Where should I post to get cct design help ?

[scottb721]

It's real basic but still like other's suggestions.
Not sure about the best site to post on for help.
I'll post here anyway if it's suitable. My apologies if not.
Thanks very much.
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I'm adding a remote trunk release to my car.
It's factory remote only locks and unlocks the car.
I can access a 12V signal, a momentary pulse, that goes to the door motors. There is actually 2 wires that go to the doors, both are normally at GND but either one get a 12v pulse to either lock or unlock.

The only idea I can come up using the factory remote is have the trunk solenoid fire when a 2nd 12v unlock door pulse is detected within a couple of secs of the 1st unlock pulse. ie 1 press unlocks the doors, 2nd press pops trunk. If 3-4 secs have passed 2nd unlock won't pop the trunk.

I could use a 555 cct but I'm wondering if there's a simpler way to do it.
I also need to make sure that no part of the 1st pulse fires the trunk. I thought perhaps the following might be a problem.
The rising edge of the 1st pulse energies a timed relay closing it's contact therefore allowing the next pulse thru it's contacts, but, since the 1st pulse is still present when the contacts close the trunk will fire on the 1st pulse.

Any ideas ?
Thanks veru much
Scott

 

[jemch72]

I dont know if you keep the unlock but. down (hold) the pulse out will be as long as you hold? or just a pulse..? if is as long as you hold, is better to use the 555 based on the hold of the 12 volts, so you have less posible truck opening by mistake...now if the 12 v pulse dont hold, I will use the 555 and a counter at the output so after 3 unlock signal in a time window of 3 seconds the truck open...4017 counter can make it and no need of 5volts as cmos, dont forget to drive the solenoid thru a relay , and put a reverse diode in the relay coil terminals....with the 555 enable the 4017 for 3 seconds, if it count 3 pulses , it open by the respective output , filter the clock input (pin 14) with a cap and resitor to clean the pulse in, not to big cap....the output 2 (pin 4) will be high after 3 clock pulses, this output should have a pull down resistor of 10 k....you got it?? this way you have a fail proof system....no false truck openings, a bit more thing, but a way more confident....

 

[scottb721]

Thanks for the suggestion. May just have to go down IC path.
The pulse is only short regardless of button press. It's driving a small motor connected to the door lock.

Cheers

 

[scottb721]

It might be better to use both the unlock and lock signals.
Get one to energise a relay thats on for 2 secs and have the other go thru the relay contacts to fire the trunk solenoid.

So if a lock and unlock signal is received within the 2secs the trunk pops. Might be the easiest way to do it.

Still have to make a cct to keep the relay on though.