Lightweight power solution

Hi,

I’m looking for a very lighweight power solution for my project.

I’m using a attiny85 and a SG515R servo motor. Now things are missing:

  • a 5v regulator than can handle at least 500mA;
  • a battery to power it all;

Concerning the battery what option are there ?

  • 9v battery, about 36g
  • 2x 18560, about 50-90g
  • 4X 1.5V AA, about 85-95g

Do you have a better idea ? Maybe use boost converter in order to use a smaller battery ?

Thanks for your input.

A huge amount depends on how much activity the servo will be doing and how long you want to go between battery charges or replacements.

You can get LiPo cells that are much smaller than an 18650 - but with a corresponding lower capacity. You can power an Attiny directly from a 1s LiPo and you could use a step-up converter for the servo power.

You could power the Attiny and the servos from 3 x AAA alkaline cells with no need for voltage regulation or step-up

Using a 9v battery makes no sense as its voltage does not suit any part of the project.

...R

Robin2:
Using a 9v battery makes no sense as its voltage does not suit any part of the project.

Nor does two 18650s, presuming you proposed to put them in series.

Thanks for you input.

I'm using the servo motor like some sort of latch. The servo arm keeps a moving part closed and once the there is a trigger, the servo arm moves out of the way. So there won't be a lot of load (moving the arm and some friction pushing against the arm), and it will have to move about 90-120 degrees (out of the way basically).

There will be about 4-5 minutes between the moment the Attiny is turned on and the release.
It would be nice is could reset and do it again at least 2-3 times (that's really just for "confort" reasons, I could very easily change the battery each time).

I like the idea of using a 1s lipo, which can be very lightweigth (around 10g) and a booster convert for the servo motor.
The 3 AAA is great too but much heavier (around 32g for 3 AAA) and not as compact as the 1s lipo.

Finding a suitable 1s lipo does seems to be difficult.
But how about a boost converter, is something like this suitable for my need ?
https://www.digikey.com/en/products/detail/analog-devices-inc/LT1301CN8-PBF/891611

I’m looking a for very compact lightweight boost converter that can boost 3.7v (1s lipo) to 5 volts and that can handle at least 500mA pick. I think it should be ok right ?

Are there better options?

RedIron:
So there won’t be a lot of load (moving the arm and some friction pushing against the arm),

I’m not sure how to interpret that. You should size the servo so the internal friction of its gearbox can hold the latch closed without needing any electrical power - that way the servo will only draw a few milliamps when it is not moving.

And it’s still not clear to me how often the servo will be required to open and close the latch between battery charges.

As with every question on this Forum it is much easier to give useful advice when you give a full description of the project so we can understand your questions in the proper context.

…R

PS … for boosting the voltage I suggest you consider MT3608 modules

The servo will move once to open the latch. Than I manually reset everything.

By "reset" I mean, close the latch back and put the servo in position to block the latch from opening.

I could change the battery after each I reset but I would rather change the battery every 2-3 reset. Battery requirements are therefore, being able to move the servo arm once and power the attiny for about 5min. But ideally 3 servo move and 15min. Knowing that once the servo is in the closed position, it won't draw any current, I could even disconnect it entirely.

The servo arm moves in the X-Y axis and the force of the latch/door is pushing in Z-axis as seen in the attached picture. In order to open the latch/door, the servo arm just need to get out the way, as seen in the second picture.

I hope it answers your questions.

The MT3608 modules are great, I've used them in many projects so far, but I would like something smaller/contact.

Thanks for help

RedIron:
I could change the battery after each I reset but I would rather change the battery every 2-3 reset. Battery requirements are therefore, being able to move the servo arm once and power the attiny for about 5min. But ideally 3 servo move and 15min. Knowing that once the servo is in the closed position, it won't draw any current, I could even disconnect it entirely.

I'm still struggling to make sense of the time involved. Are you saying that you are happy to charge (or change) the battery every 15 minutes?

I have been assuming that you would want the battery to be sufficient for a few days, at least.

Once again - what is the project?

...R

I want to build a parachute release mechanism for a model rocket that uses an attiny as a microcontroller, a BME280 as pressure sensor and servo motor (in my case a lightweight SG51R). Once the rocket reaches a certain altitude, the servo arm moves and let the parachute out.

Each flight is expected to last about 10 seconds (from launch to touch down). Each launch preparation is expected to take about 5 minutes.

As said, I could change the battery after each launch but I would rather not. I don't expect to launch more than 3-4 times. Therefore, I would like the battery to power the entire thing for about 15-20 minutes.

Since height (max altitude) is directly influenced by weight, I want to build the most lightweight "fligtht computer" possible, that's why I'm using an Attiny85 (since I don't need more than 2 I2c pins and 1 PWM pin).

I believe you can get servos that work with a 1s LiPo and if so that would be by far the simplest solution.

...R

PS ... it would have saved a lot of time if you had described the project in your Original Post.

You are right, I might be easier to get a low voltage servo motor, that way I won't need to use a booster converter. I will probably do that.

I'm still curious about the boost converter that a found, the "LT1301CN8-PBF".
the https://www.digikey.co.uk/products/en?keywords=LT1301CN8-PBF
Could it have done the job of boosting the 1s 3.7 lipo to 5v for a "regular" hobby size servo motor ?

RedIron:
I'm still curious about the boost converter that a found, the "LT1301CN8-PBF".

From a very brief look at the datasheet I think you would use it as part of a circuit that would be very similar to the MT3608

And now that I know what your project is I suspect that even a small LiPo would be sufficient for many hours of usage. The servo will only be actively moving for a few seconds on any one flight and when it is not moving the total power draw between it and the Attiny will only be a few milliamps.

For use with a 1S LiPo you should run the Attiny at 8MHz on its internal oscillator. Reducing the speed from 16MHz saves quite a lot of power.

...R

to review :
replace or charge a battery.
final assemble the parachute release canister
energize electronics
launch,
have servo move, approx 10 seconds of operation for the motor ?
touch down

entire operation to take well under 5 minutes.

============

As I see this, you have two power requirements.
power for the micro and any ancillary sensors
and then power for the servo.

also I as see this, the lightest possible battery and smallest possible.
also, that charge in place is acceptable if it can be done in a few minutes.

seems a Super cap could supply the higher voltage but not sure of the size and weight

@Robin2
Thanks for your answers. Reducing to 8Mhz is a great idea since a really don't need much processing power/speed.

@ dave-in-nj
The motor will be in operation very briefly, less than 10.
Couldn't the 1s lipo fulfill both power requirements ? As Robin2 mentionned, when the servo arm is not moving, there is barely any power drawn, so I think it should be sufficent.
Using a high capacity capacitor isn't somehting a though about. As stated, I would rather not have to charge/charge in between launches. But it is something a could add is there's any power issue by using only a 1s lipo. But then again, if I can do without it, I prefer since it adds more mass.

Thanks for your ideas

Minimal mass is key.
Charging a cap Takes literally seconds.

Lipo is. Not known for power delivery.

Testing is needed.
There are many sizes of batteries. I think an 18650 is much too much mass but they don't output high current.

I have some 80mAh LiPos that are rated at 15C which means they could produce 1.2 amps for a short period. They should easily be able to power a servo for a single movement. They measure 8x28mm and in comparison an 18x650 is a giant.

And an 18650 with an equivalent 15C rating could easily provide 25 or 30 amps. They are used in large numbers in electric car batteries.

...R

RedIron:
You are right, I might be easier to get a low voltage servo motor, that way I won't need to use a booster converter. I will probably do that.

I think the suggestion is to use a regular servo (rated 4.8-6V) and run it off a single LiPo (fully charged 4.2V). It may very well work, and in your case it seems the servo needs to deliver very little torque to move that latch, just some friction to overcome.

I also like the supercap suggestion for the quick and easy recharge. Charging it to 5V should be a breeze, have a USB connector and use a regular powerbank. Do note that the discharge cuve of a supercap is still that of a capacitor, not of a battery. Very different, voltage drops much faster in the beginning.

wvmarle:
I think the suggestion is to use a regular servo (rated 4.8-6V) and run it off a single LiPo (fully charged 4.2V).

In Reply #9 I was thinking of servos that are designed to work with 3.3v

...R

Robin2:
In Reply #9 I was thinking of servos that are designed to work with 3.3v

...R

Yep, low voltage servos are not difficult to find.

I have a discus launch RC glider, 2 servos, all up weight 113g.

Its quite happy to run off a single LiPo, the servos are designed for it.

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