Electronic parts for Mini RC Bobcat on Thingiverse

Hi guys,
I’m looking to try a new project on thingiverse: https://www.thingiverse.com/thing:1000713 building it for my nephew!

I already have the receiver and transmitter but the only place I can find the servos is from a Chinese website that takes forever to ship! Could I use this instead: FEETECH FS90R Micro Continuous Rotation Servo Australia In the thingiverse instructions they say that you have to modify the two servos that drive the wheels to make them continuous rotation, so I figure that these should be ok without any modification? But what about the third one that operates the front lifter? Do I need something different there?

For the battery, I’ve found this one: Polymer Lithium Ion Battery (LiPo) 3.7V 120mAh Australia but not sure if it is underpowered?? It’s not really clear just by looking at the pics where the battery even goes - I assume you have to pull it apart to get the battery out?!

So I’m also wondering (if I can fit it into the model) whether I could incorporate one of these: https://core-electronics.com.au/lipo-charger-type-c.html to make charging easier? I can modify the 3D model as have lots of experience with CAD, just no experience with electronics, so sorry if my questions are a bit dumb.

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Hi Melissa,

What a cool little project! :slight_smile:

That servo size is commonly know as a sub-micro servo. We’ve got a few of those, eg.:

I haven’t opened one up before, but there’s a fair chance it’ll be very similar.

Regarding the battery, all our batteries are standard LiPos. The batteries linked on the thingiverse page are high performance ones capable of 25C discharge rates. Our batteries are only good for about 2C discharge rates (perfect for powering portable electronics projects). You’d be best to source some “drone” or “RC” batteries from an RC Hobby store to get plenty of oomph (though they do run out quickly).

It looks like the battery goes in the cabin - he’s just painted his black to make it blend in.

As for the charger, yes, you could definitely integrate that without issue :slight_smile: Just wire it up to the battery in parralell with your other electronics.

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Hi Oliver,

So to clarify - are you suggesting that I use the servo you suggested for all three? Or just for the lifter with the continuous rotation servos for the wheels?

For the battery, I’m not sure what you mean by drone or RC batteries. Are the the NiMH ones? Would they be safer for a small kid to use too, rather than lipo?

Hi Melissa,

Yes all three, but only because the creator of the model used the same servo for all three. If you’ve got time up your sleeve it might be worth getting one first to pull apart and examine before going all-in on them.

Batteries have quite a few specifications, one of the key ones is their discharge rate usually quoted in C (for Capacity).

A 120mAh battery capable of a 1C discharge can provide 1x120 = 120mA max current.
A 120mAh battery capable of 25C discharge can provide 25 x 120 = 3000mA (or 3A) max current.
A 2000mAh battery capable of a 2C discharge can provide 2 x 2000 = 4000mA (or 4A) max current.

The discharge rate tells you how fast you can get the energy out of a battery, while the capacity tells you how much that battery can hold. Battery capacity is pretty much dependent on battery weight for a given chemistry.

Flying takes a lot of power, and the heavier the aircraft the more power you need. So batteries for drones and other RC aircraft need to be able to provide huge discharge rates relative to their capacity.

The take home of this, is our 120mAh battery might work, but your little dozer won’t go very fast. If you need a small powerful battery one designed for a drone is a good option. If you can redesign it to hold a bigger battery, you might be able to get away without one of the fancy drone batteries.

Got it - thanks for your explanation. I’m learning so much!
One other question, are lipos safe for kids to use? Or is there a more kid-friendly easy charge option?

I think I’ve confused myself! What I want to achieve with the usb is to allow the battery to be charged without opening up the car. So I will set it up with the female usb exposed to the outside and the lipo plugged in permanently inside. That way they can just plug a “USB charging cable” direct into the car without pulling it apart or opening it up.
However - what I have since realised is that will charge the battery, but the battery won’t be connected to the receiver! Will this one allow me to leave the lipo plugged into the receiver and the usb charger at the same time? USB LiIon/LiPoly charger - v1.2 Australia I guess I’m looking for a charger with input and output to and from the lipo.

Batteries aren’t toys, but they’re pretty safe these days. As long as they’re supervised they’ll be alright.

You could get the adafruit one, or you could just get some jst ph pigtails and solder them together. You just need to make a 1x female to 2x male cable wired in parallel. That’s all the adafruit one does, but it does it on the Apcb instead of in a cable.

Oh dear - now you’ve lost me! So sorry, but I don’t really understand the wiring you mean by “1x female to 2x male cable wired in parallel”. I can solder (clearly a bonus)! And then how does that connect to a usb charger? Do you have any links to what I’d need to do what you’ve suggested?

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Hi Melissa,

Oliver’s on the money there. What he’s saying is you can wire it up like so:

You could achieve this by using some JST pigtails. Physically, it’d look more like this:

Here’s some parts you could use to make up that cable:

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Still trying to get my head around this one guys. I’m up to speed with the connection of battery to charger and receiver at the same time using the piggy back cables. I get that bit. If I wanted to add a switch to preserve the battery charge when the car is not being driven, where would I add it?
Between batter and receiver? Or would switch have to be on if I want to charge the battery?

And one more question, if I can manage to install an on/off switch as per above post, how can I work out what current I need it rated to?The packaging of the three servos I’ll be running doesn’t give me the current, just speed (0.1sec/60° & torque 0.4 kg-cm) and weight (3.6g). Not sure if its relevant but the battery I have is a 1s lipo 300mAh 3.7V 0.3

Is there anyone out there?

Hi Melissa
What about the DF Robot Sunflower Solar Power Manager.
Will charge via USB as well as Solar. Output 5V @ 1A
May be what you are after if you just forget about Solar.
I would have thought 300mAhr might be a bit small.
SKU: DFR0559
All specs and write up on Core web site.
Cheers Bob

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See above, I’ve already bought this: SKU: ADA259 so would prefer to use it if I can. Just need a switch that will work.
With regard to power supply - if 300mAh is not enough how much would I need?

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Depends on a few factors.
What is it powering
Expected instantaneous load
Expected duty cycle
Required time between charges
It is possible that 300mAhr will be enough but it is not a big battery by any means.
I don’t know what ADA259 is and don’t have time to look just now. Will look later but someone may come up with suggestions in the interim.
Cheers Bob

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Hi Melissa
Just had a bit of a look. ADA259 is a charger only. Unless it is built in to some part of your robot you will need a boost converter to kick the voltage to the required value. For the purpose of the following discussion I will assume 5V

I am not trying to be a killjoy but you should be aware of the quirks of this mAhr figure.
The quoted 300mAhr is AT THE BATTERY or 3.7V. At 5V it is somewhat less.
Need to convert to mWhr to calculate. W = Watt
300mAhr @ 3.7V = 1110mWhr. (3.7 x 300)
mAhr @ 5V = 1110/5 = 222mAhr.
Now we have a figure called efficiency. 80% would be conservative but will use that for illustration.
Actual mAhr @ 5V then would be 222 x 0.8 = 177.6mAhr

As you can see what started out as 300mAhr is greatly reduced and you must convert to watts to establish the end result. Watts = Volts multiplied by Amps. The use of milliAmps is kosher as we finish up with milliWatts. The efficiency is a real loss and must be considered. Ignoring this usually results in running out of puff at the end of the day and probably building something which may fail dismally. This figure should be published in the device specs. If not, make enquiries.

What I am getting at here is if in doubt about battery capacity go bigger if possible. You just may avoid disappointments.
Cheers Bob

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OK - I see what you’re saying. Power supply is something I always find difficult and appreciate your help You are not a killjoy at all! I will look for a more powerful battery, but still has to be a small one to fit.

Having said all that, I am still at a loss on the power switch question. I am going to build the ADA259 charger into the model, so that is all cool. But I need a way to switch the battery off while the bobcat is not in use.

Above, Oliver suggested I use pigtail cables to connect the battery to both the charger and the receiver inside the model, which makes sense. But if I do that, I’m not sure where (or if) I can put a switch to turn the bobcat on and off. Thoughts?

Hi Melissa
I had a look at a couple of Utubes. I think I can see what you are trying to do. It would appear the receiver operates directly from the 3.7V battery so there is no need for that step up I referred to. But keep in mind what I say about conversions and efficiency for future use.

The ADA259 has 3 main connections. Battery and load are connected together on board so “Y” connection cables not needed. These connections can be made via JST connectors or soldered and are clearly marked. The other is a mini USB for charging which is self explanatory. To leave the battery in the robot to charge this connector has to be accessible.

Connection should be easy.
Charger to USB charge. Could be an advantage to be able to see indicator LEDs during charge.
Receiver connects to “Load”.
Battery to connect to “Batt”.
To isolate the battery during non use simply cut the red battery lead and insert a switch. Switch will have to be “ON” to charge.
To get reasonable use try to use the largest capacity battery you can accommodate. Don’t use multiple batteries as that charger is only designed for a single 3.7V/max 4.2V cell.
Good luck. Connection should be fairly simple.
Cheers Bob

Ripper! Thanks Bob!
Just so I understand, circuit will be fine whatever mAh I choose? Just depends on what size I can fit into the model? Will I ruin anything if I put in a much higher mAh spec battery eg. 1100mAh or 2000?

Embarrassed now - I’ve just realised I gave you the wrong SKU! Sooooo sorry!
I actually have this charger: https://core-electronics.com.au/adafruit-micro-lipo-w-microusb-jack-usb-liion-lipoly-charger-v1.html
Rather than the easy plug-in solution on the other board, can I solder the receiver into the 5V and Grd holes and plug the battery into the 2-pin connector?