How do touch sensing electronics work? - Page 1
Forum section: electronics / beginners

author: liviu.gelea
author_rank: Contributor
timestamp: December 23, 2020, 09:22:38 am
content: Howdy. I'm trying to get my head around how touch sensing works.The devices I'm drawing inspiration from are the classic 80's touch lamps and I'd like to apply a similar principle to add some power saving features to a battery powered lamp which has a full metal body. (low power 555-ish monostable timer for ON state)The only problem is that those touch lamps used to work on AC and could manage to get more current through the human body, while any contraption I make would have to use either the body's small capacitance or the low current discharge.Now I completely lack the intuitive understanding of how such a device would interact with the human body.Having played with mosfets a bit I know that touching the gate lead momentarily would randomly turn a powered mosfet on or off which makes me think that the body probably also acts like an antenna for all of the EM waves around. If I connect an LED to it, I notice the led can be put in a state which draws way more current than if I were to power the gate directly at 5volts. This makes me think the potential of the human body varies past +-5vSo basically I'm out of ideas. This seemms to be an issue of connecting two systems that are at different ground potentials (and one is AC while the other is DC) and expecting to predict how one will behave.Thisschematicsuggests just touching the damn 555 pin. How would that work? Why would that work?Thanks.

author: radiolistener
author_rank: Super Contributor
timestamp: December 23, 2020, 12:28:30 pm
content: when you touch terminal, it leads to change capacitance.Such capacitance change is detected and used to control some circuit.Another way is to sense voltage interference on high impedance input.

author: ledtester
author_rank: Super Contributor
timestamp: December 23, 2020, 12:29:49 pm
content: For touch lamps, this is a good video:https://youtu.be/TbHBHhZOglw(you can probably skip forward to 5:15 for the start of the explanation)If you are inclined to build you own touch switch for low-voltage circuits, I'd just get a ready-made module like the TTP223.

author: liviu.gelea
author_rank: Contributor
timestamp: December 23, 2020, 01:33:47 pm
content: Why would touching the lead charge capacitance if the body's potential oscilates? Is there a dc component besides the induced one?

author: atmfjstc
author_rank: Regular Contributor
timestamp: December 23, 2020, 02:09:18 pm
content: He said the capacitancechanges, not charges. A capacitor is essentially two conductive plates (usually metal) separated by an insulator. Its capacity is affected, among other things, by the size of the plates. When you touch one side of a capacitor, your body, which is also a conductor (not as good as metal, but enough) becomes part of one of the plates, changing its effective size and thus the overall capacity. This change in capacitance can then be detected, e.g. by a circuit that uses the capacitor as part of an oscillator. This is how proper capacitive sensors work.It is also possible to trigger some circuits by just touching their pins, but that's a different mechanism and not as reliable. Your body, being a large conductor, acts as an antenna, so if you touch a high-impedance pin, even the tiny currents induced in your body will be enough to change the voltage of the pin. They will do so randomly (or rather, reproduce whatever radio signal your body is "receiving"), but the circuit only needs the voltage to momentarily exceed some preset value to trigger, which is inevitable. Unfortunately if your body can act as an antenna to trigger the circuit, then so can any other antenna-like thing that connects to the pin (e.g. wires, traces), or just random electrical noise if it is strong enough.

author: liviu.gelea
author_rank: Contributor
timestamp: December 23, 2020, 03:44:01 pm
content: Thanks. In this case I will try to see if I can tune the 555 to diferentiate between the two.Probably not. I remember a video about touch lamps mention they "tune" their threshold when powered up.Still worth a shot as I have 555 ic's around.Many thanks to everyone