I can't think today.. help !

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dacflyer
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I can't think today.. help !

Post by dacflyer » Wed Aug 07, 2019 7:11 am

ok,, here's what i got in mind, i want to make a led volt meter, using 3 leds. i'm looking for a dot mode type operation,
led 1= 5v, led 2= 9v, led 3= 12v i thought to use a LM324. but it only operates in linear mode, I could use a LM3914 in dot mode and just use 3 leds so that only 1 led is on at a time. but i'll never get it to operate like i want.
I'm even open to use just transistors. but i can't think how i can get 1 led on at a time, and when voltage is out of range for 1 led, then it turns off.
only 1 led is on within a small range.
any ideas ?

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Re: I can't think today.. help !

Post by Externet » Wed Aug 07, 2019 11:18 am

Hi.
It could be better approached as three window comparator circuits.
One between 4.9 and 5.1 V to turn on led '5V' ;
One between 8.8 and 9.2 V to turn on led '9V' ;
One between 11.7 and 12.3 V to turn on led '12v' ;
or with the tolerances of your preference.
If that fits your wishes... Perhaps a LMV7231
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Re: I can't think today.. help !

Post by dacflyer » Thu Aug 08, 2019 9:12 pm

i was thinking to use the LM324 for off the shelf use.. but no way to make it for 1 led at a time. as voltage goes up,, all 3 leds will light up.
only way i can think of a way for 1 led at a time and a small range to stay on is with a micro..

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Re: I can't think today.. help !

Post by haklesup » Fri Aug 09, 2019 12:49 pm

you can also use a uController with integrated ADC and at least 3 IO lines. Likely much more accurate than an all analog thing. Try a USB based dev kit to keep the costs low, Microchip, TI, ST Micro, adafruit, sparkfun, arduino, R-Pi take your pick. Some research to select the best fit but there are many under $15 and dev tools are generally free.

https://www.digikey.com/products/en/dev ... =dev%20kit

Better selection tools and charts are on each respective manufacturer websites or datasheets on the digikey links

if you want to do it with an op amp, you'll need to use a transconductance amplifier configuration and put different dropping resistors on each LED. The voltage will be converted to current and each LED will light at a different threshold as determined by resistor offset, you won't ever get snap on and off or good accuracy or even linearity this way without simulating a but first.

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Re: I can't think today.. help !

Post by CeaSaR » Fri Aug 09, 2019 4:27 pm

Go back to the "ok, new project time" thread ( viewtopic.php?f=45&t=17714 ) and look at the LM 324 circuit. Just use 1/2 of the shown circuit, but insert a NPN transistor and possibly a resistor as a switched path to ground, effectively grounding the input to the led.
Here's a quick sketch
Image
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Re: I can't think today.. help !

Post by Joseph » Sun Aug 11, 2019 11:50 am

I think that type of way is good

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Re: I can't think today.. help !

Post by dacflyer » Mon Aug 12, 2019 6:38 am

so is that basically just shorting out the led ?

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Re: I can't think today.. help !

Post by CeaSaR » Mon Aug 12, 2019 8:34 am

Yep, but it retains some resistance to ground because the LED resistor stays in circuit to prevent overloading the output. Since the signal to turn on is taken from the next step up, it'll turn the lower one off, and as the voltage increases, the lower outputs stay active, continuing to shunt the LED power to ground, hence they stay off untill the voltage drops down again.

The only other thing you may need is a resistor on the base to the transistor if, in use, it draws down the voltage too much to light the appropriate LED.
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Re: I can't think today.. help !

Post by dacflyer » Mon Aug 12, 2019 11:23 am

ok, thanks, i'll try that

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