Team Dudge build diaries.

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Derek Dudge
Posts: 135
Joined: Thu Dec 07, 2017 9:32 pm

Re: First time building diary: Third Law

Post by Derek Dudge »

It shouldn't be too long now before I'm ready to move on to wiring up Third Law. I plan on ordering the rest of the parts I need later this week and am mostly done fiddling with the chassis and armour.

Before I get there though, I've a few potentially dumb questions I want to double check about. I *think* I know the answers but would just like to check. Better a few silly Qs now than ruined components later, right? :D

First up - I want to include a switch. Presumably between battery and ESC would be best place for it. However my esc is a nanotwo that already has a nice easy red plug connection for the battery. I figure I'd be best leaving battery side alone, so changing batteries without any faff is still easy. So, ESC side of the plug connection - I just cut the red wire and splice in the switch?

Next up - I want to incorporate a few LEDs. I read a little about it, and am aware best practice is to include a resistor with them. The formula on the site I was reading tells me a 200ohm resistor would be what I need for the battery and lights I plan to use. So, I can just wire the resistor with three LEDs I wish to use in series? Or will they be dim done this way?

Also where in the overall circuit would you guys hook up LEDs? Can I combine the switch and the lights into one series, and wire them all in betwixt battery and esc in one go?

Finally, is there any recommendation of which wire to buy? I see on ebay that it's available right down to 1mm thick, which would be awesome for weight saving. This stuff will be good for ants? Any potential pitfalls I'm missing here?

When I ordered the ESC Alex explained about connecting 3rd and 4th motors for my 4wd to the first pair, so all good there. Just those few niggling queries to double check on and hopefully I'll be good to go.

Cheers in advance!
My robots:

Third Law (antweight) - push/ram bot
Manticore (antweight) - hammer bot
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MarkR
Posts: 375
Joined: Mon Dec 18, 2017 12:46 pm
Location: Reading Hackspace
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Re: First time building diary: Third Law

Post by MarkR »

About LEDs - diodes have a "forward voltage", which is the amount of voltage they will drop (almost independent of current) - this is the opposite of a resistor which has a constant resistance.

So if you wire several LEDs in series, calculate the forward voltage (1.8V seems typical for some red LEDs; it's significantly different on high-brightness ones or different colours), then calculate the amount of current you want.

So for example, if you want 100mA (this drives some LEDs too hard, and makes others really bright!), and want 2x 1.8V LEDs, then the drop is 3.6v. using the 5v output of your regulator (aka BEC) there will be 1.4v remaining, so you need a resistor which will drop 1.4v at 100mA (V=IR ; R = V/I) so about 14 Ohms.

A bigger resistor allows less current, which makes the LED dimmer.

Putting 2 or more LEDs in parallel with a single resistor is a bit error-prone, because if their electrical characteristics aren't absolutely identical, one will take a lot more current than the other.

In general, either wire them in series with a single resistor, or in parallel each with its own resistor.

Hope it makes sense :)
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leds.PNG (44.37 KiB) Viewed 8006 times
Robots: Betsie - RaspberryPi controlled flipper bot with gyro stablisation - too clever for her own good?
Stacie - tidy flipper; 4wd driven by hair bands
Derek Dudge
Posts: 135
Joined: Thu Dec 07, 2017 9:32 pm

Re: First time building diary: Third Law

Post by Derek Dudge »

Cheers for the reply. Although sadly I confess I'm still a tad unsure here. So the ebay store I ordered a few leds from (blue ones with higher forward v than red it seems) gives me the following numbers

Forward volt 3.2 - 3.8

Forward current 20mA typical 30mA max.

You mention the voltage in your example being 5v from regulator. This part is where I'm falling down. I'm going to use the 180mA turnigy batteries (7.4v) that folks regularly recommend on here, and Rorys ESC. Where between these do I find what regulated voltage the LEDs would be getting?

I found this calculator online and entered info from battery and the LEDs (just 2 though, I'm now assuming the higher forward voltage from the blue diodes means I should just use two) and got this back. But i had to enter the 7.4v for battery as I'm not sure about the regulated value you mentioned.

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My robots:

Third Law (antweight) - push/ram bot
Manticore (antweight) - hammer bot
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MarkR
Posts: 375
Joined: Mon Dec 18, 2017 12:46 pm
Location: Reading Hackspace
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Re: First time building diary: Third Law

Post by MarkR »

Some of the speed controllers, (I don't know about Rory's) have a built-in regulator, known in RC-speak as a "BEC" or "Battery Eliminator Circuit" - because traditionally its role was to eliminate the need for a second (receiver / servo) battery on a RC car. Antweight robots usually use the regulator to power their RX and any other low power stuff. The regulator usually produces 5v.

The advantage of using the regulator is that it always makes 5v, but a 2S Lipo pack will put out somewhere between 8.4 (fully charged) and 6v (dangerously flat)

I think you've realised this - but you can't put multiple LEDs in series if the total forward voltage will be higher than your supply voltage. If you're using blue leds with a forward voltage of 3.8 or something, each will need its own resistor.

If you would feed the leds from a variable voltage, they will also have variable current, so they will be brighter when the battery is higher - maybe that's a feature so you can easily see when the battery gets low?
Robots: Betsie - RaspberryPi controlled flipper bot with gyro stablisation - too clever for her own good?
Stacie - tidy flipper; 4wd driven by hair bands
Derek Dudge
Posts: 135
Joined: Thu Dec 07, 2017 9:32 pm

Re: First time building diary: Third Law

Post by Derek Dudge »

Ok, yeah that's making a bit more sense now. Cheers!

That's seems like it's not a bad idea really, letting the lights dim as the battery discharges. I understand letting the lipos run down fully can ruin them, some kind of early indicator I'm playing around with the bot for too long would be great.

So, wired in parallel but before the ESC so it's unregulated is where I want them? Cheers again, that's helpful!
My robots:

Third Law (antweight) - push/ram bot
Manticore (antweight) - hammer bot
Derek Dudge
Posts: 135
Joined: Thu Dec 07, 2017 9:32 pm

Re: First time building diary: Third Law

Post by Derek Dudge »

Ohhh, and quick photo update of where I'm at :-)

The motor mount/bracket thingies are all hot glued together for now. I'm not convinced that glue is strong enough. Before I paint the thing and actually attach any internals, I may reglue it with something else. Not sure yet.


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My robots:

Third Law (antweight) - push/ram bot
Manticore (antweight) - hammer bot
Derek Dudge
Posts: 135
Joined: Thu Dec 07, 2017 9:32 pm

Re: First time building diary: Third Law

Post by Derek Dudge »

So transmitter handset is ordered, got a devo 7e to go with the nano. Getting there with the shopping list now, only a few things left that i need.

Small update on building - the hot glue joins for the motor mountings was nonsense. Didn't stand up well to a bit of stress testing. I have totally redone those with epoxy glue and an much happier with them.

And the armour is painted! Now....I'm under no illusions, I have no doubt that his paint job will last all of about 0.001 seconds against the spinning abominations he might face. That's fine, he's not expected to come out the arena looking quite as groomed as he went in 😂 it can be repainted haha. Couldn't resist the temptation to paint him up though, it's the hobby model builder part of me chiming in.

Weight wise, what I have + the battery I still need plus a micro switch is 147.2g. I need to fit in the connections (wire plus solder plus the little LED and resistor. There are actually a few places in the chassis I can drill if needs be, but possibly it will be ok.

Oh and the ESC and battery are getting fastened in with velcro tape I reckon.

Getting there! Waiting on the postman before I can do next bits now.

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Last edited by Derek Dudge on Thu Jan 25, 2018 3:34 pm, edited 1 time in total.
My robots:

Third Law (antweight) - push/ram bot
Manticore (antweight) - hammer bot
Kaazmin
Posts: 39
Joined: Wed Jun 14, 2017 11:02 pm

Re: First time building diary: Third Law

Post by Kaazmin »

Looks amazing, is it going to be at Ant Freeze 5 ??
Derek Dudge
Posts: 135
Joined: Thu Dec 07, 2017 9:32 pm

Re: First time building diary: Third Law

Post by Derek Dudge »

I don't actually know. I haven't signed up for the main competition because I am still waiting on things getting delivered and I dunno if it will be ready.

I want to come to ant freeze, even if I don't fight, just to see what it's all about. If it turns out I can get there (working on a lift) plus if I get wiring etc done, I'll bring him with in case there's chance to maybe test him out once the proper matches are done.

Full disclosure - I have zero experience driving these things. This competition probably came around too soon for me to be a legit competitor.
My robots:

Third Law (antweight) - push/ram bot
Manticore (antweight) - hammer bot
Kaazmin
Posts: 39
Joined: Wed Jun 14, 2017 11:02 pm

Re: First time building diary: Third Law

Post by Kaazmin »

that's all cool ^-^ my first bot was awful and slow, u can usually sign up on the day btw ^-^ it's always good to do the fights and see where your bot can improve if it needs it :D plus everyone there will surely give u some tips :D
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