View Full Version : Budget Propeller Robot: Merlin prototype on order!

10-28-2011, 05:54 PM
Just came across the ArdBot last night...

I've decided to make a low budget robot using that kit plus their wheels (I ordered the blue).
That kit is about $30 delivered.

I've got some $6 servos (digital, all metal) that I can modify for full rotation pretty easily.

If I drive it from a $25 Quickstart board (I think you can get this at RadioShack now, right?), that's $67 for just about everything...
(I'd like to make a custom board, but it's hard to imagine beating the Quickstart...)

Maybe it's slightly cheaper to hack into some mass produced product, but I think this approach has merits.

(BTW: I'm thinking about a $10 adapter board for the Quickstart to make it easier to connect servos...)

Anybody see a cheaper approach?

10-28-2011, 06:03 PM
I was starting to head down this same road except starting with the Magician from Sparkfun http://www.sparkfun.com/products/10825 for $14.95. You need to add an H-bridge of some sort (they have one for $9.95-ish) and then a micro-controller - Quickstart. So with this route, I'm at about $50 before sensors and curb feelers and fuzzy dice.

10-28-2011, 06:21 PM
Maybe that is better... Glad I asked now...

10-28-2011, 06:26 PM
The parts are sitting in my Sparkfun wishlist, I'm just waiting for the Impulsive Buying threshold to lower a bit to see if I really want to go through with it.....I sure do need ANOTHER project to work on!!

I like the servo/sensor board. It would make it very easy to plug servos and 3-pin sensors onto the QuickStart.

Duane Degn
10-28-2011, 06:28 PM

I've read digital servos aren't the best to modify to continuous rotation. I think it an issue of speed control. Digital provide full speed if the target position isn't met while analog servos vary the speed proportionally to the distance from the target position.

I have some of each. I'll try them for myself.

A buget robot is an important subject to me. I'm now a Cubmaster. I'd love to be able to find a robot cheap enough that each Cub Scout could afford to build one.


10-28-2011, 06:42 PM
There was a gentleman at UPENE who was heavily involved in school robotics competitions and youth oriented (inexpensive) robotics in Michigan (excuse me for not having his name). He had made some great, inexpensive reconfigurable robots from pencil cases/art boxes (the kind that are about 5x9x2 with nice hinged lids) $1 or so if you shop around, converted servos (or DC motors with a bridge, a micro-controller (proto-board, QuickStart,etc.), and a BIG ROLL of Velcro. It looked like a great idea and it's on my plate too. It's not sexy or flashy but it's cheap and KID PROOF! The third contact point with the ground was a ping-pong ball, if I remember correctly.

I got lazy looking for the Velcro....but have all the other parts!

10-28-2011, 07:57 PM
There are probably motors you could recover from old printers. You will still require a driver for the motors though.

10-28-2011, 08:41 PM
After thinking some more, I think I like the Sparkfun robot much better, at least in terms of being the lowest cost approach.

I've got some h-bridge chips for servos that I seem to recall only being ~$2. Can always just build a bridge with mosfets too...

My only concern is that I wanted to add a rotatable platform with a camera on top, not sure this robot is quite big enough.
I think I'll buy this one and then decide between the two when I have them in my hands...

11-02-2011, 02:04 PM
Looked into a motor driver some more. It's only maybe $2 in parts...
But, it needs 4 Prop pins instead of 1 for a modified servo.
So, I guess that'd be 8 Prop pins for the drive train.
And actually, instead of doing the usual servo "modification" for continuous rotation servos, I think I'll just rip out the control board and run the wires directly to the motor.
That way, this little add-on board I'm thinking about for Quickstart could work for both robots...

Using the 4 Prop pins also gives you the option to use "engine braking" or "coasting", which you don't have with continous rotation servos....

BTW: I got my ArdBot parts in the mail and it looks quite nice actually...

11-02-2011, 07:59 PM
I have the Ardbot chassis as well. I also added a third platform on top with the 360 degree servo for a SRF-08. I have found that my Ardbot gets stops if it is on a bare floor and then approaches the edge of a throw rug. Other than that it is a great budget chassis.

11-02-2011, 08:47 PM
Ray: I noticed a camera platform quite cheap (designed for quadcopters) on the Hobbyking website recently. Perhaps this could make a cheap camera add-on.

11-02-2011, 09:24 PM
I've already ordered two cameras from dealextreme for $11.83 delivered. Are those ones cheaper?

Duane Degn
11-02-2011, 11:45 PM
Or is Clusso99 refering to the pan/tilt modules?

http://www.hobbyking.com/hobbyking/store/__11442__FPV_Fiberglass_Pan_Tilt_Camera_Mount_.htm l


http://www.hobbyking.com/hobbyking/store/__12875__FPV_Fiberglass_Pan_Tilt_Camera_Mount_L_Si ze_.html

I wonder how HobbyKing's little color camera (http://www.hobbyking.com/hobbyking/store/__13434__1_3_inch_SONY_CCD_Video_Camera_NTSC_.html )compares with the DealExtreme camera? At $17, it's more expensive.


11-03-2011, 12:54 AM
That is an awesome price for pan and tilt! Add like $10 for servos and the total is $14...

11-03-2011, 02:06 AM
One flaw in your prices is quickstart fromRadio Shack is not$25.

Just came across the ArdBot last night...

I've decided to make a low budget robot using that kit plus their wheels (I ordered the blue).
That kit is about $30 delivered.

I've got some $6 servos (digital, all metal) that I can modify for full rotation pretty easily.

If I drive it from a $25 Quickstart board (I think you can get this at RadioShack now, right?), that's $67 for just about everything...
(I'd like to make a custom board, but it's hard to imagine beating the Quickstart...)

Maybe it's slightly cheaper to hack into some mass produced product, but I think this approach has merits.

(BTW: I'm thinking about a $10 adapter board for the Quickstart to make it easier to connect servos...)

Anybody see a cheaper approach?

11-03-2011, 02:08 AM
Noticed that too... Parallax sells direct for $25, but it's a lot more at Radio Shack. Guess they have brick&mortar costs...

11-04-2011, 02:36 AM
Duane: Yes, these were the mounts I was referring to. Hobbyking also have cheap servos that may be fine for the camera mount.

11-04-2011, 02:07 PM
Couple more thoughts...
I'm going to take a look at how cheap I can make a Prop-Platform compatible board with the motor drivers on it.
I know I can't beat $25. But, the motor driver add-in board for Quickstart would likely cost $15.
Perhaps I could make a board to sell for $40. I think I can save money by starting from my PTP1 board design and replacing touchscreen section with motor drivers.
This would also have the advantage of allowing the addition of Prop-Platform shields.

My other idea is to try to drive the motors from an I2C i/o expander instead of Prop Pins. Not completely sure that would be fast enough, but it might be... That would save 8 pins that could be used for the shields...

11-07-2011, 01:33 PM
Got the Magician chassis in. It actually looks pretty good. Only thing I'm not fond of is that the wheels are slip-fit.
I guess that's OK, considering the price. Took a look at importing the thing myself, but the Sparkfun markup looks very reasonable.

I'm back to thinking of Quickstart as the base board again... Just need a plug-in module to drive the motors...
I'll probably skip the I2C motor control and just give up 8 Prop pins. It's probably better to keep it simple.

11-13-2011, 12:44 PM
I've started the design for an addon board for the Magician with Quickstart combo. (Calling it Merlin).
I'm assuming that Sparkfun will get this back in stock. They're page says "Backorder Allowed", so I think they will...
Here's what the Magician looks like with the Quickstart on top:

11-14-2011, 06:30 PM

I looked into this idea last year in an attempt to build the very lowest cost robot I could think of to let kids make some form of robot for the absolute minimum cost.

While the cheap motors, servos, etc may be the best idea, I kept thinking about some of the designs I had done that used parts of the circuit board for 3D construction. (Placing an array of light sensors above the board for position detection). On a small scale, the fiberglass of a circuit board, and solder joints locking things together can make a pretty strong framework. If the object of the game is education... then making part of the circuit board into a motor is a great idea for teaching how such things work.

Coils, wound on a framework provided by the circuit board, combined with driver transistors to build a simple stepper motor, would let you build the motors into the design of the circuit board. (It wouldn't be very efficient, but it SHOULD work.) Magnets, glued in position to 2 DVD's that could come with the kit, would be the drive motors AND wheels. AND a demonstration on multi-purposing. Tip121 transistors sell for about a quarter each.. (2 dollars for the motor drive ) Magnets: 6 - 8 per wheel needed at about 50 cents each, let's say 8 dollars, that's ten dollars for left and right drive wheels, along with the theory AND practice of building an electric motor.

It's hard to beat the price of one of the Prop development boards... but if someone (Parallax for instance) wanted to produce this board in the same way as the other boards... ( or design the robot frame to easily adapt to one of the current boards ) it might be possible to have a working, educational robot for under $50.00.

A kit like this would let budding robot engineers play with other control/motion projects like... clocks... moving lights, etc.

Might be worth looking into.


11-15-2011, 12:25 AM
KB, well I think this gets close to $50 (not counting shipping).
The Quickstart is $25.
The Magician chassis is $15.
I'm looking at $15 for this plug-in to Magician (tentatively called Merlin).
So, that's $55 total (if I did the math right).

Where people can get creative is adding on low cost sensors...

11-15-2011, 01:40 AM
You could also mount the quickstart directly onto the $9.25 tamiya dual motor/gearbox set (http://www.pololu.com/catalog/product/114)

For added fun you could face the quickstart towards the ground, solder 3 or more LDRs onto some of the button pads, and sequence the 8 leds to help it follow high contrast lines. Or just for a cool scanning effect.

11-15-2011, 01:56 AM
I use this camera on aerial vehicles.

post 1459

Duane Degn
11-15-2011, 02:23 AM

How set are you on using the QuickStart as the controller board.

As I mentioned, I'm also interested in make an inexpensive robot kit. I can't decide if it's better to use a QuickStart or a USB Propeller Protoboard. The USB Prop Proto is also $25 if you buy four at a time. It provides a regulated 5V source as well as the 3.3V source. The USB Prop Proto's regulator are much beefier than the QS's regulator.

I've mounted a breadboard to the backside of a Prop Protoboard. I brought all the pins out to female headers to two sides of the breadboard. A third side of the breadboard has a female header with power and ground connections.

Here's a picture.


Soldering wires from the pins to the headers took more time than I'd want to spend if I had to make a bunch of these. It might be a good first project for someone just learning to solder.

I cut a larger breadboard in half to use with the Prop Protoboard. I have since seen this size (http://www.sparkfun.com/products/9567)of breadboard for sale. I think the $6 is worth it not to half to cut a breadboard.

I think the Propeller Protoboards have a lot going for them as a beginner's robot board.

I also like the QuickStart boards. It's great to be able to power the board from the USB connection while programming. I also like the LEDs on the QS board. The female header makes connecting wires easy.

I think I favor the USB Propeller Protoboard over the QuickStart board to use with a budget robot. What makes the QuickStart your board of choice for these robot?

Are you planning on having others buy the materials or would you purchase the compents and sell them as a kit?

It seems like a lot of money could be saved on shipping if one person purchased all the parts and then sold them as a kit.

Next week I'm going to show a bunch of Cubscouts how to wire up a LED circuit on a breadboard. I'm hoping I can come up with a way they could all make a robot in the near future.

11-15-2011, 03:58 AM
Armed with my new knowledge about the propeller's driving fets, and how worse case fault current would be only just over 40mA (per pin), I thought I'd give parallel motor driving a go.

And guess what, the wheels turn! And nothing blew up - not the laptop usb port, not the regulator on the quickstart, not the propeller itself. The prop didn't reset. The quickstart regulator hardly gets warm. However I don't dare try stall current while drawing usb power from my laptop.

Tested here is 8 prop pins in parallel forming each leg of an h-bridge. With 2 motors you could have 3 half bridges, using 24 pins. However I would only drive one motor at a time, because their no load current (incl gearbox) appears to be ~170-200mA at 3 volts. When being driven by the prop they will draw less than that due to the internal fet resistance.

*** don't try this with anything you would miss should it fail! ***


Duane Degn
11-15-2011, 04:30 AM
I guess this is maginally kinder than taking a belt sander to a Propeller chip but oh, the poor chip.

You're powering the motor directly from the Prop? Ouch.

I thought a read about limiting the current to groups of pins. I wonder if it would be better to spread out the pins used a bit so the current wasn't all coming from the same set of eight pins?

Edit: I just read your post (http://forums.parallax.com/showthread.php?135845-P8X32A-Pin-driving-fet-resistance-graph&p=1051322#post1051322)about the Prop's driving fets (good stuff). I still think it might help to spread the load over different sets of pins.

11-15-2011, 05:06 AM
Duane, yes I think it would be good to spread the current around. But we're not talking about very large currents, at least not yet (light loading of motor). When the motor was up to speed my estimate is each of the 8 pins was contributing about 25mA, ie not much more than we use to drive a LED without thinking too much about load distribution.

I need to better understand the short circuit condition, because something doesn't quite add up with only being able to get 42mA out. And of course the situation with 8 high side switches on and only 1 low side active would be very bad indeed as all that current would flow through the single low side switch.

But I'm prepared to sacrifice a prop (maybe not a quickstart) investigating how long it runs for. Perhaps we could run a sweepstake.

11-15-2011, 05:25 AM
tubular: what motors are you using?

11-15-2011, 06:37 AM

build this bot 2 years ago with a group of kids.(10 /12 years old)


Gadget gangster propeller platform

Tamiya twin Dc motors (like in Tubular note )

SN754410 (H bridge)

220 , 22 and 0,1 microF condos

2 LEDs

plastic sheet

IR receiver + TV remote

It is fun to make competitions through a course with red and green round boxes

Jean Paul

11-15-2011, 07:59 AM
Jean Paul: very nice work. Cluso: The tamiya kit (from jaycar) comes with motors but tires are additional.

Rather than hijack Rayman's thread any further, how about I investigate the stall current in detail, and start a new thread

11-15-2011, 02:53 PM
Duane, The Proto USB is also a good option.
But, the Quickstart gets power over the USB cable, making it easier to use.
Also, it has buttons and LEDs on it, so people can experiment with it by itself without a soldering iron.

The real advantage of Quickstart (and PropPlatform USB too) is that I can make a board that simply plugs in and requires no soldering.

Tubular, that's an interesting way to save money. motors wouldn't have as much power, but maybe that's not so important...
Main problem I see in tying Prop pins together is that somebody is going to drive one low and one high and poof...

11-15-2011, 08:23 PM
Rayman, actually it seems to take more effort than that to hurt the prop. I struggled to get much more than 40mA in short circuit situation with "one low and one high". But still, it does need further investigation and/or testing to the death.

Realistically my suggestion is better suited to a raw prop chip, 2 AA batteries, perhaps a protection diode, cheap 40 pin socket, and no pcb. If (when) it fails it's at most a $8 socketed chip to replace.

I look forward to seeing what you come up with following the quickstart path

11-16-2011, 12:00 AM
You might be right about not hurting the Prop. I remember with a DIP chip that I had a Pin shorted and was surprised that it survived.
But, I've also had a QFP chip die on me... I've always wondered if DIP is stronger than QFP for some reason...

11-16-2011, 04:48 AM
I would have thought DIP, having less rail tie points to the die, would 'sag' more. Perhaps that helps, in this case...

11-17-2011, 02:08 AM
Well, I've completed the PCB layout for "Merlin". I still think I can provide it for $15.
I was pretty proud of my quad nand circuit that took 2 Prop pins and created 4 motor states: free, brake, forward, reverse.
Just the quad nand and the h-bridge was enough to drive a motor...

But, then I found the Toshiba TB6552 that does the same thing, but drives 2 motors and costs half....

So, I've just redone my circuit for that chip. It's actually much better now and and drive pretty beefy motors (1A)...

11-17-2011, 12:13 PM
Just ordered some test "Merlin" boards... Should have them next Tuesday...
Here's what they look like:

Merlin plugs into the Quickstart board and has a dual motor driver.
It has the jack for the battery plug.

It also is a half Propeller Platform adapter, for pins P16..P31.
P1..P8 are also brought out to a header.
P9..P12 go out to four servo headers.

BTW: I just noticed this low cost distance measuring thing on Digikey: 425-2616-5-ND

11-18-2011, 12:07 AM
Each way I look at it, the most useful seems to be this...
* Tamiya 89918 clear twin 4 speed gearbox with motors are usually the cheapest at ~$8.50. The 70168 is the non-clear version and is usually +$1.
* Tamiya 70101 wheel set 4@ 36mm ~$4.70 (only require 2)
* Optional Ball castor for ~$4
All available from www.robotmarketplace.com (http://www.robotmarketplace.com) (USPS $10.40 First Class to Australia for #89918 & #70101 - omitted ball castor)

The Sparkfun Magician Chassis ROB-10825 at $14.95 is certainly attractive, but long term I am unsure about the versatility of the motors.

Then of course, there is the electronics and mounting platform.

11-18-2011, 01:56 AM
DC motor gearbox directly to a board does sound like the cheapest method.
But, your total is $8.50+4.70x2+4?
The magician seems like a much better and cheaper way to go...

Still, my Merlin board should be applicable to your setup too.
I'm also still fond of the ArdBot. Merlin should work well with that too and some lobotomized servos...

11-18-2011, 04:20 AM
Rayman: The dual motor/gearbox is $8.50 plus $4.70 for 4 tyres (only 2 are required) = $13.20. I have seen the ball castors cheaper (~$2), but I am sure this could be done quite simply and cheaper. Big cost to me is shipping $10.40 which makes it similar to buying here. But by me buying from the USA I have verified a USA source.

IMHO the motor/gearbox with 4 speeds makes this a much better solution in the long term.

I was given a couple of Robots that were built in a project in conjunction with a local newspaper (in South Australia anyway) that ran a few years ago. The project was quite neat. Here is a link http://www.i-bot.com.au/ibot/page.php?sId=1 .

Now, where did I put them??? Oh yes, they are on the boat :(

11-18-2011, 10:19 AM
I was just thinking that there must be some cheap RC car or something at the toystore that could be used as the robot chassis...