Hermetry

Posted October 26, 2026 at 1:30 pm · in Sensors on campus lamp posts with no wiring, can we power them wirelessly? #5

OK so batteries. Does Hermetry City handle that or do I need something else?

Posted October 12, 2026 at 1:44 pm · in Eight meters on 30 m of RS-485 at 9600 baud: do I need termination, or is it something else? #3

Does cable length really matter at only 9600 baud?

Posted October 4, 2026 at 1:47 pm · in Night-time bed-exit alert for a care home: which sensor, and how do I only alert between 22:00 and 06:00? #4

How do the night staff work, one person on a fixed station or walking around? That changes what kind of alert makes sense.

Posted October 1, 2026 at 7:06 pm · in Reading a 9-bit multidrop bus on a Pi gateway: can the UART even do that? #2

What’s the 9th bit for exactly? I’ve only ever seen 8N1.

Posted September 29, 2026 at 7:39 pm · in Python call to the latest-values endpoint hangs for two minutes on our Pi #4

Had exactly this at one of our properties after the ISP turned IPv6 on. The Pi gets an IPv6 address, tries it first, the route is broken and it waits for the timeout before falling back to IPv4. Your curl test shows that.

I didn’t want to switch IPv6 off for everything, so I told the Pi to prefer IPv4. Uncomment this line in /etc/gai.conf:

precedence ::ffff:0:0/96  100

No reboot needed for new processes, just restart your script. If you only want it for the script, you can force it in Python before importing requests:

import socket
import urllib3.util.connection as conn
conn.allowed_gai_family = lambda: socket.AF_INET

And Owen’s idea is still a good one for later. Put the hotel’s public IPv4 in the key’s IP allowlist so a stolen key is useless elsewhere.

Posted September 29, 2026 at 4:33 am · in SX1276 in FSK mode: what AFC, AGC, RSSI and FEI do, and why my links fail on cold mornings #3

Is this the same radio as LoRa? I thought the RFM95 was LoRa only.

Posted September 23, 2026 at 1:19 pm · in Why doesn’t LoRaWAN mesh? Four sensors behind a steel warehouse never join #3

Can’t you just put the antenna higher?

Posted September 20, 2026 at 12:32 am · in Uno as an I2C slave sending floats to a Mega and taking commands back: values come out garbled #3

Maybe you need pull-up resistors on SDA and SCL? I read I2C needs them.

Posted September 3, 2026 at 1:09 am · in Switching Wi-Fi bulbs from a board Switch through my Pi, no vendor app #3

Will the switch show if someone turned the light on at the wall?

Posted August 22, 2026 at 1:13 pm · in Inverters on RS-485 and serial-radio sensors: what gets non-IP gear into Hermetry? #6

so the gateway counts as one device or 26?

Posted August 16, 2026 at 6:55 pm · in MCP3008 gives 0 or 1023 only: how do I prove SPI works on a Pi 3B+? #2

following, I have an MCP3008 in a drawer I never got working either

Posted August 3, 2026 at 3:27 am · in Pi 3 in a steel box on the shed keeps dropping off Wi-Fi: can I add an external antenna? #3

I’m new to IoT but we had exactly this with a Pi in a steel box in our hotel’s pool plant room. Our IT guy talked me out of soldering. A closed steel box is close to a Faraday cage, so the onboard chip antenna inside it will always struggle, and the Pi 3 is not designed for an external one.

What we did instead, cost about 20 dollars:

A USB Wi-Fi adapter with an RP-SMA connector.
A short RP-SMA extension cable through a cable gland in the box, antenna mounted outside on top.
Onboard Wi-Fi switched off so the dongle is the only wlan0. Add dtoverlay=disable-wifi to config.txt and reboot. Then put your Wi-Fi settings on the dongle as usual.

Our signal went from about -84 to -58 dBm.

On the Hermetry side I also lengthened the offline alarm. The docs suggest an Open after time longer than a normal reconnect for devices that can drop briefly, we use 600 s, so a 30 second Wi-Fi dip no longer wakes anyone. Keepalive is 60 s in our script.

Posted August 2, 2026 at 1:06 pm · in Can a Pi Zero get Ethernet through its mini-HDMI port (HEC) for a sensor node? #6

Yep, laptop shares its connection (NAT) and has to stay on. That’s why I went with the adapter.

Posted August 2, 2026 at 4:15 am · in Optocoupler plus high-side MOSFET into a 24 V PLC input only gives me 9 V #5

Probably the least qualified person in this thread, but our electrician sorted almost the same thing in our boiler room, and Emily’s point is why yours sits at 9 V. Your 10k pull-up and 10k pulldown make the gate about 12 V, minus the threshold gives the source a bit under 10 V. The MOSFET is half on, so it gets warm.

His fix was to throw the MOSFET out. A PLC input like that only draws a few milliamps (ours was about 5 mA at 24 V, the manual says), and a PC817 transistor can switch that by itself:

Pi side:   GPIO17 -- 330R -- PC817 anode (pin 1),  cathode (pin 2) -- Pi GND
24 V side: +24 V -- PC817 collector (pin 4)
           PC817 emitter (pin 3) -- PLC input I0.5
           PLC input common -- 0 V

Opto on, the input sees close to 24 V. Opto off, nothing. Isolation stays. If you want something with a DIN rail clip and a label on it, a ready-made 24 V opto or relay module does the same.

On the Hermetry side he set the hold command as High impact so whoever sends it has to confirm. A board control won’t send a high-impact command though, so it goes from the device’s Commands tab. For a hold request that stops a line, we found that was fine.

Posted August 1, 2026 at 2:07 am · in Can a Pi Zero get Ethernet through its mini-HDMI port (HEC) for a sensor node? #4

I’m a facilities guy, not an EE, but I chased this exact idea for a lobby display last year and burned an afternoon on it. The Pi doesn’t implement HEC. On HDMI that’s pins 14 and 19, and the Pi has no Ethernet PHY behind them, so there’s nothing to switch on in software. No overlay will fix it.

What I ended up with, and what I’d try in your order:

A micro-USB to Ethernet adapter on the Zero’s USB port (not the PWR one). ASIX AX88772 based ones work out of the box, eth0 just appears. Mine was about ten dollars.
USB gadget mode: the Zero pretends to be a network card to a laptop over the USB cable. Add dtoverlay=dwc2 to config.txt and modules-load=dwc2,g_ether after rootwait in cmdline.txt. Then the laptop has to share its connection.
Swap for a Zero 2 W if Wi-Fi ever becomes allowed.

Once eth0 is up the MQTT side is the same as any other Pi, TLS on 8883 with the device token.

davidp11

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