Cheap Accelerometer

[Paolo1986]

Hello all,

I wanted to see if anybody has attempted to use the following accelerometer;
MMA7660FCR1 by Freescale semiconductors
http://gb.mouser.com/Search/ProductDetail.aspx?qs=sGAEpiMZZMt9EC5K82p3yomYDgdldlKH

I want to use it to determine the angel /direction of movement of a control stick. This is one of the cheapest accelerometers I can find £2.18 and wanted to know if it is compatible with the Picaxe 28x2? Any advice on this area would be great. I have used I2C before with a digital compass so have some knowledge in that area.

I am also unsure how I would prototype/interface such a small IC with no soldering tags! is there a method anyone can recommend?

Look forward to any response

 

[BeanieBots]

I've never used that particular one but see no reason why it could not be interfaced to a PICAXE.
Note, it is a 3v max device.
Also note, it is only 6-bit resolution, however, it does have some interesting features including a fast 120 samples/sec refresh rate and orientation change interrupt.

For proto work, I'd solder it (using wirewrap bird's-nest) to a turned-pin 8 way DIL socket. (it's a 10 pin device but only 8 are used).

 

[MartinM57]

Agreed - a quick sqizz at the datasheet (page 28) shows it has poor angle resolution so it may not be what you want. It's more of a tap/double tap/I'm up this way type of accelero-o-meter....and it's very small

 

[hippy]

Soldering 4 leads within 3mm is probably beyond my eyesight and steady hand these days. If you can find a suitable breakout board that will probably help. Getting it pre-assembled and delivered as tested, even better. Note the difficulties alband had in getting a similar sized accelerometer working. ...

http://www.picaxeforum.co.uk/showthread.php?t=12842

Once assembled it should be easy to use with almost any PICAXE. Getting there is perhaps the hardest part.

 

[Paolo1986]

Thanks BeanieBots! I have ordered two of these to mess around with just waiting for delivery.

Do you have a photo of the described method;?
‘I'd solder it (using wirewrap bird's-nest) to a turned-pin 8 way DIL socket.’

What is wirewrap bird’s nest? I have never come across this before at least not the terminology. Do you think the heat from the soldering iron will kill this IC? I do not have a temperature controlled soldering iron so am wary of this.
In regard to the 3V issue I want to use a potential divider (as demonstrated in picaxe_manual3, Page 19 section 3)? However I am unsure how I would calculate what resistors would be required to step down from 5V regulated supply to 3V? Does anybody have the equation?

 

[hippy]

I've never had any problems with using a non-temperature controlled soldering iron so would guess you will be okay if you don't dawdle over it - what temperature is your soldering iron and what does the datasheet say about soldering temperatures and time ?

Note that the potential divider in the PICAXE Manual is for dropping the level of a signal and isn't usually appropriate for dropping a power rail. Regulator or Zener plus R would be recommended but with little current draw a simple potential divider can work. Standard potential divider calculations apply.

How are you planning on interfacing the I2C bus between the 5V PICAXE and the 3V chip ? I suspect it would work with the pull-ups to 3V as it's an open collector bus but have never tested the theory. It would be simpler all round to run your PICAXE at the same 3V as the accelerometer.

 

[BeanieBots]

DO NOT USE A POTENTIAL DIVIDER for a 3v power supply!
The current used by the chip depends on what it is doing and hence it is impossible to design a divider for such an application.

Use a proper regulator chip (including required capacitors) such as this one.
http://194.201.138.187/epages/Store.storefront/?ObjectPath=/Shops/Store.TechSupplies/Products/REG004

Wire-wrap is special type of wire which many hobbiests use for quick connections without the need for solder. It is very thin and single core.
However, what I am suggesting is that you use that type of wire but actually solder it to the chip such that it looks like a dead spider.
Then solder those wires to an IC socket. It will now look like a birds nest.
That's were the terms actually come from. A circuit made without the use of a PCB is often called a birds nest because it can and often does look like one.
The chip can then be tacked with hot melt glue or similar to the socket to add a little security.

Soldering to the chip without a temperature controlled iron should be OK as long as you don't spend too much time doing it. Let it cool between each pin.
A rough (and convienient) guide is, if it gets too hot to hold, STOP.

 

[Paolo1986]

Great stuff BeanieBots, thanks for the explanation, I have some these 3V power supplies lying around!

I will update this post when I receive the accelerometers and try to keep this post up to date.

Thanks again all