Voltage regulators

[Joe Keaveney]

Hi all,

Thanks for all the advice you guys have given me, I hope to get some time to come up to the hackspace again soon when the kids are a little older.

I'm currently trying to figure out a design for a lap timing system for RC cars. This type of thing has been done before (http://rcarduino.blogspot.co.uk/p/project-index.html) but I would like to improve on this.

The project itself consists of a transponder/transmitter, and a receiver. The transmitter (ATtiny-based) continuously sends the same message over IR using the IR-remote library. A button on the transponder enables the message to be changed, so other users with the same system can use the RC track without spoiling the results of other users.

The receiver (Arduino) receives the emitted signals and counts number of laps, lap times etc. It also allows the minimum lap time to be defined, as well as selecting the message to look for (essentially the 'channel' that the transmitter will operating on)

Anyway, my issue is that I would like to keep the transponder/transmitter as small and light as possible. It will comprise:

-ATTiny45/85

-Push button

-Resistors/Caps/Xtal etc

- IR LED & Visible-spectrum LED for identifying 'channel' being used

The ATTiny45/85 CPUs have an input voltage of 1.8V to 5.5V - however the voltage supplied by my RC car's BEC (Battery Eliminator Circuit) is 6V, so is it better to use a 5V LDO regulator, or a 'normal' linear 3.3V regulator? Or is there no difference? And do any of you guys have any recommendations for small SMD or through-hole regulators that would achieve the above aim?

Cheers

Joe 

[Damian Axford]

HI Joe

Cool project and that rcarduino site is full of great stuff - going spend a while reading the back catalog :)

For a battery powered circuit, LDO regulators make more sense, as they waste less power, and similarly 3.3v should be marginally more efficient.  Not sure what the value is in taking the feed from the BEC through... might as well take it straight from the battery?  Given the motor noise, and current/voltage spikes, you may need chunking smoothing cap(s) to avoid brown-outs.  

The ATTiny has an internal oscillator, so you shouldn't need an external xtal, especially given the timing accuracy for IR packets isn't that tight.  At 3.3v, with short bursts, you can probably get away without resistors to the LEDs as well.  Would make for a super low part count.

I've not got any experience with SMD, so can't help on that front, but for a prototype (and given the low part count), I'd just prototype on stripboard - it'll still be tiny!  I suspect you could get that entire circuit within a 25mm square using normal through-hole parts.

hope that helps

D

[Joe Keaveney]

Hi Damian,

Thanks for the tip about the oscillator on the ATTiny! Definitely saved room and money on the design.

Taking the power from the BEC via the receiver means I can quickly plug and unplug the transponder when I need it. Should be a very simple and small circuit! Will keep you posted.

Cheers

Joe