Let's Make Robots!

a few questions regarding picaxe 28x, ,voltage regulators and SRF08


well im working on my first robot and i have the picaxe 28x1 board from the start here guide

i am trying to connect an SRF08 to it, i understand i should connect it with i2c,

i looked in the picaxe 28x1 manual (http://www.rev-ed.co.uk/docs/picaxe_manual1.pdf page 28)

and it says that the scl and sda are legs 14 and 15

my question is, where are these legs on the board?

is it really input 3 and input 4?


my second question is about the v1 and v2 on the board

i want my board (v1) to be connected with 5 volt

as for v2 it is tricky, i want my SRF08 sensor and servos to get 5 volt

but i want my dc motor to get 7.2 volt

how can i make it that nothing will get burned?


thanks in advance, shaked

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

well first of all, thanks for posting a reply.

second, lets see if i got what you're saying:

1. remove the jumper

2. connect v1 with 5v, v2 with 7.2v

so far i knew before asking


3. connect the servos+ sensor to a 5v regulator  from the board to the servos and sensor while connecting the data wires (all the  wires that are not voltage wires, actually)  as normal


3. connecting the servos and sensors voltage wires to any v1 (5v) and ground in the board (even parallel) directly.


for the pull-ups part: what you are saying is that the 10kohm resistor can take too much voltage so the sensor ( or any i2c  component for that matter) wont work properly?

now i can see some serious disadvantages for buying a complete board with all the connections, i cant really change anything.

that's what i get for being lazy.

thanks again guys.



The AXE020 board does not have a 5 volt regulator. So this would need to be added, or a seperate 4.8 volt (4 rechargable AA batteries) pack could be used. A regulator could supply V1 or a 4.8 pack could, your choice.

If the servos are smaller, under a light load, they may be ok using the same 5 volt supply as the board. If under heavy load, their draw on the supply might reset the processor.

Another option is to use the 7.2 volt pack, attach one regulator to it to supply the AXE020 board and sensors with 5 volts, and use another regulator to supply the servos with 5 volts, plugging in only the servo signal lines to the AXE020 board. Each regulator would need the standard capacitors recommended. 

Just wanted to say, this was an awesome way to ask questions, by linking the manual (and page number!) where you had read, helping others to find what was of concern. Thanks, it was really cool, and hope you have a great experience building your robot.

For question 1, yes,  In3 is the I2C SCL line and In4 is the I2C SDA line on the 28x PICAxe. The only question on using these on an AXE020 board (page 2), is that the board has all the inputs, including In 3 and In 4, pulled down with 10k resistors. For I2C,  4.7 k resistors are needed as pull ups. I see no easy way to remove the 10k strip resistor to have the In 3 and In 4 only pulled up by the 4.7 k resistors. It might work anyway, or it might not, I think Chris the Carpenter was using a AXE020 board communicating with a I2C EEPROM, so perhaps he has some ideas on how it works on those pins. There is a PICAxe I2C manual here.

For voltage seperation, just remove the jumper on the bottom right of the AXE020 board, that connects V2 to V1,  This link has a picture about a third down, shows the V1, V2 and G pins around the AXE020.It talks a little about this on page 1 of the AXE020 manual.  Just plug in the 7.2 volt battery on the outside (ground) and third inside (positive) pins where the jumper had been to have all V2 pins be 7.2 volts. 

Use 2 power sources. If you have too much power on the 5 volt power source then use a voltage regulator. There is a post about how to use them on this site.