BasicPI

Ferrite RAM is one of the older RAM techniques used on old mainframes dating back to the 60s, but have in recent years also been popular as a replacement for EEPROM and flash because it is fast and have close to unlimited rewrites. I recently purchased a bunch of FRAM chips with the same interface as SPI flash in SOP8 format. This is excellent for the Blackpill that is a STM32F401CC  breakout board with space for a SPI Flash on the back since they use the same footprint. These FRAM chips cost around 4.- USD and will only have 32Kb (kilo byte), but that is excellent for my usage.

STM32F401 is not ideal for a low power battery operated application, but with the current MCU shortage I decided to give it a try since it is available with a breakout costing around 3.- USD in dip40 format.

Despite being hight performance and not low power the STM32F401CC is still capable of dropping down to uA while using only RTC and executing sampling using < 10mA. Reading an ADC (temperature) can be done once a second at very low power on a 1000mAH, 3.7V battery. And with a FRAM we can wake it up, read and write to it and be done within 5ms giving the battery a decent lifetime.

My first application is a simple temperature sensor that uses radio. I will sample to a FRAM and then connect to a server and transfer the FRAM content. To do this I create a simple array on FRAM storing time and temperature every second and every 10 minute or so I wake up a LoRa and send the content starting over.

I considered using Wifi (ESP), but realized that connection time on any Wifi unit would be 3-7 seconds with 150ich mA – far to high for battery usage. So I will test LoRa. The first usage will be 1-2 units only for testing.

I have also purchased STM32L4P5CE MCU’s that is much better on low power than STM32F401CC, but these are also more expensive and difficult to get by these days. And it is not the current usage of the MCU that is my main concern – it is everything around the MCU – Temperature sensor, FRAM, LoRa sender etc.

I plan to add far more sensors than temperature, but I need to experiment with low power solutions and temperature is a simple, good start.

In the Blackpill case I will need to put FRAM in sleep (12uA) and wake it up which takes ca 0,5 ms – while operating it uses 2.3 mA and I think I can finish this part within 1-2 mS which will be great for my battery budget.

One of the challenges I face using wireless communication is that it requires a bit of power/time to send signals.

Wifi : ESP-M1 is the first solution I evaluate. This is the smallest of the ESP modules and can send using 120-170mA. Simply speaking if you have a battery with 1000mAH or( 1AH) you can operate non-stop in 5-7 hours. So the classic technique is to switch off and only send then you need to. But, even in Rx mode we use typically 20mA and using that as base we have 50 or so hours of operation. We are still far off where we need to be. Deep sleep mode for this ESP module is <10uA according to the datasheet. 1000 / 0.01 = 100,000 hours meaning we suddenly talk about 10+ years in that mode assuming we use a 1AH battery. But, I can’t do much in deep sleep mode and I am surprised over how long time I use to connect and send. Testing on my table suggest up to 10 seconds since the ESP-M1 needs to connect to Wifi first.

Assuming I send once per minute (using 10 seconds to send) that gives me a ruff calculation 1000 / 200 * 6 = ca 30 hours of operation. Every 5 minute is 150 hours of operation etc – every hour ca 1800 hours operation etc. These calculations are a bit ruff, but they show that with 10 seconds connect & send time we will be struggling unless we upgrade once per 24 hour in which case we would be up in ca 43200 hours. The key here is to see if we can reduce that connect and send time. I think that reporting once per minute is a minimum for sensors.

ESP-M1 have a deep sleep mode that uses 20mA that remain connected to Wifi, but that alone will reduce us to 1000/20 = ca 50 hours of operation. The reality is that we need an average current consumtion on ca 100uA or lower for a 1AH battery to last a year on that battery.

Testing ESP32 connection time I get connected within 5 seconds, while using ESP-M1 I need closer to 10 seconds, so a classic ESP32 might be more optional that ESP-M1 for low power. But, we are far away from any solution that gives us 1 year+ on a 1000mAH battery. I need to dig further into this as using Wifi + battery is my first choise.

A lot of IoT/Cloud solutions focus on access through internet, while I want a closed network of sensors in my house with a switch that enable very secure and restricted access from outside that network. I don’t want it to be fysical possible to break in without fysical access. But, I want to use Wifi and Wired solutions to connect sensor units in my home. Using a combined multi-sensor as described before means I can get away with one unit in each room.

The advantage of this is that I can use the information to provide simple automation logic at low cost. The most obvious start is temperature in each room and the capability to save electricity by lowering temperature in unused rooms and at times of day based on simple logic.

If I go on holliday I can enable my own phone or tablet to access this through a secure connection where only my physical device can be used. The thing is that with internet you never know as some smart hacker might always find a way in, so the best security is to disconnect internet completely. Wifi is a bit more secure as you need to be in ca 100 meter range and we can have the units to auto-change security passwords etc. Even better, we can also use wired, secure connections if we want to. Just keep in mind that a wired connection also is wireless on ca 50cm distance with correct equipment.

As for functionality I can add camera and voice units as needed, but me in person I believe things like Alexia or similar is useless crap at this point.

I have my PLS stack that I will use for actuators, but I need to create the sensor unit and I want to start with a low, cost battery modul with Wifi. 3,7V battery have the advantage that I can go direct without a regulator and use something like ESP32 as core for a simple, cheap start.

I have always dreamed about automating my own home and one of the key starting components are low cost sensors. This technology is available today, so it is not rocket science, but I want to create a low cost practical system. Starting with sensors only I would like to monitor temperature in every room in the house as a start. If I can monitor temeperatures I can control heating – it is a simple application that any modern electric owen can do, but I fancy doing it through a centralized system and to deal with some of the challenges I face getting there.

The concept drawing abobe illustrate 3 different communication options as well as some sensors and a battery solution.

Battery is a challenge because you will either need to recharge batteries or swap batteries on regular intervals. But, battery combined with Low Power Wifi/Lora is the most elegant solution. The option to use a cheap network like RS485 (or similar) and power cables. You then get 4 cables to each sensor. The drawback with the RS485 solution is that you need to drag cables around the house. I think it’s ok to have a system that can do both specially since the RS485 option hardly require PCB footprint at all.

The pink part – BMS and batery is quite simply a voltage sensor on a 3,7V LiPo or similar. We don’t charge, but we can monitor battery state and give a low power alarm.

This block diagram is rather generic, but I think I can manage it all in one circuit which will be better for higher volumes. I have a L4 Nucleo board I can use as a start. Standard for all the sensors are that it is of little extra usage to monitor them constantly, so we need to work out a time scheme. Starting with temperature I would say that measuring the temperature every 10th second is more than sufficient. That means we can sleep in 9.9 seconds and just switch on a minimum of power for a little while. Being intelligent we can also decide if temperature have changes sufficiently for use to switch on the radio and report anything. If we do this with every sensor we should end up with a pretty good low power scheme where a seleceted battery cam last a predictable period.

As for form factor I am thinking a bigger board this time. Most of the cost here can be lowered more with volum than with size, so having a larger PCB that can do more makes sence. I can then configure that as I please for each room. Lets at least start with this.