small load on a voltage divider?

Other Hardware General Electronics
1

Hey everyone reading this, I have a question I would like some clarity on and maybe even a better understanding of.

I have an experiment that involves powering a lithium coin cell (3v) powered device.(a car key fob)

I have 5 volts already in my circuit that i am using to power the Arduino pro micro, it is supplied by a three legged voltage regulator(3amp).

I measured a brand new lithium coin cell, and it measures about 3.3v, and my personal experience leads me to believe that when you finally replace the cell, they measure under two volts. so I don't think that they are too sensitive to voltages being exactly 3v.

I don't really want to use another regulator, as I don't have one.

I am considering using a couple of resistors to make a voltage divider, as a key fob must only use a TINY amount of power(current) being that they can run off said coin cell for years.

I calculated that a 1k and a 2.2k resistor puts me at a close enough voltage to mimic a coin cell voltage, but I am wondering, what are the repercussions of doing this?

Using what I have, the only other thing that I can think of using based on what I have at hand, is a couple of diodes in series to drop it from 5vdc to 3.6vdc(ish).

What do you guys think? the Voltage divider route is way cheaper than diodes. the resistors I have are 1/4watt

I would love to hear what you guys think, as I have never tried powering anything from a voltage divider before. It is such a tiny load, and when my circuit presses a button, it will only be transmitting for a fraction of a second so I am thinking that there wouldn't be enough time for it to even come close to warming up the resistors.

The other thing that I was curious about re voltage dividers is does it matter which position the different value resistors are in? higher value closer to ground? other way around? I googled this and can't really find a clear answer.

Thanks in advance, Michael

2

michaeli:
I calculated that a 1k and a 2.2k resistor puts me at a close enough voltage to mimic a coin cell voltage, but I am wondering, what are the repercussions of doing this?

A voltage divider "output" voltage is only stable if is it feeding into a high impedance input, thus leaving the resistance ratio intact. So the repercussion is that, the second you start drawing current from your voltage divider, the voltage will change to some indeterminate value that you can't know unless you know exactly how much power you're drawing.

michaeli:
The other thing that I was curious about re voltage dividers is does it matter which position the different value resistors are in? higher value closer to ground? other way around? I googled this and can't really find a clear answer.

The order they are in completely changes the value of the output voltage. I'm surprised you couldn't find the answer through online searches. The simple equation for the output is Vo = Vi*R2/(R1+R2) so that should tell you that the order of R1 and R2 matters.

3

michaeli:
as a key fob must only use a TINY amount of power(current) being that they can run off said coin cell for years.

Keyfobs with e.g a sc2262 chip use no power in standby.
The buttons work as selector and power switch.
Leo..

4

saximus:
I'm surprised you couldn't find the answer through online searches. The simple equation for the output is Vo = Vi*R2/(R1+R2) so that should tell you that the order of R1 and R2 matters.

Now that I think back a little more on my searches, I was looking at voltage divider calculators, and not the formula itself. Thanks for sharing it, nice to know how they come up with those values.

I guess I too am a victim of fingertip information. why learn something when you can google it when you need it. Sorry, it is easy to find online when I search for the formula now.

5

Wawa:
Keyfobs with e.g a sc2262 chip use no power in standby.
The buttons work as selector and power switch.
Leo..

It sounds to me like you know a little about this topic.(key fobs)

What I have built is a sim800l connected Pro Micro to press the button which matches the sms message sent. I have this working now on perf board, but I just KNOW that the battery will go dead in the fob, and then its useless. So, I want to run the whole thing off of the truck battery in my 2013 Ram 1500.

The sim800l takes a max 4.2vdc, so I am going to put a diode in series to drop the input voltage down a bit. My research and trials have demonstrated to me that the sim800l can actually handle 5vdc without any issues, but since the data sheet says max 4.2vdc, the voltage drop across the diode should suffice.

do you think my plan of 2-3 diodes in series is a better choice than the voltage divider?

Do you think that the voltage divider will provide adequate power for long enough for the key fob to do its thing?

I did meter out the pins on the fob, and it appears that the buttons just pull to ground activate. can you add anything to this?

I am going to use a Darlington array chip to do the pulling to ground. I currently have a little micro 5vdc relay shorting the button to do the auto start function. With the 7 switches in the darlington array available to me, I am adding the other functions, lock, unlock, locate(panic) I have designed a PCB in fritzing that I just can't stop tweaking and redoing long enough to etch it

6

michaeli:
The sim800l takes a max 4.2vdc, so I am going to put a diode in series to drop the input voltage down a bit. My research and trials have demonstrated to me that the sim800l can actually handle 5vdc without any issues, but since the data sheet says max 4.2vdc, the voltage drop across the diode should suffice.

do you think my plan of 2-3 diodes in series is a better choice than the voltage divider?

Do you think that the voltage divider will provide adequate power for long enough for the key fob to do its thing?

I'm not as knowledgeable as some others here, but thought I'd pitch in. Figure out the maximum current requirement for the rest of the circuit, size R1 on a voltage divider accordingly, and calculate R2 to suit.

Using diodes or a voltage divider are going to lose some power to heat. No way around that - except if you have room for a switching DC-DC converter...

Cheers!
Dirk

7

Not enough time right now to read all the datasheets (I don't know the sim800l well enough).

The Arduino can be powered from 4volt, no problem. The keyfob (without battery) too.
It seems to me that powering everything from a single 4volt DC/DC converter is the easiest answer.
The sim800l datasheet even mentions an LM2596 ($2 from China) supply, adjusted to ~4volt, on page 22.

Another option is a 433Mhz or 315Mhz transmitter module. Half the size of a keyfob, cheaper, and runs on 3-12volt.
Leo..

8

Maybe I should explain a little better. my mistake btw, sorry

The whole idea behind the project was so that I could start my truck from my cell phone via a SMS message. this offers me basically unlimited range which is helpful as the building I work in won't pass a signal to my truck even if I can see it

Like I was saying, the whole thing works on a perf board now, but with only one function (auto start) via a 5vdc relay. these don't like to work at anything less than 5vdc, I learned this the hard way.

so the new pcb that I have designed uses a darlington array IC which has 7 switches inside is going to be used so that I can have all functions available to me via sms.

I was thinking of attaching the PCB layout, but without components overlaid, which I don't know how to do, I don't know that it will make sense

9

Excuse me asking a stupid question, why would you want to remotely start an unattended motor vehicle ?

10

its not a stupid question at all.

When it's minus temps outside, you remotely start it to warm up the engine and the interior.

In the summer, it's nice to get the cab cool with Air Conditioning.

Or are you asking why I want the LONG RANGE?

if its the second, its because I can't use a key fob from where I park to my office. The unattended portion is a factory option that I am in no way modifying, so it still maintains all safety features provided by the manufacture.

Hope this clears up your question.

i have it working now, but it is a perfboard project at the moment, and now that I have proven the concept, I want to make a proper pcb for it and install it permanently in the truck.

Thanks for the question.

Off to bed now, good night! :slight_smile: