Picaxe and Relays Help Please

[GreaseMonkey1980]

Hi there, im a complete newb to picaxe, having done stuff the hard way with circuits, ive decided to try micro controllers instead of standard components, and the flow charts are so much simpler...
Anyway i have several questions...
I want to make a control system to power / run my dads Diesel Tractor.
I have made a flow chart composed of following outputs and inputs:

Key to Picaxe INputs / OUTputs
Input 1 - Push Button to start or stop (depending which state its currently in.)
Output 1 = to a relay which controls fuel solenoid
Output 2 = to a relay controlling starter motor solenoid
Output 3 = to a relay controlling heater / glowplugs

The PROBLEM is these all require 12volt automotive relays and are quite heavy duty in comparrison to electronics and picaxe hardware.
So my question is, how could i connect up 3 x 6volt small relays, running from the micro controller to power the bigger relays running 12 volt higher amperage.
Could i just connect picaxe outputs direct to the 6 volt relays or do they need extra power or components, transistors etc... Also what is the most practical way to connect the picaxe control board to a 12 to 13.8 volt vehicle power supply as it needs only 5.5 volt im led to believe. Hope you can help a total NEWB.
Grease-MONKEY-1980

 

[Andrew Cowan]

You can drive 12V relays via a suitable driver - posssibly a MOSFET?

Look at the suggested diagrams for solenoids in manual 3.

6V relays would also need a driver anyway, so you might as well forget them.

For connecting a PICAXE to a vehicle power supply have a search on here. Note that it is dangerous, could interfere with onboard systems etc.

A

 

[BeanieBots]

Welcome to the forum.

Don't use relays to drive relays. (you'd an interface for them as well!)
Use a transistor or transistor array to drive the 12v relays.
Have a look in the interfacing manual. It's all explained in detail there.

 

[Dippy]

Do the above suggestions and look in Manual 3 pages 7 , 8 and 13 'Output Device Interfacing'.
The precise component choice depends on the relay coil current.

The figure on page 13 of the on-line Manual 3 shows a typical relay circuit.

The bit you will be doing is the transistor and diode.
BUT you need to know the relay coil resistance to make a choice of transistor (and even the diode),
Do the calcs then err on the side of caution.
'Manly' components are usually only pennies cheaper than tiddly ones.

Many places like Farnell and RS Componnets have pages of transistors and they have parametric searches to make life easier.


Mentioning vehicular supply and PICAXE will excite a few people.
You will need suppression and a suitable regulator. The supply line should be fused and many suggest using a diode to prevent damage on reverese polarity.
ALWAYS read the data sheet on the regulator - manufacturers supply them so you can choose the correct components.
There are 'automotive' rated regulators which basically have a higher input voltage spec.
It is easy in principle. But your construction is important. It must fail safe.
On a car I believe the fuel pump solenoid is arranged to cut out on impact (well in my car it does). It would be silly to override that safety feature.

I assume your level of electronics is 'new'?

How are exisiting relays connected?
i.e. do you have access to both sides of coil , or will your circuit just be taking one side to ground or supply?
i.e. the switching at present; how does it do it?