Electronics > Beginners

MOSFETS as voltage controlled gates

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SolderOcelot:
Very grateful for everyone's input :) I've decided to with an IC after all.


--- Quote from: Hero999 on November 18, 2018, 10:47:34 am ---That's the way to go for 99% of designs nowadays. The great thing about the LM3914 is it has current regulated outputs, so the brightness remains constant, irrespective of any changes to the supply voltage.

--- End quote ---
I actually really like this feature.


--- Quote from: Hero999 on November 18, 2018, 10:47:34 am ---An alternative would be to use an IC such as the the TL431, which has a sharp turn on threshold of 2.495V, as a voltage monitor.

--- End quote ---
Is the LM3914's turn-on treshold less sharp?

Also, does the LM3914 know when I've attached 5 LED's instead of 10 and use that as a scale?

mvs:

--- Quote from: Faloude on November 18, 2018, 11:57:16 am ---Also, does the LM3914 know when I've attached 5 LED's instead of 10 and use that as a scale?

--- End quote ---
No, it does not. LM3914 is dumb as bread. You need to define voltage window, LM3914 will make 10 equal intermediate steps.
Then you can decide which outputs to use.

If RHI=1.5V, RLO=0.5V and only 5 last leds are connected, you will get 1.5V, 1.4V, 1.3V, 1.2V and 1.1V thresholds.

SolderOcelot:

--- Quote from: mvs on November 18, 2018, 02:51:53 pm ---No, it does not. LM3914 is dumb as bread. You need to define voltage window, LM3914 will make 10 equal intermediate steps.
Then you can decide which outputs to use.

If RHI=1.5V, LHI=0.5V and only 5 last leds are connected, you will get 1.5V, 1.4V, 1.3V, 1.2V and 1.1V thresholds.

--- End quote ---
Got it! How are those intervals (0.1V) set? I know, I should read the PDF.  ^-^

Audioguru:
You might be too late. The LM3914 is not made in an old DIP package anymore, they are only surface mount now. You might still find a few old ones somewhere.
The LM3915 logarithmic display driver is not made in any package anymore.

MarkF:

--- Quote from: Faloude on November 18, 2018, 01:06:57 am ---
--- Quote from: ArthurDent on November 18, 2018, 12:34:37 am ---Here is an IC that is designed for this type of application and I'm sure there are some others.

https://www.ti.com/lit/ds/symlink/lm3914.pdf

--- End quote ---
On one hand I think it's pretty cool that IC's come onto my path now but on the other hand it's quite a buzzkill to just slap an iC on there and be done with the project. I was hoping to build this a bit more primitively.

--- End quote ---

If you would like to make it more complex, you could add a microcontroller to read the voltage and define any voltage steps you like. 
A pic18f1320 and constant current blocks made of LM358 op-amps and IRFP250 MOSFETs.
Here is my idea with minimal components:

   


Edit- 
   If you add some MCP4802 DACs controlled by the PIC, you could have dynamic brightness for the LEDs. 
   You would need to go to a different PIC (like a pic16f886).

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