# Thread: Convert a flashing signal to a constant signal?

1. trace the led back to the source
see if its connected to a timer relay or something
its probably not but you never know

2. Ok...

If you are tapping from the LED most likely its a 5V/0V cycle not 5V/open circuit.

You also need to determine if the signal is 0V/open circuit...this can be also the case.

The voltage will be also lower due to current limiting resistor by the time it get to the LED, 2V perhaps. How is the LED connected? signal/0V or +12V/signal? theres a big difference.

Try this circuit :

Put a small diode (1N4148) just after the output of alarm that connect to the LED.

A capacitor from the diode to the ground. A resistor across the capacitor.

The value of the capacitor and resistor is such that it cover the time when the LED is off. The diode ensure that the capacitor is only charged by the flashing signal and never discharged. The resistor does the discharging (try without the resistor first)

You should get a 2V constant across the capacitor/resistor if you get the values correctly. However you cannot get much current out of this circuit, as I said before there must be a limiting resistor in the alarm.

You must then convert this constant 2V into a constant 12V...more to come later...Im late for work

Hope I didnt confused you

3. Ricky327,

Thanks for the reply, I'm a bit confused, but much better off then I use to be. I am tapping the LED voltage before a circuit that looks to have just a resistor and disc capacitor. Before the circuit I get a 12v signal when the light is on and after its 1.8v both flash though. The flashing signal is generated by the Body Control Unit, which from what I can gather from the service manual is more a computer then simple a circuit board with 70+ lines feeding into it controlling everything from lights to the climate control system.

I'm not sure I understand by what you mean by signal in +12V/signal or 0V/open all I know is that when I hook up my multimeter when the light is on, I get 12v and 0v when its off. Does signal / open just mean whether it has / does not have continuity?

I shall put the circuit together when I go home today and play around with it. Thanks!

Also, on the capacitors I bought, there is both a voltage and a number in microfarads inversly proportional to the voltage. I know the microfarads is for the capacity of the capacitor, but why is there a voltage? I thought capacitors could take any voltage and would just fill up / discharge at a different rate depending on the current of what it is fed and discharged at.

Originally Posted by Ricky327
Ok...

If you are tapping from the LED most likely its a 5V/0V cycle not 5V/open circuit.

You also need to determine if the signal is 0V/open circuit...this can be also the case.

The voltage will be also lower due to current limiting resistor by the time it get to the LED, 2V perhaps. How is the LED connected? signal/0V or +12V/signal? theres a big difference.

Try this circuit :

Put a small diode (1N4148) just after the output of alarm that connect to the LED.

A capacitor from the diode to the ground. A resistor across the capacitor.

The value of the capacitor and resistor is such that it cover the time when the LED is off. The diode ensure that the capacitor is only charged by the flashing signal and never discharged. The resistor does the discharging (try without the resistor first)

You should get a 2V constant across the capacitor/resistor if you get the values correctly. However you cannot get much current out of this circuit, as I said before there must be a limiting resistor in the alarm.

You must then convert this constant 2V into a constant 12V...more to come later...Im late for work

Hope I didnt confused you

4. ^^^^
Voltage on caps is the highest voltage a cap is rated on. Don't go over it.

5. Rather than screwing around with the LED idea, which I think is better to give up on now, find a diagram of the power locks.

6. I have a diagram of the power locks. They run off a simple 5v line... I could do that and when I eventually fail doing the alarm armed thing, I probably will. This will introduce the problem that if I lock my doors while in the car, the computer will go off, not a real deal killer, possibly even a feature for a built in power switch, but at this point its more of not letting my car beat me and I wouldn't mind learning a little more about circuits in the process.

Originally Posted by P4_2.66GHz
Rather than screwing around with the LED idea, which I think is better to give up on now, find a diagram of the power locks.

7. Originally Posted by god_of_cpu
I have a diagram of the power locks. They run off a simple 5v line... I could do that and when I eventually fail doing the alarm armed thing, I probably will. This will introduce the problem that if I lock my doors while in the car, the computer will go off, not a real deal killer, possibly even a feature for a built in power switch, but at this point its more of not letting my car beat me and I wouldn't mind learning a little more about circuits in the process.
I wonder if it is possible to get input from the lock/unlock buttons. You could use it not to send any signal to your PC. In my car, it would be virtually impossible [edit: figured out how a few minutes later ] due to digital bus signal running from the driver side door. I am not sure how G35 is.
You could also add some logic to it, such as if the car is on, ignore lock input. Also, if the PC is on, ignore unlock input, if it is off, ignore lock input.

8. Originally Posted by P4_2.66GHz
Rather than screwing around with the LED idea, which I think is better to give up on now, find a diagram of the power locks.

oo thats a good idea

9. I thought about the LED, and alarm and then the door locks, then I come upon the same pitfalls as you have, the door locks are no good either as they lock automatically on my car after starting the car, then it hit me, install a seperate door open/closed switch on the drivers door, wire it up with the ignition switch and a relay or two and bang you open the door with ignition off pc begins booting, close door with ignition off no effect, ignition on no effect, then work out what you want it to do when shutting down (and also what power supply you have etc) you will need some other circuit that if the ignition is on and door is open nothing happens to the pc, I think three relays, a door switch and some wires will see you done

Hope this helps

10. My car doors don't lock automatically after I start moving, so I'll probably just hook into them with a single relay or so if I can't get the alarm armed thing working...

I was just talking to an engineer at work about this and he wasn't too optimistic about being able to do it with a capacitor since the on time is like only .5s and the off time is 2.5s. He did have what seems to be a very good seemingly simple idea... Use a timer circuit that would be triggered and to stay on for more then 2.5s (the flash interval of the alarm) it would be retriggered everytime a flash was recieved, so the only time it should go off is if the car is in the disarmed state and so off for more then 2.5s. This seems like the most practical idea so far, but I have yes to find some hardware to do it. Any suggestions? Pitfalls?

Originally Posted by SAScooby
I thought about the LED, and alarm and then the door locks, then I come upon the same pitfalls as you have, the door locks are no good either as they lock automatically on my car after starting the car, then it hit me, install a seperate door open/closed switch on the drivers door, wire it up with the ignition switch and a relay or two and bang you open the door with ignition off pc begins booting, close door with ignition off no effect, ignition on no effect, then work out what you want it to do when shutting down (and also what power supply you have etc) you will need some other circuit that if the ignition is on and door is open nothing happens to the pc, I think three relays, a door switch and some wires will see you done

Hope this helps

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