Circuit to ignore 12v pulses below certain duration.

[Chalky]

I have a rear-view camera circuit in my car that I'm triggering from the reverse lamp 12v feed.  All good so far, works a treat...except my cars lighting system periodically sends short 12v pulses to the reverse lamps to check the bulbs haven't blown.  Those pulse make my reverse camera flash on briefly.

How can I stop that from happening?  I thought a simple capacitor might do the trick, but couldn't get that to work.  A delay-on  timer circuit would do it I guess, but I'd like to have an infinitely long on time, as long as the 12v trigger is still present.  I've not seen that on any 555 timer circuits - any ideas?

Thanks everyone, any ideas much appreciated!

Edit: will post specifics of pulses later, have got them stored in my scope.  Appreciate all solutions, but the simpler & cheaper the better!  Thanks again!
Edit: pulses are only just over 8 V DC, might be able to filter them out on that basis?  They're about 1.2 ms each.  See attached image.

[grumpydoc]

Inductor+cap to filter the 12V power?

[rx8pilot]

How long is the pulse?

A simple MOSFET with a voltage divider driven gate slowed down with a capacitor could work. That would be a slowly rising voltage. Add a zener to make the gate turn on only at a certain voltage which will quickly turn it on, bot only after the delay set by the RC time constant.

[denelec]

Just use a time delay relay.
http://www.allaboutcircuits.com/textbook/digital/chpt-5/time-delay-relays/

Safe and simple.

[tautech]

Run a separate wire from the gearbox Reverse sensor switch to enable the camera.
You'd need to check it was at the voltage levels needed not logic levels or similar.

[Delta]

If the camera is powered separately and the 12v from the reversing lights is just a signal line, then a simple resistor capacitor circuit should do it.

[tautech]

Without knowledge of the bulb test cycle you'll just be stabbing in the dark trying to suppress it from enabling the camera.
Do you have a DSO that you can capture the test pulses?
Then it might be possible to design an RC to prevent it from enabling the camera.

Using an enable signal from the gearbox reversing switch, while a little more work, will be a trouble free solution if the voltage available is correct to enable the camera.
BTW, as it seems this is an aftermarket installation, do you have a minimum voltage spec for the camera enable?

[KerryW]

There is ab IC that does exactly that!  MC14490

[Chalky]

Run a separate wire from the gearbox Reverse sensor switch to enable the camera.
You'd need to check it was at the voltage levels needed not logic levels or similar.

Yeah not an option, I need something that can work off the existing signals.

[Chalky]

Just use a time delay relay.
http://www.allaboutcircuits.com/textbook/digital/chpt-5/time-delay-relays/

Safe and simple.

Thanks, they seem a bit pricey, ideally I'll come up with a simple R-C circuit to do this, something super simple!

[tautech]

pulses are only just over 8 V DC, might be able to filter them out on that basis?  They're about 1.2 ms each.

At what frequency?

[Psi]

Is the camera powered from the reversing lamp feed, or does it only use that as a on/off signal?

If its just a on/off signal you can use a simple resistor and cap to add some delay

You may have to play around with the values, since i dont know exactly how much voltage is required before the reversing camera turns on. But this should get you started.

ReversingLampPower ----- 10k resistor  -- +---  on/off signal into reversing camera
                                                                    |
                                                            100uf cap
                                                                    |
                                                               ground

[Ian.M]

*ANYTHING* that uses ordinary ICs and must run off an automotive '12V' (or '24V') supply, needs to be protected against the very nasty transients that commonly occur on an automotive raw DC supply.   You can assume that a good quality reversing camera designed for automotive use is designed to cope with them but any 555 circuit or CMOS logic circuit that powers the chip direct from the raw '12V' supply, is doomed to early failure.   Depending on the number and amplitude of the spikes on the supply, it may last months or it may die before you can get the car out of your driveway.

You have two choices apart from directly tapping into the reversing switch:
* implement a delay circuit using discrete components, taking care to make sure everything has at least a 100V voltage rating (preferably 200V),
  or:
* design a proper automotive rated regulator circuit  to power your delay timer circuit or time delay relay.

Google: "load dump" transient
and: "ISO 7637-2"
to see what you are up against.

If your camera has a permanent 12V feed, and the reversing light wire is only used to trigger it, a simple RC filter will probably work.  However if its powered from the reversing light wire, it gets a lot more difficult.

[nali]

I'd have thought a plain simple relay would be slow enough not to respond? Especially a big clunky automotive type.

Put a flyback diode across the coil as you don't want to upset whatever is switching the lamp supply.

[tautech]

*ANYTHING* that uses ordinary ICs and must run off an automotive '12V' (or '24V') supply, needs to be protected against the very nasty transients that commonly occur on an automotive raw DC supply.   You can assume that a good quality reversing camera designed for automotive use is designed to cope with them but any 555 circuit or CMOS logic circuit that powers the chip direct from the raw '12V' supply, is doomed to early failure.   Depending on the number and amplitude of the spikes on the supply, it may last months or it may die before you can get the car out of your driveway.

You have two choices apart from directly tapping into the reversing switch:
* implement a delay circuit using discrete components, taking care to make sure everything has at least a 100V voltage rating (preferably 200V),
  or:
* design a proper automotive rated regulator circuit  to power your delay timer circuit or time delay relay.

Google: "load dump" transient
and: "ISO 7637-2"
to see what you are up against.

If your camera has a permanent 12V feed, and the reversing light wire is only used to trigger it, a simple RC filter will probably work.  However if its powered from the reversing light wire, it gets a lot more difficult.

From the OP:

I have a rear-view camera circuit in my car that I'm triggering from the reverse lamp 12v feed.

This implies the reverse lamp enables the camera, right?
As yet we don't know the frequency of the lamp test pulse that is the root of the problem.

[Chalky]

pulses are only just over 8 V DC, might be able to filter them out on that basis?  They're about 1.2 ms each.

At what frequency?

Not measured but I'd say about 2 Hz.  In summary: a cluster of 5 pulses of +8V of 1 ms duration, spaced approx 500ms.  One cluster every minute.

[Chalky]

Yes, reverse lamp only triggers the camera (i.e. enables that video input to the LCD monitor) - camera is supplied by car's accessory 12V. 

[Chalky]

Looks like a simple RC setup would suffice, I think I just need to get the values correct to ensure pulses can't charge the capacitor too quickly.


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[tautech]

It'll be interesting to see if the cars onboard diagnostics will run with that.
Let us know how you get on. 

[Ian.M]

I'd keep the resistor value in the RC filter quite high, probably around 100K, and use an emitter follower using a 2n5550 or similar high voltage NPN transistor that can withstand the transients to provide a low impedance signal for the camera trigger input.  As long as the trigger input doesn't draw more than a couple of mA, the transistor will be capable of surviving the 12V system ISO 7637-2 test pulses without damage

[Delta]

I'd have thought a plain simple relay would be slow enough not to respond? Especially a big clunky automotive type.

Put a flyback diode across the coil as you don't want to upset whatever is switching the lamp supply.

That is the simplest solution, and therefore the best!

Why didn't I think of that?

[Chalky]

Yeah that would work, am keen to build a solid state version though!


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