LM3914 Circuit for the mixer
Hi,
I've decided to finally get around to finishing this two channel mixer.
I've done a post on using the OPA2134 op-amp so the next step is some VU-meters to monitor the gain levels. For this I've decided to use an LM3914 Dot/Bar Display Driver chip. I know there's the LM3916 which is more suitable for VU meters but I happened to have the LM3914 on hand and i'm only after a rough indication that the gain is at its maximum. The datasheet can be found here
For the mixer the circuit will closely follow the typical application circuit from the datasheet shown below.
There are a few things that we need to change. First of all the range in this example is 0V - 5V. Because this mixer will be USB powered and therefore 5V, we will run into problems with the signal hitting the rails of the op-amp and the LM3914 having to also operate at its rails so we need to change the range from 0V to 4V (which is pushing it). The other thing that we need to change is the LED current.
The equation for R1 ,which is the LED supply current, is ILED = 12.5/R1. To keep things simple I decided to set the LED current to 10mA so the rearranged equation would be - R1 = 12.5 / 10mA. Therefore R1 = 1.250K.
To set the upper range of the bar graph (R2) you use the formula - Ref Out V = 1.25 (1 + R2/R1) from the datasheet. This formula rearranged ends up being - R2 = (Vref/1.25-1)*R1 where Vref is the upper range. For this example the upper range is 4V so once that's put into the equation we get an R2 value of 2.75K.
After this you can breadboard your design. Below is the schematic I drew up for our circuit:
Here's the final design on the breadboard:
Using an oscilloscope and wave gen I ran a couple of little tests to see at what voltage the first and last LED's turn on.
Minimum Test:
Maximum Test:
As can be seen, the first LED turns on at 740mV peak to peak and the last at 7.36V (x10). This is more or less where I wanted it as 740mVpp is the typical line level audio voltage and we're going to use a gain of 10 in the op-amp.
So that's it. The next blog will focus on the power supply for the mixer and how to generate the negative rail for the circuit.
Thanks
Justin Ryan
I've done a post on using the OPA2134 op-amp so the next step is some VU-meters to monitor the gain levels. For this I've decided to use an LM3914 Dot/Bar Display Driver chip. I know there's the LM3916 which is more suitable for VU meters but I happened to have the LM3914 on hand and i'm only after a rough indication that the gain is at its maximum. The datasheet can be found here
For the mixer the circuit will closely follow the typical application circuit from the datasheet shown below.
The equation for R1 ,which is the LED supply current, is ILED = 12.5/R1. To keep things simple I decided to set the LED current to 10mA so the rearranged equation would be - R1 = 12.5 / 10mA. Therefore R1 = 1.250K.
To set the upper range of the bar graph (R2) you use the formula - Ref Out V = 1.25 (1 + R2/R1) from the datasheet. This formula rearranged ends up being - R2 = (Vref/1.25-1)*R1 where Vref is the upper range. For this example the upper range is 4V so once that's put into the equation we get an R2 value of 2.75K.
After this you can breadboard your design. Below is the schematic I drew up for our circuit:
Here's the final design on the breadboard:
Using an oscilloscope and wave gen I ran a couple of little tests to see at what voltage the first and last LED's turn on.
Minimum Test:
Maximum Test:
As can be seen, the first LED turns on at 740mV peak to peak and the last at 7.36V (x10). This is more or less where I wanted it as 740mVpp is the typical line level audio voltage and we're going to use a gain of 10 in the op-amp.
So that's it. The next blog will focus on the power supply for the mixer and how to generate the negative rail for the circuit.
Thanks
Justin Ryan
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