Thread: 555 Timer Problem.. Need it To Trigger when key removed

Never had that much luck with 555 circuits, but if you want a pulse for when the car goes off, think about a cap.

The cap charges and then acts like an open circuit, when the power is removed, it can drain through the trigger input.

The problem I see is that it's going to hit the trigger twice.

This second circuit should work fine for you...

http://www.mp3car.com/vbulletin/show...53&postcount=9

Hmm, let me rethink that.

The 555 is triggered when the input goes low correct? If that's the case and you've got it wired to ACC, how is it triggering before the car is turned off? I'm assuming the 555 isn't running off the ACC line

What is the output connected to? Can you give some more details?

Originally Posted by blessani

This second circuit should work fine for you...

http://www.mp3car.com/vbulletin/show...53&postcount=9

Shouldn't there be quenching diodes on those?

Originally Posted by fire

ok... ive had a look at using a seperate cap.. but this double pulse stuffs up the system... i need a single pulse somehow only when the car is being turned off or else the timer already has began its counting.

sorry for the lack in details..

ok..ive used a relay for the trigger... (the coil is connected to acceccories and ground)... and the switch acts as the trigger as demonstrated in the circuit.,. only when accesories has power..... [sorry if this confused you into thinking i was giving it a high not a low..]

The timer's power source is directly off the battery +12v and ground..
and its output +12v triggers a relay to close the circuit for the inverter to be on (thus switching on the inverter)..



this circuit im using is known as a 555 monostable circuit...

I'm a bit familar with the 555, but it's been awhile. A couple points for clarification.

Are you powering the relay that drives the inverter from two circuits or are you somehow expecting it to both toggle on and off the inverter via the status of ACC?

BTW another good 555 page.

http://www.kpsec.freeuk.com/555timer.htm

Note this...

Like all ICs, the 555 and 556 must be protected from the brief high voltage 'spike' produced when an inductive load such as a relay coil is switched off. The standard protection diode must be connected 'backwards' across the the relay coil as shown in the diagram.

However, the 555 and 556 require an extra diode connected in series with the coil to ensure that a small 'glitch' cannot be fed back into the IC. Without this extra diode monostable circuits may re-trigger themselves as the coil is switched off! The coil current passes through the extra diode so it must be a 1N4001 or similar rectifier diode capable of passing the current, a signal diode such as a 1N4148 is usually not suitable.

Well it is soooo simple you probably overlooked it.

Depending whether or not the trigger is set from the high-to-low pulse, or the low-to-high pulse, just take the ign line and NOT gate it or feed directly. It doesn't need a pulse, it relies on the high/low or low/high. So if you continuosly feed it IGN, then it goes from 0 at start to 1 all the way until you shut it off, then the 1-0 stage will trigger. I think this is the stage you want. High to low. If you want low to high, then just NOT gate the thing to ~IGN, and you have 1 at beginning, 0 until you shut it off, and then the 0-1 stage triggers.

Now this will work to switch when the power is cut to the car. If you want to be able to kill your car, and leave your key in, then I suggest using your KEYSENSE wire to activate it instead of the IGN. I know some cars' keysense wire is data, not regular 0/12 so you need to get an immobilizer bypass of sorts to convert the data pulses into 0/12

Originally Posted by 2k1Toaster

Well it is soooo simple you probably overlooked it.

Depending whether or not the trigger is set from the high-to-low pulse, or the low-to-high pulse, just take the ign line and NOT gate it or feed directly. It doesn't need a pulse, it relies on the high/low or low/high. So if you continuosly feed it IGN, then it goes from 0 at start to 1 all the way until you shut it off, then the 1-0 stage will trigger. I think this is the stage you want. High to low. If you want low to high, then just NOT gate the thing to ~IGN, and you have 1 at beginning, 0 until you shut it off, and then the 0-1 stage triggers.

Now this will work to switch when the power is cut to the car. If you want to be able to kill your car, and leave your key in, then I suggest using your KEYSENSE wire to activate it instead of the IGN. I know some cars' keysense wire is data, not regular 0/12 so you need to get an immobilizer bypass of sorts to convert the data pulses into 0/12

Or just get a PIC