Circuit to ignore 12v pulses below certain duration.  - Page 1
Forum section: electronics / projects

author: Chalky
author_rank: Regular Contributor
timestamp: July 12, 2015, 08:16:24 pm
content: 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.

author: grumpydoc
author_rank: Super Contributor
timestamp: July 12, 2015, 08:19:37 pm
content: Inductor+cap to filter the 12V power?

author: rx8pilot
author_rank: Super Contributor
timestamp: July 12, 2015, 08:24:09 pm
content: 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.

author: denelec
author_rank: Regular Contributor
timestamp: July 12, 2015, 08:48:07 pm
content: Just use a time delay relay.http://www.allaboutcircuits.com/textbook/digital/chpt-5/time-delay-relays/Safe and simple.

author: tautech
author_rank: Super Contributor
timestamp: July 12, 2015, 09:42:47 pm
content: 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.

author: Delta
author_rank: Super Contributor
timestamp: July 12, 2015, 10:29:19 pm
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: July 12, 2015, 10:54:47 pm
content: 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?

author: KerryW
author_rank: Regular Contributor
timestamp: July 13, 2015, 03:25:58 pm
content: There is ab IC that does exactly that! MC14490

author: Chalky
author_rank: Regular Contributor
timestamp: July 14, 2015, 09:33:03 am
content: Quote from: tautech on July 12, 2015, 09:42:47 pmYeah not an option, I need something that can work off the existing signals.

author: Chalky
author_rank: Regular Contributor
timestamp: July 14, 2015, 09:34:07 am
content: Quote from: denelec on July 12, 2015, 08:48:07 pmThanks, they seem a bit pricey, ideally I'll come up with a simple R-C circuit to do this, something super simple!

author: tautech
author_rank: Super Contributor
timestamp: July 14, 2015, 09:58:31 am
content: Quote from: Chalky on July 12, 2015, 08:16:24 pmAt what frequency?

author: Psi
author_rank: Super Contributor
timestamp: July 14, 2015, 10:01:22 am
content: 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 delayYou 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

author: Ian.M
author_rank: Super Contributor
timestamp: July 14, 2015, 10:12:00 am
content: *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" transientand: "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.

author: nali
author_rank: Frequent Contributor
timestamp: July 14, 2015, 10:18:34 am
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: July 14, 2015, 10:19:34 am
content: Quote from: Ian.M on July 14, 2015, 10:12:00 amFrom the OP:QuoteThis implies the reverse lampenablesthe camera, right?As yet we don't know the frequency of the lamp test pulse that is the root of the problem.

author: Chalky
author_rank: Regular Contributor
timestamp: July 14, 2015, 10:24:30 am
content: Quote from: tautech on July 14, 2015, 09:58:31 amNot 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.

author: Chalky
author_rank: Regular Contributor
timestamp: July 14, 2015, 10:36:06 am
content: 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.

author: Chalky
author_rank: Regular Contributor
timestamp: July 14, 2015, 12:18:45 pm
content: 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.Sent from my iPhone using Tapatalk

author: tautech
author_rank: Super Contributor
timestamp: July 14, 2015, 12:29:15 pm
content: It'll be interesting to see if the cars onboard diagnostics will run with that.Let us know how you get on.

author: Ian.M
author_rank: Super Contributor
timestamp: July 14, 2015, 12:43:50 pm
content: 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

author: Delta
author_rank: Super Contributor
timestamp: July 14, 2015, 11:12:34 pm
content: Quote from: nali on July 14, 2015, 10:18:34 amThat is the simplest solution, and therefore the best!Why didn't I think of that?

author: Chalky
author_rank: Regular Contributor
timestamp: July 15, 2015, 12:21:57 am
content: Yeah that would work, am keen to build a solid state version though!Sent from my iPhone using Tapatalk