Voltage comparator for dashcam - Page 1
Forum section: electronics / beginners

author: Mikey
author_rank: Regular Contributor
timestamp: February 28, 2016, 06:13:02 pm
content: I am working on making a combined voltage regulator and "switch" for my dash cam. My idea is to either use a 7805 or an LM2596 (this option is probably preferred) to convert the 12V down to 5V for the dash cam to run on.To turn the dash cam on and off, my idea was to use a voltage comparator, and have some hysterisis to remove the unstable voltage while the engine isn't fully started yet.The voltage comparator I have been looking at, is this onehttp://www.ti.com/lit/ug/tidu020a/tidu020a.pdfbut it feels like a lot of information, and it is making my head spin.Then to make it worse, on page 19 there are different variants of it, each with what seems to be very different specs, so now I am not only confused about how to use one, I am also not sure which one to look at.But if I am correct, the TLV3401 should be able to do what I need, and can be powered from 2.5V to 16V, but where did the hysterisis now go? And what about the output?If I go with a voltage comparator that can be powered directly from the 12v-14v in the car, it would be handy to just have it pull the ON/OFF pin high on the LM2596 to power up the dash cam.But right now, I am confused about which one to look at, and how to get the hysterisis done.

author: Macbeth
author_rank: Super Contributor
timestamp: February 28, 2016, 06:29:49 pm
content: All the dashcams i've experienced have a rechargeable battery in and a cigar lighter 5V USB adaptor that provides power and starts/stops the recording using the ignition. This feature doesn't work in my car as the lighter socket is permanently on so I just have to find an ignition switched supply to run off - perhaps from the map lights.

author: Mikey
author_rank: Regular Contributor
timestamp: February 28, 2016, 06:36:02 pm
content: Quote from: Macbeth on February 28, 2016, 06:29:49 pmThat is what I had hoped to do too, but there are only one permanent 12V up behind the mirror, and then two ground where one is permanent and the other goes to the door opening switches.I could also find some 12V from the ignition, but that would mean I would have to detach panels containing things for the airbags to get the wires hidden, and that is something I would like to stay far away from.

author: Macbeth
author_rank: Super Contributor
timestamp: February 28, 2016, 06:48:04 pm
content: The back of the car radio is another good place in the vicinity to get a permanent 12V and an ignition switched source

author: Mikey
author_rank: Regular Contributor
timestamp: February 28, 2016, 06:49:45 pm
content: Quote from: Macbeth on February 28, 2016, 06:48:04 pmIndeed, it is just mounted on the dashboard under the front window, and I want my dash cam to be behind the rear view mirror, in the top of the front window, and the panels on each side I dont want to touch because of the airbag stuff.

author: SeanB
author_rank: Super Contributor
timestamp: February 28, 2016, 07:06:23 pm
content: If you are not willing to do wiring then what about simply using a cheap IR demodulator ( 38kHz type which is in a 3 pin package, often free in old electronics with a remote control already on a PCB with appropriate connector, IR window and wiring loom) looking into the car interior, and use a 38kHz IR from a IR LED to give the power signal to it. Just mount the transmitter where it can get a switched supply, and use the receiver output to control the dashcam power supply with some delay so momentary interruptions do not trigger the camera off.Otherwise simply use a high pass filter on the 12V rail, a capacitor to pick off noise and amplify and detect for this, so you run the dashcam when the engine is running and the alternator is generating a lot of noise on the 12V rail. A lot more reliable than detecting battery voltage which will vary with battery temperature, though you can be fairly sure if the battery voltage is over 13V the engine is running, but heavy loads ( rear demister, defrost running using fan on full with AC running as well, electric seat heaters) can drop the voltage in the instrument console down below the level of 12V quite easily.I mounted my dashcam there as well, drilling and tapping the rear view mirror pillar M5 underneath to hold a hand made mounting adaptor which put it close to the window, mounting by the standard 1/4in camera mount insert. Power supply is tucked up under the hood lining, with the little SMPS taken out of the case to put better and bigger capacitors there ( original was mystery branded, so put in bigger slightly better brand versions and put in a fusible resistor for protection) with it wrapped up in a sleeve. Power is from a wire dropped down the lining ( hardest part was getting it under the lining to the A pillar) to a supply tapped off a 12V switched line.

author: Macbeth
author_rank: Super Contributor
timestamp: February 28, 2016, 08:08:40 pm
content: I've had a look at the wiring diagrams for my Ford Focus Mk 1 as I also fancy hardwiring my next camera. The interior front/map lights come from a "battery save relay" controlled by the central timer module (no doubt via the ignition). There is also a permanent as well as switched and timed grounds available.This seems like a better solution to me as my cam will remain recording for a short while after the ignition is powered off and I've exited the car, at least until the battery save relay kicks in. I will also have the recording starting as soon as I unlock the car from outside using the remote and the interior lights switch on.

author: Mikey
author_rank: Regular Contributor
timestamp: February 28, 2016, 08:22:31 pm
content: Quote from: SeanB on February 28, 2016, 07:06:23 pmThat sounds pretty good too, but I have absolutely no idea about how to do that

author: Mikey
author_rank: Regular Contributor
timestamp: February 28, 2016, 08:32:56 pm
content: Maybe an LM567 could be used?

author: Macbeth
author_rank: Super Contributor
timestamp: February 29, 2016, 12:11:00 am
content: Are you intending to hardwire the cam and hide the wiring behind all the trim, or run it from the cigar lighter? If doing it from the cig lighter then I can understand your problem if you have a permanently on connection like my Ford Focus has (most cars don't).But if you are trying to make a neat wired install then I really don't understand why you want to find some way of detecting the alternator overvoltage instead of just tapping off an ignition sourced signal/source. There is absolutely no reason to interfere with airbags. You can easily run wiring through the A pillar which is pretty much just a tug off job on any car, not even a screw to undo or those plastic prize clips at worst.What car do you have anyway? Pretty sure some owners clubs forums will have everything you need.

author: zapta
author_rank: Super Contributor
timestamp: February 29, 2016, 05:19:14 am
content: Quote from: Mikey on February 28, 2016, 06:36:02 pmAs Macbeth said, it's easy to tack wires under the trims all they way to the windshield mirror area, no need to remove trims.If you want an ignition signal near the windshield mirror, chances are it's available on a linbus that goes to the LEDS and button in that area (depending on the model of your car of course). You can hook a small linbus sniffer that will extract that signal. This is what I did to have a reverse gear signal near the center mirrorhttps://github.com/zapta/linbus/blob/master/beeper/eagle/beeper.pdfFor the dashcam and radar detector I passed wires from the fuse box as Macbeth suggested. For the radar detector I had to install a booster that preserves ~12V even when the voltage drops to ~8V. The dashcam camera has a rechargeable battery and takes care of conditioning its power.http://www.valentine1.com/v1info/bob.asp

author: Macbeth
author_rank: Super Contributor
timestamp: February 29, 2016, 05:51:11 am
content: Valentine Radar Detectors!! Do they still use radar these days? blimey!I had a "Snooper" detector 15 years ago. It just bleeped constantly at traffic lights, garage doors, and who knows what stuff. Useless. It had a laser detector which basically let you know the feds gotcha, so not much use at all.The pocketgpsworld database is the best for me now (thankfully I have free sub for life). Though I understand it is illegal in some countries like France. So perverse...Best of all is my retrofit cruise control. Great in average speedcam zones and roadworks.

author: zapta
author_rank: Super Contributor
timestamp: February 29, 2016, 06:29:06 am
content: Quote from: Macbeth on February 29, 2016, 05:51:11 amStill going strong. It also has laser detector.The cool kids use active counter laser measures. Out of FCC's jurisdiction.

author: haveissues
author_rank: Contributor
timestamp: February 29, 2016, 11:14:09 am
content: If you have a MK I you probably don't have airbags in the pillars so just stuff the wire behind there. You can pull off the rubber weather strip around the door and run it down that way all the way to under your dashboard. It really should not be a big of a deal and then you can just grab a ignition switched power under the dash. Make sure you fuse it and play with things like the headlights, power mirrors, etc while you have a test light on that line to make sure you just have a switched ignition and nothing else.I just used one of those cheap LM2596 modules from ebay along with a 1 amp fuse. I think my camera peaked at 600ma or so.

author: Seekonk
author_rank: Super Contributor
timestamp: February 29, 2016, 01:38:24 pm
content: If not missing something, all you need is a TL431 driving a relay when voltage is 13.8V. that will tell you when the engine is running,