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Voltage based relay

litustim

New Member
Hi all
Need help again, i have coming home feature on my car, now i installed bixenon lens and dont want them to run everytime i lock or unlock my car i want.them to run obly whrn car is running.
So i need a simple circuit that just turns a relay on or off (relay controls bixenon and led bulb) depending on battery voltage, example if battery voltage is over 13v, relay turns on and closes circuit for a ballast to turn on, when car is shut off, voltage drops to 12.7, relay deenergizes and led bulb turns on for a coming home feature.
 

Pommie

Well-Known Member
Most Helpful Member
No idea what any of that means but why can't you just power the relay from the ignition switch?

Mike.
 

Diver300

Well-Known Member
Most Helpful Member
You could also run the relay from the alternator output.

It would be a very bad idea to turn off the headlights if the battery voltage dropped below 13 V. There are lots of things that can cause that to happen when driving.
 

gophert

Well-Known Member
Most Helpful Member
You could also run the relay from the alternator output.

It would be a very bad idea to turn off the headlights if the battery voltage dropped below 13 V. There are lots of things that can cause that to happen when driving.
Unless he has two sets of headlights, XenonHID and LED
 

Diver300

Well-Known Member
Most Helpful Member
Unless he has two sets of headlights, XenonHID and LED
Even so, you wouldn't want a set of headlights turning on and off when driving.

I nearly crashed a car when the headlights turning on and off was mistaken as a signal by another driver.

It was a non-abs manual gearbox car. Someone started to pull across in front of me and I hit the brakes, and locked the wheels, stalling the engine. I was late pressing the clutch, so the engine was stalled. While still slowing, I turned the ignition off and back on because of those stupid anti-restart keys that demanded that, and the other driver took that as a signal that I was letting them go ahead, while I was actually controlling a skid.

I should have just bump-started the engine by releasing the clutch, but according to the handbook, that wasn't allowed. Something about too much fuel in the catalytic converter, which certainly wouldn't apply in those situations.

Also, if you turn on headlights, there will be a voltage dip, which could cause the headlights to turn back off, so you could easily get an oscillation.

The A/C fan starting will also dip the voltage, as will the glow plugs and screen heaters if those are fitted
 

gophert

Well-Known Member
Most Helpful Member
Unfortunately, in the case of the OEM, aftermarket modifications are poorly regulated and, of regulated items , few are enforced. Even states with annual inspection processes, a few dollars easily convinces a certified mechanic that the modification doesn't exist.

your arguments assume a circuit that can't ALWAYS differentiate between the states the OP has defined.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
HEADLIGHTS:

The deer that stopped in front of the car and your on a back 2-lane road in the dark.
You have to:
Stop the car (deer).
Put car in park.
Turn off the ignition. (Light will still be on)
Put on the parking brake.
Open the door, Close the door.
Put foot on brake. Start the car. Lights will turn off..
release parking brake - Lights turn back on.

Shorted filament
In one car (1982) (Japanease) each filament had a fuse.
In another (US 1965) thermal breaker in headlight switch, All headlights blink every 30 s. Happened in this vehicle.
 

litustim

New Member
Oh my god, didnt ask you for regulations or any of that, i have my bixenon lens in high beam reflector, and led in low beam reflector so even if my bixenon cuts out my low beams will turn on. But thats only if my alternator craps out. I just asked for a circuit that switches off when voltage is below 13v. So i will use my low beam reflector only for comin home feature, so my battery doesnt get used for nothing.
 

litustim

New Member
Alternator charges at 13.8 to 14.2v and under full load it falls to 13.6v so voltage 13v is perfect voltage that wont allow them to turn off or on based on load, accidentaly. And actually wiring it somehow to the ignition switch is not a bad idea
 

gophert

Well-Known Member
Most Helpful Member
You can just capacitively couple the battery to an amplifier. Rectify the output into a filter to keep the output high any time an AC signal is present on the power rail.

that way, every time the alternator is running, the signal will be sensed and the relay coil will be energized. When just battery is powering, then the relay coil is not powered.

that way, load on the battery or charging variation is not important.

note the switch before the signal generator allows straight DC or, when switch is flipped, DC with a 0.25v AC signal on it.

A5636360-850E-4980-9BF0-795D050E00FB.jpeg
5A05DF91-F815-42F2-A702-E3060F92CFAD.jpeg
 

Diver300

Well-Known Member
Most Helpful Member
The ignition feed or the alternator warning light feed would be much better than measuring the battery voltage.

A lot of modern cars will adjust the alternator voltage depending on the battery condition and what the car is doing. My wife's 2004 MINI turns the alternator off briefly when full acceleration is called for. That would be annoying if linked to the main beams.
 

gophert

Well-Known Member
Most Helpful Member
The ignition feed or the alternator warning light feed would be much better than measuring the battery voltage.

A lot of modern cars will adjust the alternator voltage depending on the battery condition and what the car is doing. My wife's 2004 MINI turns the alternator off briefly when full acceleration is called for. That would be annoying if linked to the main beams.
Only annoying if this is intended to be a consumer product for ALL cars, ...which I doubt. I'm sure the OP could check if his specific vehicle has the feature/design issue/unAmerican-sized engine that can't accelerate without dimming the headlights. Geez, all your worries about flashing/dimming lights and then you tell us that cars in the UK are DESIGNED to dim their headlights 20% (14v drop to 12.7v or less) when you floor the accelerator. In other words, come up with better/consistent arguments when you tell us what won't work. Arguing about possible intensity changes on posts 1-10 is just so funny when you tell us your alternator stops when you floor your wife's little go-cart. Too funny.
 

Mickster

Well-Known Member
Most Helpful Member
Only annoying if this is intended to be a consumer product for ALL cars, ...which I doubt. I'm sure the OP could check if his specific vehicle has the feature/design issue/unAmerican-sized engine that can't accelerate without dimming the headlights. Geez, all your worries about flashing/dimming lights and then you tell us that cars in the UK are DESIGNED to dim their headlights 20% (14v drop to 12.7v or less) when you floor the accelerator. In other words, come up with better/consistent arguments when you tell us what won't work. Arguing about possible intensity changes on posts 1-10 is just so funny when you tell us your alternator stops when you floor your wife's little go-cart. Too funny.
Just saving this post for posterity, in case it ends up being edited after the booze/meds wear off...

:D
 

litustim

New Member
Lol lots of cars turn off their alternator when full acceleration or when cruising(fuel economy), some real patients on this site‍♂. How can lights dim if ballast regulates the output to the bulb
 

Diver300

Well-Known Member
Most Helpful Member
Lol lots of cars turn off their alternator when full acceleration or when cruising(fuel economy), some real patients on this site‍♂. How can lights dim if ballast regulates the output to the bulb
The MINI was just an example that I've experienced and actually measured. The car is one the BMW designed ones.

I've used alternator voltage to see when a car engine is running to control when a vehicle tracking unit should be on all the time, or turning on and off every now and then to save power when the vehicle is not in use. That works OK for that application, but there are timers as well, so brief drops in voltage won't cause the tracking to stop.

There are problems with that approach, and it can cause problems when trying to show when journeys start and end. The apparent start can be delayed when the alternator can't get the voltage to 14 V initially if the heated screen and glowplugs are on. The apparent end can be delayed a lot in warm weather when nothing is run after the engine is shut off, and it can take a long time to get down to 13 V. On the car with voltage reduction during cruise, a simple voltage level just doesn't work to show when the engine is running.

The voltage reduction on cruise causes problems charging caravan batteries.

On voltage dips causing visible headlight brightness reduction, I've seen far worse in a production car than what we've seen the in the MINI. I had an Audi with a 4 speed auto box and diesel engine. In cold weather the glowplugs would still be running when the car was on the highway. The glowplugs were turned off above 2000 rpm. With the auto box, the engine speed would exceed 2000 rpm before a gear change and fall below 2000 rpm after the gear change for most rates of acceleration. There was an audible click form the fusebox each time the glowplugs turned on or off. Once the battery got a bit old, the glowplugs turning on or off caused a visible change in headlight brightness, so each time the car was accelerated from stationary to highway speeds, there were 7 changes in headlight brightness, until the engine warmed up.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Lol lots of cars turn off their alternator when full acceleration or when cruising(fuel economy), some real patients on this site‍♂. How can lights dim if ballast regulates the output to the bulb
This is the 21st century, I'd like to imagine that most modern cars will use LED headlamps anyway? - mine does (but at the time - 2019 - only the higher spec models did), however all the range of that car now use LED headlamps and indeed DRL.

I can't say I was aware of cars disabling the alternator?, but seeing as modern cars are all software controlled, and with the never ending search for lower emissions, I can see why they might do so?.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
If it's high beam only & not affecting low / dip headlights, why not enable it via a relay from the ignition power, as Pommie suggested in an early post?

Simple, direct, no complications and no added electronics to get confused.

Or enable for both accessory power AND ignition on; that would disable high beam while the engine is being started on many vehicles, as accessory power switches off during starting to reduce battery loading.
 

Diver300

Well-Known Member
Most Helpful Member
This is the 21st century, I'd like to imagine that most modern cars will use LED headlamps anyway? - mine does (but at the time - 2019 - only the higher spec models did), however all the range of that car now use LED headlamps and indeed DRL.

I can't say I was aware of cars disabling the alternator?, but seeing as modern cars are all software controlled, and with the never ending search for lower emissions, I can see why they might do so?.
Some things like that are surprisingly slow to spread across all cars. 2019 is quite recent.

My 1998 car had an LED centre brake light. Many cars still have incandescent tail lights 20 years later.

A 1983 car of mine had one touch electric window on the driver's door, and all the windows controlled by an ECU. 20 years later, the Nissan Micra that my daughter owned was built, It had a trip computer and keyless entry, but the electric windows were just motors and switches, with no electronics.
 

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