Hello,

I need to reduce automotive voltage down to CMOS level. My project is converting a garage door remote opener to an integral part of my motorcycle and powered from the motorcycle electrical system. I decided to build a simple voltage divider using resistors. I read that it is not good to use high resistance so I purchased 2 and 7 Ohm resistors.

(14 VDC * (2 / 9)) = 3.111 VDC.

The circuit will only be energized when I honk the horn so heat from the resistance should not be an issue. Am I on the right track or headed for a train wreck?

Ron

 

Conventional wisdom is to use a voltage regulator. Using a voltage divider for your application is a waste of power.

There is no single voltage for CMOS, but you can get a voltage regulator for the common voltages.

 

I have to agree with dl324, a voltage regulator is the way to go. However if you insist on using a voltage divider I would definitely pick a higher resistance than that. That combination will be wasting at least about 1.25A. Not to mention your larger resistors power rating would have to be almost 13 watts. One of the big downfalls of a voltage divider, aside from wasting power, is that it limits current into the device, likely the reason someone suggested lower resistance values. Just make sure the resistor connected to +14V isn't so large that it chokes your device, add some headroom, and then size the other resistor to match the desired ratio. Again, a voltage regulator is a much better option.

 

I have to agree with dl324, a voltage regulator is the way to go. However if you insist on using a voltage divider I would definitely pick a higher resistance than that. That combination will be wasting at least about 1.25A. Not to mention your larger resistors power rating would have to be almost 13 watts. One of the big downfalls of a voltage divider, aside from wasting power, is that it limits current into the device, likely the reason someone suggested lower resistance values. Just make sure the resistor connected to +14V isn't so large that it chokes your device, add some headroom, and then size the other resistor to match the desired ratio. Again, a voltage regulator is a much better option.

 

I thank all of you for your input.

I started out with Pololu dual output voltage regulator boards and one had no output and the other had 5vdc for a short while. My multimeter was the only load and I was within specifications on the input voltage. Would a potentiometer be any better than a divider?

 

I started out with Pololu dual output voltage regulator boards and one had no output and the other had 5vdc for a short while. My multimeter was the only load and I was within specifications on the input voltage.

It would be helpful if you gave more information regarding the voltage regulator. Some require a minimum load current before they'll regulate, but that usually gives an unregulated voltage, not no voltage.

Would a potentiometer be any better than a divider?

Not likely. Pots are typically low power.

What is the current requirement for your CMOS logic? A zener diode would be better than a resistive voltage divider.

 

We know 22mA should be slightly warm if that.
let's say you would like to try current in the neighborhood of 22 mA at 3.3V
we know V/I=R so that 13.8/.022=627
627/3.3=190 a rough approximation for a two resistor divider.
R1,R2 = 630,200 Ohms

 

What is the nature of what you want to power from that low voltage, and more importantly, what is the range of voltages over which it will operate (min and max voltage) and what are the minimum and maximum currents will be required.

 

Hello,

I need to reduce automotive voltage down to CMOS level. My project is converting a garage door remote opener to an integral part of my motorcycle and powered from the motorcycle electrical system. I decided to build a simple voltage divider using resistors. I read that it is not good to use high resistance so I purchased 2 and 7 Ohm resistors.

(14 VDC * (2 / 9)) = 3.111 VDC.

The circuit will only be energized when I honk the horn so heat from the resistance should not be an issue. Am I on the right track or headed for a train wreck?

Ron

If you can get a hold of an op amp, a voltage-follower circuit would probably work well for something like this.





The best thing about that particular setup is that the op amp draws very little power with those big resistor values.

Couple of caveats to keep in mind though. You can't attach it to a bunch of separate loads and expect a stable output; It's one load per op amp. Second, you need to find out how much current the device uses to determine if the op amp in question can handle the throughput.

 

If you add a transistor to the voltage follower, you can get more current; but the transistor may require a heatsink.


We still don't know what voltage you're trying to obtain; 3.11V is a strange voltage.

 

I think the TS (thread starter) wants to use a remote garage door opener to open the garage door. One that works on a coin cell like a 2032 battery. If so - it's fairly low current since a coin cell can't give more than 30 mA (guessing at that number).

Hooking that up to the horn means when you come home late at night and want to open the garage door you have to wake up your neighbors.

 

He said he will honk the horn on the motorcycle to wake all the neighbors and open the garage door.
Oh, the motorcycle itself will wake all the neighbors, not the horn. Most motorcycles can be heard blocks away.

 

Hooking that up to the horn means when you come home late at night and want to open the garage door you have to wake up your neighbors.

He said he will honk the horn on the motorcycle to wake all the neighbors and open the garage door.
Oh, the motorcycle itself will wake all the neighbors, not the horn. Most motorcycles can be heard blocks away.

I've heard of some stupid things. If true, this would be near the top of the list. And inconsiderate to boot...

 

I think the TS (thread starter) wants to use a remote garage door opener to open the garage door.

I've heard of some stupid things. If true, this would be near the top of the list. And inconsiderate to boot...

I said "I THINK"! Lets not jump to judgement. I recall one motorcyclist wanting to do something similar but use the left turn blinker to open the left garage door and the right turn blinker to open the right garage door. At least he wasn't honking a horn.

A few years ago I wanted to put a GDO remote in my truck and wire it into vehicle power. Of course, the GDO Remote uses a coin cell battery. 12V (13.8 to 14.4V) would blow the remote to heck. So I used an old cigarette lighter adapter that puts out 5V for charging cell phones and modified the circuit to give me 3.3V. It worked for a while; but now it doesn't work at all. Perhaps a Voltage Follower and a resistor to limit the current to something like what the coin cell puts out will revive my old project.

Some scolded me for solving a problem that didn't exist. I get it. But having just a little more power means I don't have to drive up to the garage door before pushing the button. I can push the button when I'm a couple doors away from home and by the time I reach the garage the door is fully open.

Sometimes an electronics hobbiest means doing things because they can be done. Doesn't always mean it's practical. My TV remote - the wife is always dropping it behind the pillows and then sitting on it, holding buttons pressed. With the IR obscured, the TV does nothing. But come the next morning I'm replacing batteries. So I thought since I keep a wireless charger for my cell phone by my chair, why not make the TV remote wirelessly rechargeable. Again I got scolded. We all do things for our own reasons. Lets let the TS decide what he wants to do on his own and see if we can help him solve the problem he presented.

 

My daughter needed to move because a neighbor's noisy motorcycle woke her at 4:00 every morning.
Hee, hee, it was driven by a policeman.

Police in my city recently gave tickets to many noisy car drivers. There are still millions of those inconsiderate idiots.
I think a new Ford Mustang 5L V8 has a button for loud, louder or loudest. The very expensive Lamborghini and Ferrari cars driven by Chinese students at my nearby university are also extremely loud.

 

The TS never said he'd be honking at unreasonable hours. Of course we all know what drunk people do. AND I'm not saying the TS is an alcoholic either. Just speaking in general.

 

Sheesh, and I was the one who was told not to bring my hot rod 69 Camaro with 3" exhaust and turbo mufflers back to town until I "toned it down". One of my neighbors called it his alarm clock when I cranked it up to go to work in the mornings. And it DID HAVE mufflers! The good ole days...

 

Perhaps a Voltage Follower and a resistor to limit the current to something like what the coin cell puts out will revive my old project.

You may not need a resistor there at the output. Remember, current is usually driven by the load. So unless it's something that specifically needs to be current-limited (like an LED), you can simply hook it up to the required voltage level and it should work fine.

 

Sheesh, and I was the one who was told not to bring my hot rod 69 Camaro with 3" exhaust and turbo mufflers back to town until I "toned it down". One of my neighbors called it his alarm clock when I cranked it up to go to work in the mornings. And it DID HAVE mufflers! The good ole days...

Wow, coming from a guy who looks like a member of ZZ Top I would have expected a little more consideration.

 

Been thinking about this one. When a car with a mid life battery starts up, it's voltage can go up to 14.4 volts. But when cold, after sitting over night, the battery voltage may be at 12.4 volts. Using the scenario in post #9, at 14.4 volts the output would be 3.323 volts. But before starting the engine the voltage (12.4V) will have an output of 2.862 volts. A better way to set a voltage at a given setting and hold it there regardless of the battery voltage would be to use something like a TL431 voltage reference. I'm looking at how to construct such a device now.

[edit] a "Mid-life battery" is an older battery, about half its useful life. A new battery tends to hold 12.6 volts over night and when charging it rarely goes up over 14V. At least that's what I observe daily with my battery and battery voltage monitor.
[end edit]