Suggestion needed for dc/dc boost conversion.

Thread Starter

bmcgee

Joined Jan 2, 2020
6
Hi,

I'm working on a project that I could use some expert advice with please. I'm new to designing my own circuits, but I did take both discrete and digital electronics courses in college (many years ago).

My project is using a very low power Arduino-like device to monitor a driveway vehicle sensor and send a LoRa (low power radio) signal when one is detected. Since this is my driveway, it will spend the vast majority of the time idle -- perhaps 4 vehicles a week. I'm having difficulty figuring out the best solution to power both the Arduino AND the sensor:

The Arduino is this. It's designed to use 3.7vdc batteries. Idle current will be about 20uA, and maximum current (while transmitting) about 200mA.
The sensor is two of these. 8-30vdc. Idle current is 30-70 uA, maximum current 40-60 mA.

I'd like to get about a year of standby time out of two 3.7v 2600mAh batteries in parallel, which puts my standby energy budget at about 400 uA (I think).

Also, although I can solder, I can't work with SMT components.

I would appreciate any thoughts or advice people have to offer me.

Thanks!

Brian
 

pmd34

Joined Feb 22, 2014
503
Hi Brian, I had a look at your components your RF module is a bit expensive and you could probably find a cheaper arduino and a 433MHz transceiver unit if you wanted.

You could simplify things more actually and use one of the cheap RF switch modules such as:
https://www.aliexpress.com/item/4000518556359.html?spm=a2g0o.productlist.0.0.acd666aaZhuHdT&s=p&algo_pvid=448cbcd6-c16c-4e74-9fe9-3dc03bb2e32c&algo_expid=448cbcd6-c16c-4e74-9fe9-3dc03bb2e32c-9&btsid=a2e9d2fe-a35c-492b-913e-9d8903d36845&ws_ab_test=searchweb0_0,searchweb201602_10,searchweb201603_53

And have your sensor simply activate the button.


The 8V of your gateway sensor is a bit of a pain actually. Having to run a DC/DC converter continuously would be a major power drain on your batteries, as it would only be about 90% efficient at conversion plus potentially some additional "quiescent" supply current to keep the converter running. I might actually be tempted to see if I could reverse engineer the sensor and make it run off a lower voltage, as the voltage range specs of the sensor suggest there is a voltage regulator in it anyhow.
Most logic runs at 5V tops, so a higher voltage might just be for a relay. You might even be able to replace this with a MOSFET to do your switching to decrease the power demand even more.

Failing that, my favored option would be to have 3 batteries and have them in series, powering the gateway sensor.. when this sensor activates, it should switch on the power to the Arduino, which should then immediately send off its RF signal. This way there is no need to power the Arduino continuously.
 

ronsimpson

Joined Oct 7, 2019
679
The sensor is two of these. 8-30vdc. Idle current is 30-70 uA, maximum current 40-60 mA.
There is a good chance the electronics runs at 5 volts and a voltage regulator is on board. Seems inefficient to start out at 3 volts, boost to 8 and then drop it down to 5. Better to only boost to 5.
 

Thread Starter

bmcgee

Joined Jan 2, 2020
6
There is a good chance the electronics runs at 5 volts and a voltage regulator is on board. Seems inefficient to start out at 3 volts, boost to 8 and then drop it down to 5. Better to only boost to 5.
I thought much the same thing. I contacted the manufacturer and they said that if I supplied less than 8 volts the device would have considerably less sensitivity.


Failing that, my favored option would be to have 3 batteries and have them in series, powering the gateway sensor.. when this sensor activates, it should switch on the power to the Arduino, which should then immediately send off its RF signal. This way there is no need to power the Arduino continuously.
I like the idea of using batteries in series as my voltage boost.
 

Thread Starter

bmcgee

Joined Jan 2, 2020
6
Correct, I'd ignored self-discharge. If I use 3 2600 mAh LiOn batteries in parallel, for 7800 mAh total, at 3% self-discharge, after 12 months I'm down to 5411 mAh that I can use. Divide that by 8760 hours in a year, and I have a budget of 618 uA.

Is my math right?

After doing some reading about putting a tap between batteries in series, I set aside the suggestion of using 3 batteries in series and tapping one of them to power the arduino. Right now my current best option is a TI TPS6734IP -- a 12 volt buck converter. http://www.ti.com/lit/ds/slvs127a/slvs127a.pdf

B
 

pmd34

Joined Feb 22, 2014
503
After doing some reading about putting a tap between batteries in series, I set aside the suggestion of using 3 batteries in series and tapping one of them to power the arduino. Right now my current best option is a TI TPS6734IP -- a 12 volt buck converter. http://www.ti.com/lit/ds/slvs127a/slvs127a.pdf

B
This wont work im afraid, you have to run the sensor continuously, so that means the converter is running continuously too. In the data sheet the converter uses 1.2mA in operation!

There is no need to worry about an additional voltage regulator for your Arduino as it has a 3.3V regulator on it anyway, and you need only power up the arduino for 1 second or so when your sensor actually detects a car.

If power is a real problem for you, you might consider having a small solar cell on your system, there are some nice very low voltage boost converters that would allow the solar cell to maintain the battery voltage.
 

Thread Starter

bmcgee

Joined Jan 2, 2020
6
Okay, I hear your suggestion.

In terms of switching the arduino on and off -- I don't think that's necessary as it's essentially what my software is doing. It's putting the arduino into standby mode (20uA) until a falling edge on D7 or D8 causes an interrupt, at which point it wakes back up (and retains the volatile memory).

Here's the schematic I think we're talking about. Anything else I should be considering? Test points?

Capture.PNG
 

pmd34

Joined Feb 22, 2014
503
But there is really no need to keep the Arduino running all the time, and would be simpler to just make it transmit once when it powers up.
It's always best to avoid uneven loading on batteries, as you will end up with one cell always more depleted, and there will be more need for cell balancing.
 

Thread Starter

bmcgee

Joined Jan 2, 2020
6
If the Arduino is totally powered off, the volatile RAM is flushed and it takes a second or two to power back up, and then up to 30 seconds to re-authorize on the LoRaWAN network before it can send the payload. I am using the second sensor to determine direction of movement, and that wouldn’t be possible if the Arsuino were fully off, and it wouldn’t be able to send timely alerts.

I think those things make low power standby a requirement. If that will cause too much battery imbalance we’ll have to take another look at boost converters.

When fully charged, the battery pack will be losing about 9 mA/hr from self-discharge - about 3 orders of magnitude more than the arduino draws. Will the 20 uA/hr present an issue we should consider?

B
 

pmd34

Joined Feb 22, 2014
503
Hmm your 9mA/hr sounds exceptionally high, your battery certainly wont last long at that rate!
I've used an atmel microcontroller myself with a battery application that has to latch its own power on and the code is up and running in less than 10ms plus I guess you wont be keeping your network connection active all the time as the RF transmitter will guzzle power anyhow.
 

ci139

Joined Jul 11, 2016
1,459
I guess you wont be keeping your network connection active all the time as the RF transmitter will guzzle power anyhow.
? log your data locally then do network traffic at most likely "best" time of the day/week/month
______________
there are not rechargeable battery species that have significantly lower self-discharge rates -- !!! if i remember right
related
https://blog.epectec.com/why-should-you-use-alkaline-batteries-in-your-application
https://en.wikipedia.org/wiki/Rechargeable_alkaline_batteryIN THE MARKET
 
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Thread Starter

bmcgee

Joined Jan 2, 2020
6
Hmm your 9mA/hr sounds exceptionally high, your battery certainly wont last long at that rate!
I've used an atmel microcontroller myself with a battery application that has to latch its own power on and the code is up and running in less than 10ms plus I guess you wont be keeping your network connection active all the time as the RF transmitter will guzzle power anyhow.
You’re right, sorry. I went back and re-read the article and it’s 2-3%/month, not per day. Oops.

Are you familiar with LoRaWAN? I’m not an expert by any means, but my understanding and experience is that each time the arduino powers on it has to reauthenticate with the server by sending a request and receiving an OTAA message. This is because the authentication token it receives is stored in volatile memory. That process, even on my test bench where I’m just 20 feet from the gateway, takes a lot of time. 30 seconds isn’t unusual. During that time the arduino has to have the radio on and is consuming about 200 mA.

Can we for now continue with the assumption that standby is a better solution? What would be your suggestion if we stipulate this?

B
 
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