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lf351

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fkuk

Member
hi

i need help with this project im doing

i have a lf351
lf351

and i have 20mV pk to pk and i need 12V max out for a lm3916

am i right in saying that i need a gain of 600? anyways

i need a schmeatic or wiring help so i can get the gain of 600 but at the same time, i want to be able to change the gain so if i have a input of 10mV or less
it still shows a 12V max

also i need to power the lf351 from one 12V supply so only half of the wave will exist

could someone help me and create a schmeatic or something very similar

thank you
 

MikeMl

Well-Known Member
Most Helpful Member
Not bloody likely :(

The LF351 is only capable of swinging its output within the range (Vcc-3V) to (Vee+3V), meaning that if you operate it on a single 12V supply, its output will only be valid in the range from ~ 3V to 9V. You will need an amplifier whose output swing includes ground, like an LM324 or LM358.

Why do you need 12V to the LM3916? It can be configured to progressively turn on the LEDs as the input signal goes from 0-1.25V. Also be aware that you cannot just amplify an audio (AC) signal and apply it to LM3914. You have to "rectify" the audio to turn it into a filtered slowly-varying signal proportional to the "average" or "peak" audio level before applying it to the 3916. Read the Typical Applications section of the LM3914 Data Sheet.
 
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fkuk

Member
Not bloody likely :(

The LF351 is only capable of swinging its output within the range (Vcc-3V) to (Vee+3V), meaning that if you operate it on a single 12V supply, its output will only be valid in the range from ~ 3V to 9V. You will need an amplifier whose output swing includes ground, like an LM324 or LM358.

Why do you need 12V to the LM3916? It can be configured to progressively turn on the LEDs as the input signal goes from 0-1.25V. Also be aware that you cannot just amplify an audio (AC) signal and apply it to LM3914. You have to "rectify" the audio to turn it into a filtered slowly-varying signal proportional to the "average" or "peak" audio level before applying it to the 3916. Read the Typical Applications section of the LM3914 Data Sheet.

how do you make the lm3916 turn on leds at 0-1.24V then
 

MikeMl

Well-Known Member
Most Helpful Member
how do you make the lm3916 turn on leds at 0-1.24V then
Read the Typical Applications section of the LM3914 Data Sheet.
 

audioguru

Well-Known Member
Most Helpful Member
The4 LM3914, LM3915 and LM3916 have an adjustable voltage reference with its output at pin 7. It can be set from +1.25V to almost as high as you want.

If pin 8 is connected to 0V then pin 7 is +1.25V and can connect to pin 6 so that the 10th LED lights when the input voltage is +1.25V or more.
 

Artificer

New Member
The basic formula you want is on page (2) of the insert. An explanation of how the innards work is on page (7). Page (16) and (17) go into some esoteric adjustments. This is from the 3914 docs. The 3915 and 3916 merely have the appropriate resistor stacks internally for non-linear applications.

For 20 mV input, you'll need a gain of around 150. Target is 3 volts, 3000 mV, then use a divider network or pot to tweak the input level. Best bet is an op-amp so you don't need to compensate for linearity of a transistor. And gain tweaking is a snap with them.

Best circuit would come from Op-Amp CookBook, in maybe the second chapter?.... If you don't need to offset the signal, you're talking two resistors(and a pot to tweak gain), the op-amp, and split power supply.

Don't forget to average out the signal coming in or all you'll see is a blur of LEDs. The human eye integrates anything over about 50 mS. What you need to imagine is the audio signal overlaid on an AM carrier. You need to slow down the amplitude changes to a visably usable speed.
 
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