emonTH V2 - now in the shop

I think they never were in parallel. It makes little sense to lower the voltage for a boost converter since the lower the voltage the worse the efficiency. Plus it’s better practice to use them in series. OTOH, a DC-DC converter essentially conserves power (in contrary to LDOs) - so indeed, the full capacity is closer to 2x2700 mAh than 2700 mAh. I still would find a stepper-less device a simpler design when the parts can run on very low voltages.

The emonTH was launched in Nov 2013…so three years! Also to our excuses the Si7021 has only been available for the past 24 months…I do my best :confounded:

This is what I’m currently measuring, I might well be hitting the limits of my multimeter. I’ll have another go at measuring. Agree, we should be able to do better. In theory, this is what’s possible:

  • Atmega382 WDT sleep: 4uA
  • Si7021 sleep: 0.06uA
  • RFM69CW sleep: 0.1uA
  • DC-DC: 7uA

Total: 4+0.06+0.1+7 = 11.6uA sleep current should be theoretically possible.

Agree, I’m playing it very safe here as we have had trouble with RFM reliability due to sleeping in the past. I was trying to ensure it has plenty of time to wake up. Since making that scope trace, I have actually reduced the wakeup time to 50ms. I can probably reduce further, I will do some more testing. Reliability… is a top priority over an extra few weeks battery. Thanks for your suggestions, I’ll report back!

How do you mean? The MCU needs to be awake to read from the sensors and take an ADC reading of the battery life. Here is the code during those first 17.5ms. A DS18B20 was not connected during my testing & scope trace capture only si7021. One idea could be to only take an ADC reading of the battery life once per day since this is quite an energy intensive operation. What do you think?


Yes, it would be a nice cost saving to ditch the DC-DC. The reason for using the DC-DC originally to hold the voltage at 3.3V as the batteries discharged was the DHT22 cannot operate below 3.3V. The SI7021 can work down to 1.9V, RFMC9CW down to 1.8V. HOWEVER many users connect external DS28B20 sensors to the emonTH and plug in optical pulse sensor into the terminal block connectors, these sensors require a min of 3V / 3.3V thefore the DC-DC converter is here to say I’m afraid.

Sample Interval?

What are your thoughts on sample interval? Currently, emonTH default is once per min. If the default sample rate could be decreased to once every 2 min this would have a BIG effect on battery. My feeling is once every 2min is perfectly fine for monitoring domestic room temperature (emonTH target application). However, I am aware that some users use the emonTH heat-pump monitoring and control applications where 2min might be a bit slow. What do you think? I’m leaning towards making 2min the default sample time.

You forget the efficiency at that current, which is around 50% IIRC. That would about double the amperage needed in sleep - possibly a lot more.

Well, the Eneloops are NiMH, that have bad self-discharge characteristics. Obviously the best would be using Lithium batteries (like the Energizer Ultimate Lithium or similar) since their shelf life/self dc. is astounding and have the best voltage retention in their lifecycle. But they cost a bit more.

I mean, start T/H sampling, sleep, and at wakeup check/read out the results (I remember making it work with DHT22 but I’m avoiding that sensor now). Considering other factors it might not worth this single saving, though.

I might be missing some details, but why is taking ADC readings such a penalty? I don’t recall it being one. ADC naturally needs to be switched off when sleeping. I don’t think it sohuld be a problem.

I don’t experience such issues but you can use the modeready flag in a loop to check if the radio is in the desired state, instead of fixed waiting periods. I think it’s not only a lot shorter wait but a more robust approach. I generally use the LowPowerLab RFM69 lib and it works quite nicely, with all these nuances handled.

Booster: OK, I can understand why you need the boost converter - I was not aware how important this is IRW extra sensor usage, but it apparently is :slight_smile:

Sampling interval: for room temperature monitoring even 5 minutes is fine. Other usages do require different frequencies - not a simple question. I’d say an ability to configure it would be nice, but it has implications on cost and simplicity of usage…

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Does the DC-DC converter have an enable pin? If so you could power the bits that go to a low voltage directly from the battery and only switch on the DC-DC converter when using the external sensors.

Yeah, might be a feasible approach. According to the datasheet, it does have a shutdown pin with <1 uA shutdown current.

Interesting idea, it won’t save any cost on components but would result in slight power savings when only using si7021.

It would require a hardware change to connect the enable pin. I’ve committed to a first manufacturing run with the current emonTH V2 hardware. I’ll keep it in mind for the next revision.

You could always not populate the DC-DC converter components (and terminals) and sell a lite/internal only version :wink:

Guess we can circle back to this on the next revision :wink:

I’d personally buy a bunch of the lite, should it ever see daylight. But it seems that the addons requiring the 3.3V booster are much more important so cost saving is not possible, sadly. It’s not simply leaving off some parts, though, the power path is different. So either a slightly modified design or some parts shorted/populated with 0 Ohm resistors.

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Just to put the cost saving in perspective the LTC3525 is £1.60 1K qty, therefore the lite version would probably come in at about £2-£3 cheaper.

I’m keen to keep variation to the minimum, manufacture is hard enough already!

Power consumption calcs have been re-done using a figure of 2700mAh for the AA capacity, the batteries are in series:

I also agree with @kobuki, 5 minutes is fine for normal temperature measurements, that’s how my emonTX reports now, and I haven’t changed it’s battery since!
If it’s clear in the firmware/read.me how to change this, then people using this for bespoke purposes can set it to what they need, or better still, make is user selectable via a board link?


If you use the booster, the power is conserved irw. input and output. So you have effectively 2xAA capacity (~5400 mAh) minus losses, as I noted earlier, fixing the presumption of not using a booster. It could still do better with a single Si7021. The 225 ms is excessive. Radio wakeup is around 0.5 ms IIRC. Waiting for 100 ms when you know the TX finishes in 4 ms is not a good idea either, IMO. Though, realistically speaking, after 2 years people forget when they swapped the batteries the last time :slight_smile:

Ah, ok I have reverted the calculations and added a note regarding the losses.

I’m a little confused, is the new EmonTH product discussed above that is sold here? Also, did an option exist earlier without T/H sensor? It might be that only the descriptions haven’t been updated yet…

This thread is discussion and update to the emonTH not emonTx. The updated emonTH V2 as discussed has not yet been released into the shop. We estimate that it will be in the shop in 2-3 weeks time. We are currently setting up manufacture.

Sorry, it was a typo, of course I meant to ask about the EmonTH, not the EmonTX. Thanks for the availability clarification. (I also fixed my post above.)

emonTH V2 is now available for pre-order in the shop:

We plan to start shipping emonTH V2 in the next couple of weeks.

Today we received the first 10 production (test) units from the factory which all worked :smile: :tada:

Therefore, we’re pushing into qty production. We will have 1K units in the shop in the next few weeks.

Inspecting the wave solder thu-hole soldering:

Ordered mine :slight_smile:

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Looks great!

I placed my order when I saw the shop go to the V2. It shipped today, awesome! But wanted to check - they are the V2 right? Did I get some of this very first batch then? Very cool if so!

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Yup emonTH V2 units started shipping last week :grinning:.

We now have plenty of stock. Here’s a photo of the new V2 units:

The Si7021 sensor can be seen in the top right-hand corner of the PCB. The white film on top of the sensor is a dust film and should not be removed. The dust film is factory fitted and will protect the sensor from dust and air contaminants.

I have just published a blog post to launch the emonTH V2:

Introducing emonTH V2 - Blog | OpenEnergyMonitor

emonTH -Temperature & Humidity Node - Shop | OpenEnergyMonitor