense with AC,but as you want just the one "beep" and then the device will be removedand the terminals allowed to dry, DC will work just fine.There will be sufficient conductance in "Mate" for this to work, I am sure -the only time I had difficulty was with Vodka - the purity meant that therewas very little conductance. I found that interesting.