Relay will switch earth.

Just got home guys! Shall have a shower and tinker with this. If using transistors, make sure you use some diodes in reverse bias across the relay poles or the tranny won't last long...

But from the top of my head, I would use a DPDT relay, put capacitor positive on the common (neg to cap neg), and have the NC pole going to constant positive. The value of this cap will determine your delay on time of the next relay, which can be connected to the NO pole of the first. The second relay acts as the inhibit for the crank, and inhibits thru its Common and NC terminals. It will power up for a split second when power is applied to the coil of the first relay, the cap will discharge thru the second relay, and then it will relax again, and allow the original circuit to do its thing

Its basically turning a momentary signal into a timed pulse, and you just use the appropriate relay contacts if you want to reverse it. You could also do it with a single relay, and use a cap in series with a pole of the relay coil. While the cap is charging thru the resistance of the relay coil, it will be energized, and will relax once charged. Put an appropriately valued resistor in parallel with the cap to discharge it once power is removed (or you will only get one cycle out of it!)

Keep the resistor value high or it will act to keep the relay energized all the time. Don't go too high or it will take ages for the cap to discharge after power is removed. Have a look at my Audio controller schematic on this site for the same circuit.

I'll cad it if I get time, but currently doing some paid cad work so may not be for a while. Maybe an envelope sketch will do

using R/C circuits to time relays is a thing of the past... do it properly with a pic

I dont get it, i've done electronics, and now every1 seems to be an expert on it all, bloody braidy knows more than I do

Hey! I am not that thick! =P On a serious note, I have learnt a s**t over the past couple of months about this sort of stuff though, it's awesome.



I would love to solve the cold start issue. I'm used to killing the ignition and pressing the button again, but I'd love to have to just do it once.

I think it would be better to hold the crank a lot longer though, because I know when my car used to use a key, when dead cold it would still take a second or two. Having the crank time going for a few seconds isn't a problem because the relay that monitors the alternator will cut it off when the car starts. I reckon this is the easiest and most fail-safe method, however if I am wrong, say so! Like I said, learnt alot, but there's always more to learn!

Pm me with an exact sequence of events you need, and i'll see if i can give you some pointers... im offering info only if you want it

thats the way i have made mine so that it will crank untill the car starts. the timer is so the pump can prime and doesnt need to crank as much. ive tried so many ways with a couple of different transistors but cant seem to get it to work consistantly.

i wasnt using a relay at this stage just a multimeter. but i think the cap was somehow chargeing through it as i had the earth on and the multimeter and it was charging

im not quite jigging onto what your saying, do mind making a small diagram for me?

had a look at DS and jaycar and both seem to use a 555 chip. i was thinking of using a relay running off the fuel pump to cut the starter relay out and when the fuel pump stoped priming it brings in the starter relay

Last edited by efgiar on Wed Nov 28, 2007 5:39 pm, edited 2 times in total.

too many relays mate... there are much easier ways... ie only 2 relays, one for the ign and one for the starter... ive givien braidy a bit of info on this

Using a PIC would be great - but it involves a lot more than just some parts and a soldering iron. You need to know a little about programming, have a board to program the PIC, and then build the rest. Its well worth learning, but its a big step for someone who's never done this sort of thing before.

found another problem, i was going to use the fuel pump as a cut out untill it primed, but when you crank the pump runs and will cut the starter out... so back to square one. the only other thing i can think of to prime the pump is to drun something like 25000uF of capacitors on the c/o relay to cut out the starter side.but then you would need double that for the signal to trigger the relay. i think i will scrap the priming side.

how much worth of capacitor was used for the boot release signal? to extend 1 relay for 1 second is like 7000uF so im guessing 10000-12000uF will be needed?

Hey, guys. I am sorry to hijack this post, so to speak. But I was just wondering what you meant, braidy, by:



Are you saying that I can deliver power to the starter for say 5 seconds or more and the starter motor will stop turning immediately once the car starts and the alternator is turning?

Because I have been working on an Arduino controlled Bluetooth remote car starter and my biggest problem was figuring out when to shut off power to the starter motor. My approach was to use a simple voltage divider that would give a 0V-5V output corresponding to the alternator being off (~12.5V) and on (~14.5V), respectively.

I am almost certain that my voltage divider approach would work, in letting the Arduino know (through an analog input pin) that the alternator was running (i.e. system voltage is ~14.5V and output from voltage divider is ~5V). But I will not waste time designing the voltage divider, if I can simply rely on the starter cutting out automatically once the engine and alternator are running.

NOTE: I would actually tune the output of the voltage divider to 0V-4V, as this would prevent any small voltage spike from the alternator from providing a voltage output (of the voltage divider) of more than 5V, thus protecting the 5V maximum rated analog input pins on the Arduino.

Thanks for your time, mate!

Hey mate,

What I mean is using a cap or a timer or something to extend the length of the pulse when you push the boot button on the remote so it continues cranking for longer. Currently, the pulse only lasts for a second or so which usually isn't long enough to start a cold motor.

The Smartlock remotes are disabled once the car has started so it won't continue cranking the starter motor after the motor starts. Don't know if this will apply to your situation, best of luck!

Thanks for the explanation. That doesn't actually apply to my project. But, just to confirm, will my car start without the key in the ignition? Will Smartlock allow this? So if I was to simple bridge the connection between the Ground and accessory and On position on my key (ignition) barrel, then cranked the starter motor for long enough. Would the engine start? Or does Smartlock check for the key before it will let the car start?

Thanks again for your time, mate It is very much appreciated.