IAN LANG ELECTRONICS

Electronics can be very expensive to pursue. Rudebot below is not yet even halfway finished and he has swallowed up so far the best part of £100. It's because I'm using new kit, and some of it is very costly indeed.

Robots- Scrapbots

Go Back Philosophies Steering a Robot Driving Motors CCOG/Wheelbase Powering a Robot rudebot Introduction

For instance, for his remote control system Rudebot uses a pair of Xbees, one for the receiver in the vehicle itself and the other for the trasmitter with which I'm controlling him. The pair costs £42, and then there's the expense of an explorer board with which to program them. Xbees have many drawbacks besides the cost not the least of which is that it takes a few seconds for the pair to establish a network, so you can't just switch Rudebot on and play - you have to wait for established contact between the Xbees to happen. They are also on 2.4 GHz, a very high frequency requiring very precise trimming (hence the cost).

So why am I using them? It's because they have certain features that I require. Xbees are incredibly stable once a network has been established and by using a single frequency I can send commands in the form of data rather than relying on an analogue voltage or a series of radio channels. In fact I could send billions of different commands simply by using a numerical system and I'm limited only by what my microprocessor (Arduino Pro-Mini) can do rather than my radio-control system. This is a significant advantage. I could do the same thing with a cheaper wireless link but the exclusivity of the Xbees means that any signal detected that's not in the network is rejected, and thus one transmitter and one transmitter only can work Rudebot and I get no interference from anywhere else and so in the course of development I can be sure that if Rudebot is doing something strange it's my error somewhere and not something they are doing next door or indeed in my house that I don't know about.

Furthermore the Pro-mini costs another £15, so we are up to £57 already. Transistors, regulators etc take it up to about £60 and the chassis and drive are made from a box of bits that comes from Maplin and costs £30, though you do get a heap of stuff in that box including the wheels you can see (four big ones and four little ones) and all manner of gears and axles plus the perforated metal sheet out of which his chassis has been fabricated. The Maplin stock code is UZ15R. Add in a servo and we are at £95. I dread to think how much something like Ecobot III or Asimo has cost.

But it doesn't have to be like this. You can make robots out of anything that's got a motor, gearbox and a space in which to put some batteries and circuitry. If you are making a ROV then you don't even need a microcontroller, just a receiver and some sort of driver. Allow me to introduce to the proceedings BAMbot, which cost me £1:85 to make (for the batteries) and three hours on a Sunday afternoon. There he is below.

BAMbot is what's known as a Scrapbot, aka Junkbot, Bitbot,

DIY bot or pile of old bits cobbled together. His chassis and gearbox/wheel assembly is from an old toy and from the same old toy came a radio board with four channels and an integrated motor driver. The original old toy had stopped working and was going to be thrown out, so I salvaged it for the motors. When I looked, all that was wrong was that the wire feeding a positive voltage to the radio receiver had come adrift, so I soldered it back on and voila, working toy. The owners did not want it back as the body was broken too, and so I got a cost-free chassis, gear/motor assembly, driver and radio-receiver to play with. It's always worth investing in broken gadgetry; I've lifted tons of stuff out of them that have come in useful for other stuff and once I paid a pound for one of those FM transmitters that you can plug into an MP3 player and have it play through the radio.

It came from a junk-shop where they sold all sorts of broken bits and originally I had intended to salvage the lead and maybe the OLED screen, but when I got it home I found that all that was wrong with it was that the fuse had gone. I took out the 12V section, attached a 9V battery snap, and voila- working transmitter. So, I attached a switch and put the lot in an enclosure and got a device for about £3:50 that a major chain store wanted to charge me £28:99 for.

 

You can do the same with your bots. Here's how an old radio control board with drivers usually leads out:

M1

M2

M3

M4

GND

Vcc

V

RF

This sort of design is very old; it's been kicking around since the 1960s but it's still used by a lot of manufacturers because it's so cheap. On modern systems you'll find an integrated circuit to do the motor driving, older ones use a series of transistors and possibly relays to achieve the same thing. you may also find an IC to do the radio receiving though usually it's a series of transistors, coils and capacitors. They usually operate at 27 and 40 MHz, so ensure your transmitter and receiver board are the same frequency or it will do precisely nothing.

So what do all those leads do? There are two power supplies, Vcc (which could also be marked as Vs or V+) and VRF which is usually marked as just that. Vs supplies the chip doing the driving and VRF not unreasonably supplies the radio receiver. On posher boards there may be only one supply, Vs but it's usual to find two. Usually there is one common GND wire that returns current to the negative terminal of the battery or other power supply; sometimes there may be two. Link them together at the negative terminal if there are. M1 and M2 are a motor pair. I've looked at loads of these boards and it's always been the case that two consecutive numbers are a pair. So M1 and M2 go to one motor and M3 and M4 go to the other, and if there's M5 and M6 and so on then they pair up too. One lead will usually be one colour (say green) and one another colour (say yellow) and so each pair needs to be put on the motors in the same way; if it's yellow on the right on the first motor then yellow goes on the right at the second too. Here's a tip- if the wires are not already on the motors, clip them on and check your controls; it's very easy to wire up motors the wrong way. I usually use a detachable pair of clips on my motors- experience has taught me not to solder directly for the very reason outlined above.

BAMbot's body is made of some old bits of foamboard I had kicking around, but it could have been balsa wood or lightweight metals or plastics salvaged from old packaging or Tupperware- that's the beauty of Scrapbots, you use any old junk you can find and as long as they are mechanically and electrically sound what they look like doesn't matter. As with BAMbot, the most expensive thing about them could well be the batteries.

 

BAMbot is a dozer-bot and he's made to amuse the anklebiters (alright then to amuse me as well) by playing a game of  "Shove Loads of Stuff Off from the Table On to the Floor as Quickly as You Can without Running the Bot Off the Table Whilst You're Doing It ". He works on 3 AAA batteries so 4.5V and he does not have all that much power (what do you want from 4.5V?) but he can move cigarrette packets and other similarly weighted junk easily. He can move a coffee cup with some revving if it's empty, and if he gets a good run-up when it's full too. Upgrading the batteries a bit would give him more speed and power but I didn't try it because I didn't want to have regulate for the radio board.

 

So, where do you scrounge the bits for this sort of thing? Check out cheap shops such as Poundland, and charity shops can be a rich source of cheap electronics. Last year I picked up a small radio controlled car in, of all places, Primark, for the sum of £6. If you get on with your neighbours, ask if they have any electronic toys they want to throw out. You'll be amazed how many bits you can garner. Any sort of stiff packaging can be pressed into service for chassis and bodywork and wheels (if you cant get them with the motors) can be made out of anything that's round and solid- I've seen them made out of wooden buttons that formerly adorned a lady's cardigan.

 

Aesthetics is not a primary concern in scrapbots; in fact some schools of thought say the junkier they look the better.  The primary goal is to make a ROV or autonomous bot that does something useful or entertaining or can be used to demonstrate a theory or technique that can be used on more stylish (and expensive!) devices.

Ian Lang, September 2013

Battery Concerns bambot

CAVEAT:

 

The text on the left is the usual layout for motor wires : M1 pairs with M2, M3 with M4 etc but just to confuse the issue some boards have M0. If it has M0, then M0 pairs with M1, M2 pairs with M3 and so on. Nothing's ever simple, is it?

Brains Practical 1 ROV Controlling