How to detect this door latch?
T Chap
Posts: 4,223
I want to check if a patio door is locked or not remotely. I have a Prop that will check a switch/sensor and communication to the home automation system whether the lock is engaged or not. There are two latches that flip out and go into a receiver slot. I want to put a switch or sensor of some sort inside this plate to know when it is latched. I suppose the options are a plunger or lever switch, optical interruption(emitter below and detector above), proximity or inductive sensor. Optical interruption may be less reliable as the light can bounce around and still hit sensor. The sensor needs to have a long lifespan, the simpler and dumber the better.
I use this Avago Gesture sensor APDS-9960 which measures distance and outputs distance in a value in the Prop, so I can set a threshold to check if there is a reflection in less than 5":
https://www.sparkfun.com/products/12787
McMaster sells metalic sensors like this which I assume are inductive:
https://www.mcmaster.com/#73635k81/=1bhiahp
https://www.mcmaster.com/#7364k24/=1bhibzj
The photo I uploaded shows the latches in the Extended Lock Position.
I can remove the plate with two screws and add something inside the plate. Any ideas on a good way to do this?
I use this Avago Gesture sensor APDS-9960 which measures distance and outputs distance in a value in the Prop, so I can set a threshold to check if there is a reflection in less than 5":
https://www.sparkfun.com/products/12787
McMaster sells metalic sensors like this which I assume are inductive:
https://www.mcmaster.com/#73635k81/=1bhiahp
https://www.mcmaster.com/#7364k24/=1bhibzj
The photo I uploaded shows the latches in the Extended Lock Position.
I can remove the plate with two screws and add something inside the plate. Any ideas on a good way to do this?
Comments
Sorry I just had to say it.
Here is one I have somewhere in my shop. This could easily be glued in.
Surely you are both joking. Get a slotted optical switch with the proper configuration for screwing it to the strike plate. Drill and tap the strike plate, and truly make it reliable.
-{hil
Then I'd mount it to the door frame, couple screws will do nicely.
Then glue a neodymium magnet to the door face, even with the sensor.
Close the door read the sensor, open it a crack and read the sensor again. That's your threshold.
That's the simplest and least invasive.
-Phil
+1. K.I.S.S.! Nothing to go wrong and good for a million cycles.
Simple, yes. Nothing to go wrong for a million cycles? Maybe....or maybe not. I replaced an awful lot of micro switches used in low voltage light switches for building automation systems every year. Far far more than should have been replaced if they were actually good for anywhere near a million cycles.
+1 give me something without moving parts any day of the week and twice on Saturday
I have a similar application that will not allow for wires. I want to know if a latch is present but there can be no wires ran to the switch or sensor. My home alarm has door contact reed or hall sensors with a single cr2032 that has not dropped hardly any over 2 years since it was installed. It wirelessly talks to the home alarm lcd brain. My guess is a PIC with a very low clock speed watching the contact and it turns on full speed if at all just to send data to the main box. Anyone got an idea how to watch a contact and get years of life like this example? I have the Nordic NRF24L01+ working well for other applications on the prop but that board only gets a few days of life with 2 AA but at 80mhz. One option is use wires but run the wires to an area where there can be a through-air power transfer. This might even possibly be preferred over replacing batteries every few years. What to search for to find this type of power transfer to end up with 5VDC.
OK. At the extreme end of the Weibull probability curve, you get the Smile switch which fails at 1% of a million cycles. That's 10,000 cycles. Activated once a day, that will last over 27 years. I could live with that.
Yikes. I could die with that.
Newhaven Display LCD datasheet: 50,000 hours Backlight lifetime, rated as Hours until half-brightness, under normal operating conditions.
An LED for a photo interrupt will have a life span too.
BTW my xfinity alarm front door reed/hall and magnet(whatever type it is) goes off randomly every day. It is mounted perfectly. Something is wrong they have to replace it.
I think the early failures of the light switches has more to do with with the use case rather than the quality/cycles of use of those switches. By that I mean that the lights tend to be turned on or off with a tap or slap rather than a more gentle press. The same type of closure action is probably going to happen with that style of latch.
Edit:
The issue with this part looks like with low carbon steel you need a distance of around .1" to get it to switch. That's a tight tolerance in this case but I want to check them out for future reference anyway.
In regards to your reliability question, yes, very reliable. I know of a product used by firefighters that utilizes these switches.
I have a sliding door with a similar latch and also wondered what kind of sensor I could use to make sure it was fully latched...
Those are impressive - I briefly looked at the specs and thought the air gap distance was about 1 inch but a second look showed that was with NEO 35 magnet...
With ferrous metal it is closer to .1" as T Chap found out. Like he said - might be handy in certain situations.