OK, here we go!
Total noob here, and I'm trying to teach myself electronics 101. It's overwhelming, btw.
I'm trying to make an analogue voltmeter (of sorts) from a few LEDs and resistors. I'm actually building this to monitor my e-cigarette voltage. I got it from this thread, but know you all over here are pretty sharp at these sorts of things.
Long story short, every LED has a turn on voltage, and you can exploit that by setting up a voltage divider to create steps, causing the LED's to turn off at certain voltages.
I know the formula for a simple divider is Vin=((R2)/(R1+R2))*Vout
But figuring out what value resistors I need is just kicking my ace. I understand the principle, but don't know how to figure out the specifics.
My schema is attached, and here is some data on the LED's
(I measured a cut on of 2.7v @ 31uA)
Size (mm) : 3mm
Peak Wave Length (nm) : 395 ~ 405
Forward Voltage (V) : 3.2 ~ 3.8
Reverse Current (uA) : <= 30
Luminous Intensity Typ Iv (mcd) : 3000(Typical) ~ 5000(Max)
Max Power Dissipation(PM): 80mW
Max Peak Forward Current(IFP): 75mA
Max Continuous Forward Current(IFM): 20mA
Thanks, ya'll!
Total noob here, and I'm trying to teach myself electronics 101. It's overwhelming, btw.
I'm trying to make an analogue voltmeter (of sorts) from a few LEDs and resistors. I'm actually building this to monitor my e-cigarette voltage. I got it from this thread, but know you all over here are pretty sharp at these sorts of things.
Long story short, every LED has a turn on voltage, and you can exploit that by setting up a voltage divider to create steps, causing the LED's to turn off at certain voltages.
I know the formula for a simple divider is Vin=((R2)/(R1+R2))*Vout
But figuring out what value resistors I need is just kicking my ace. I understand the principle, but don't know how to figure out the specifics.
My schema is attached, and here is some data on the LED's
(I measured a cut on of 2.7v @ 31uA)
Size (mm) : 3mm
Peak Wave Length (nm) : 395 ~ 405
Forward Voltage (V) : 3.2 ~ 3.8
Reverse Current (uA) : <= 30
Luminous Intensity Typ Iv (mcd) : 3000(Typical) ~ 5000(Max)
Max Power Dissipation(PM): 80mW
Max Peak Forward Current(IFP): 75mA
Max Continuous Forward Current(IFM): 20mA
Thanks, ya'll!
Attachments
Nigel gave you excellent advice. What you're trying to do is actually tricky since the load keeps changing as LEDs turn on. Can you do it your way? Yeah. Is it a PITA? Yeah. If you insist on avoiding IC's I would recommend a 2 resistor divider for each LED to avoid the varying load. With no comparators to detect thresholds, your LEDs are ALL going to vary in brightness as the currents go up with varying voltage and there is no way to avoid that 'quirk' unless you use comparators to keep the LED drive constant. That is how the LM3914/3915/3916 chips (that you don't like) work. If you want to change the LED color you'll have to re-calculate all the resistors because different color LEDs have different operating Voltages. Or you could go to Jameco and pay the $2.75 for the LM3914, use any color LED - even mixed colors for different voltages - and move on to something more fun. Or not.
G²
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My apologies, I wasn't trying to be a dbag.
It's not that I don't like the IC's, I'm just frustrated; electronics is something I've dabbled in here or there on a very basic level. but the last several projects I've sought out to accomplish have been beyond my immediate capabilities. No one likes not being able to do something.
As far as the changing current dimming the LEDs, cool; as the voltage creeps down from use, they will dim their way down and turn off as voltage drops. That's what I want it to do; I"m not trying to measure the voltage on some critical lab equipment, just a general idea of where my battery is at (+/- 0.1v).
I though it would be fairly easy to do this one; guess not.
Thanks ya'll.
It's not that I don't like the IC's, I'm just frustrated; electronics is something I've dabbled in here or there on a very basic level. but the last several projects I've sought out to accomplish have been beyond my immediate capabilities. No one likes not being able to do something.
As far as the changing current dimming the LEDs, cool; as the voltage creeps down from use, they will dim their way down and turn off as voltage drops. That's what I want it to do; I"m not trying to measure the voltage on some critical lab equipment, just a general idea of where my battery is at (+/- 0.1v).
I though it would be fairly easy to do this one; guess not.
Thanks ya'll.
I'm surprised. I'd of thought a microcontroller would have been suggested over a bargraph chip. Anyway, your schematic is not correct for what you want to do. If all the LEDs are identical, the 4V one will be the only one to light up. To be more precise, the LED with the lowest Vf will be the one that lights, regardless of battery voltage. Try thinking of the resistors not as a voltage divider but instead as ballast. They drop excess voltage, and limit current thru the LED. It may then become apparent that your voltmeter would be easier to implement with 5 LEDs with different forward voltages. They would be lowest to highest moving right from the battery and thru a single resistor. +/-0.1V accuracy is asking a lot from a small passive circuit, not to mention the error generated by the mismatched load.
Which reminds me... you really ought to determine the load on the battery during normal use, then design a voltage tester around that load figure. Hope this helps.
Which reminds me... you really ought to determine the load on the battery during normal use, then design a voltage tester around that load figure. Hope this helps.
Thank you.
To keep it short, I'll just get too it (not to be rude)
Yes, and IC would be easier to implament; follow the wiring diagram and throw it in. But now it's just the point (for me) that I can't figure this out. I do intend on doing a 101 course at the local community college.
I meant 0.2v +/-0.1v at each LED. Maybe I'm just not reading it right, here's the idea behind it as posted on that other forum:
To keep it short, I'll just get too it (not to be rude)
Yes, and IC would be easier to implament; follow the wiring diagram and throw it in. But now it's just the point (for me) that I can't figure this out. I do intend on doing a 101 course at the local community college.
I meant 0.2v +/-0.1v at each LED. Maybe I'm just not reading it right, here's the idea behind it as posted on that other forum:
I did that, it's 2.7vuser RJG at ECF said:
I think this is where I get lost. Is he saynig just make a voltage devider for each one, or string them together? My intent on my attached schema was to string them together, maybe I just don't get it.
My e-cig draws 2.8amps (3.6v, 2ohm load), and I was trying to do a <3v to 4v scale (the 18650 "3000mah" battery runs about 4v, no load fresh charge. and yes, I know it isn't 3000mah, but $10 for 2 from china, who cares, it will still last longer than the 14500 700mah battery I'm using in my other ecig).
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Sorry, but I'd have to see a schematic, at the very least, to accept this as a working idea. Or I'm totally missing something.
"When it's dead, only the bottom one lights up."
A dead battery that lights an LED? To repeat, the first LED to turn on and the last to turn off will be the one with the lowest Vf.
I'd love to see the working prototype.
Don't feel bad. There's only one point you want to monitor - the + terminal of the battery (ground referenced). And "a voltage divider with a couple resistors that makes exactly that voltage" only does so at an equally exact voltage from the battery. Obviously, that's counter to the entire point of the circuit.
"When it's dead, only the bottom one lights up."
A dead battery that lights an LED? To repeat, the first LED to turn on and the last to turn off will be the one with the lowest Vf.
I'd love to see the working prototype.
I went back to find the pics of the unit operating, but they got deleted (it happens). But then I started re-reading the thread (for the nth time), HERE I hate it when I prove my own stupidity. I guess after having read up on some of the concepts and tried a couple circuits on the bread board myself, and then re-reading the thread, I picked up on some things that didn't make sense before.
Even though it's not super impressive, nor anything to do with diy audio, I'll post a clip when i get it done.
OH, btw, "dead" to a vaper (that is, one who uses and e-cig or "personal vaporizer"), is about 3.2v for "low resistance". after that, the vapor production is almost nothing, really "wet" and the flavor is off. Optimally you want around 7watts for "low resistance" (2ohm attomizer @ ~3.6v) to 12 watts for "high voltage" (3ohm @ ~6v), also depending on what juice you're using and personal preference.
Even though it's not super impressive, nor anything to do with diy audio, I'll post a clip when i get it done.
OH, btw, "dead" to a vaper (that is, one who uses and e-cig or "personal vaporizer"), is about 3.2v for "low resistance". after that, the vapor production is almost nothing, really "wet" and the flavor is off. Optimally you want around 7watts for "low resistance" (2ohm attomizer @ ~3.6v) to 12 watts for "high voltage" (3ohm @ ~6v), also depending on what juice you're using and personal preference.
You're not alone. I don't get the designer's "voltage divider" stuff. It just isn't coming together in my head. I think what is happening is that the resistors are simply setting the current thru the LEDs (as they're supposed to). The 4V LED has the highest series resistance, therefore the least current, so its light is visible only when the battery voltage is at maximum. The 3V LED has the lowest series resistance and most current, so its light is visible until the battery reaches the recharge voltage (That looks to be what your schematic is attempting, though the voltages are in reverse order). A schematic could show an LED and series resistor times n in parallel. Each resistor value set so that ~2mA flows at the designated voltage. 2mA may be too much, or it may not be enough. You have to determine the values for your battery & LEDs. The designer's circuit may be a stroke of genius, or just plain dumb luck. I can't say either way.
YES.
All it is is multiple voltage deviders set up to make a set voltage from varying inputs........yeah, that's it.
You(pl) have to figure out the individual voltage dviders for the different input voltages (4v, 3.8v, etc) that all make the necissary output voltage (in this case, 2.8v).And then, yes; when it's at 4v, they all light up, even though the rest are above their target voltages (the divider set up for 3.8Vin and 2.8Vo, will actually have a higher Vo, until Vin is 3.8v)
Also, they don't just cut on/off, they dim off, and if you throw a cap in there too, when you let off the button, it will dim down sequentially instead of just cutting off. It's as much cool factor as function.
Yeah, it's late, and that was wordy.
schema and vid coming, just as soon as I figure out what values to use in my dividers (I keep coming up with wierd values, like 1kΩ+429Ω, and don't want 40 resistors in there for 5 LEDs)(OH, waitress, can you get me the world in a hand basket, please?)
And I"m gonna go with genius: the OP in the thread on ECF is one of those old timers who probably built his first 8888 computer when they were still $4k. He has time to just sit around and think **** up.
All it is is multiple voltage deviders set up to make a set voltage from varying inputs........yeah, that's it.
You(pl) have to figure out the individual voltage dviders for the different input voltages (4v, 3.8v, etc) that all make the necissary output voltage (in this case, 2.8v).And then, yes; when it's at 4v, they all light up, even though the rest are above their target voltages (the divider set up for 3.8Vin and 2.8Vo, will actually have a higher Vo, until Vin is 3.8v)
Also, they don't just cut on/off, they dim off, and if you throw a cap in there too, when you let off the button, it will dim down sequentially instead of just cutting off. It's as much cool factor as function.
Yeah, it's late, and that was wordy.
schema and vid coming, just as soon as I figure out what values to use in my dividers (I keep coming up with wierd values, like 1kΩ+429Ω, and don't want 40 resistors in there for 5 LEDs)(OH, waitress, can you get me the world in a hand basket, please?)
And I"m gonna go with genius: the OP in the thread on ECF is one of those old timers who probably built his first 8888 computer when they were still $4k. He has time to just sit around and think **** up.
Another flaw with your Voltage divider idea is the continuous current draw of the LEDs will run the batteries down faster. Remember batteries are rated in mA Hours for capacity. Wasting even 1 mA can cause a noticeable change in battery life. You really should use a chip designed for this purpose as those are designed for very low power consumption and can do things like blink an LED which draws your attention and only wastes power when lit.
G²
G²
Stratus,
Thanks for the advice, however, the LEDs will only light up when I push the button to take a toke on my ecig, for 2-10 seconds.
The person from whom I got the idea, had 4 leds and it drew 9ma. when the atomizer draws 1.8amps, 9ma isn't anything.
I wanted to build this circuit because I wasn't able to understand it; but now I do. in light of the difficulty in getting the precise values necessary to do what I want, i'm considering using a chip. do you have a recommendation? low power, small form factor, and low cost (of course).
EDIT*
I just remembered, one of the requirements is also that the ic has to run off of the voltage it's measuring. I just started googling chips, came across a few and realized it could be an issue.
TIA!
Thanks for the advice, however, the LEDs will only light up when I push the button to take a toke on my ecig, for 2-10 seconds.
The person from whom I got the idea, had 4 leds and it drew 9ma. when the atomizer draws 1.8amps, 9ma isn't anything.
I wanted to build this circuit because I wasn't able to understand it; but now I do. in light of the difficulty in getting the precise values necessary to do what I want, i'm considering using a chip. do you have a recommendation? low power, small form factor, and low cost (of course).
EDIT*
I just remembered, one of the requirements is also that the ic has to run off of the voltage it's measuring. I just started googling chips, came across a few and realized it could be an issue.
TIA!
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Radio Shack probably has LM339 comparators. The problem with this voltage divider method is that it is difficult the predict the LED behavior (at least for me it is). That's why I suggested the series/parallel approach. Move the resistors in your schematic so each is in series with its own LED. Then you can calculate the current flow, using higher resistance values to display higher voltages. Don't use the LED on the far left without a resistor between it and the battery, or just leave it out altogether.
BAM!
LED Voltmeter ranging from 2.8v-3.6v+
(I couldn't figure out how to just emebed the video...)
Video
Remember, it had to be functional, but only to a point. I'm not using this for precision, more for general indication, and coolness.
For the E-cig, I also rigged up a resistive touch switch, and wired in a usb lipo charge board with micro usb. you can look around that photo album to see the other parts. Gen 1 has some issues. Gen II will be wayyy better!
LED Voltmeter ranging from 2.8v-3.6v+
(I couldn't figure out how to just emebed the video...)
Video
Remember, it had to be functional, but only to a point. I'm not using this for precision, more for general indication, and coolness.
For the E-cig, I also rigged up a resistive touch switch, and wired in a usb lipo charge board with micro usb. you can look around that photo album to see the other parts. Gen 1 has some issues. Gen II will be wayyy better!
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