Stereo VU-meter with blue LED's
Here is
a rather simple project, showing off the usage of the well known IC LM3915.
This little handy circuit is a 10-segment bargraph driver with open-collector
outputs capable driving almost everything that can be lit up (direct or via
additional drivers). It gives a logaritmic readout with 3dB per segment.
In this application, I have made a very generic design that can easily modified
to fit any custom requirements.
To be able to follow the text, open up the schematics
(goes for it's own window).
The circuit consists of four sections:
- The VU-meters based around the two LM3915 (U1 and U2). Two resistors for each
are programming the LED current to 10mA and a full scale of 10V. Almost any LED
will run fine at that current. I'm using very bright blue 5mm LED's in my prototype,
but any other color (or combination of colors) will work fine. The LED's are run from
a separate 5V DC feed (the rest of the circuit runs at +/- 12V DC) to reduce the
power dissipation of the VU-meter IC's.
- The rectifier and peak detector based around a LF353 (U5). The rectifier also
amplifies the input signal ten times to match the full scale of 10V (for 1V
input). The two straps J6 and J7 makes it possible to remove the peak filter
capacitor to give the meter a more fast (but not that correct) behaviour.
- Input/output buffers (U3 and U4). These are needed if the circuit is put between
two other units. The buffers eliminates impedance errors when connecting two
inputs (the VU-meter and the other unit) to one output. These can be left out
if the meter runs at it's own output (the signal is fed directly to the rectifier
section.
- Power supply. There are two versions of it - one for single winding AC feed
and one for dual (or center tapped) winding. The selection depends of used transformer
type. Both versions uses a full-wave rectifier followed by filter capacitors
and regulators. When using the single winding type, the negative voltage needed
by the operation amplifiers is created by a ICL7662 (U6). The inverter aren't
needed if a dual winding transformer is used (and this is prefered in better
HIFI-setups due to risk of audio interference from the inverter). The components
shown in blue are needed if dual winding transformer is used. All blue straps
(J8 & J9) have to be closed and J5 must be left open. If the single winding
mode is used, then it is also possible to run the meter from a external DC source.
Pictures will come...
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