mike11298

Hello all!

As some might remember, I was making a VU meter for an audio amp some time ago. I've since had time to play with it and I have a great result.

I dunno about anyone else, but the first time I made this, It was pretty crap. The LEDs were unresponsive, flickering and just really bad. BUT: I've fixed it all up- with only a few components and no ICs! (except for the LM3915, of course)

Heres how you do it. ::

Right, so you have your Not-so-great VU meter circuit with everything attached, but not working all too well.

1) put a non-polarized cap on pins 4&5 - I've used a 563 (56n)
2) place a 1N4004 or similar rectifier diode forward to the signal input (pin 5)
ie:

(pin 5)------|<------(new signal input)

3) put a 1uF MKT/Ceramic cap directly after the diode. Add a 210k resistor (value not critical) accross the cap.

Now the signal input wire is connected to the other end of the cap.

And there you have it! much, much better performance!

Check out the Results on Youtube!

(btw, anyone know what the capacitor & resitor actually do in technical terms?)

__________________
-michael11298

audioguru

hi Mike,
I think your previous circuit did not work properly because the pins on the BC548 transistor were backwards.

The 1uF capacitor at pin 5 to ground charges quickly through the diode then it discharges slower through the resistor so your eyes can see the peak. They are called a peak detector circuit. The diode should be inside the feedback loop of an opamp.

My Sound Level Indicator project also uses an LM3915 and the range extends to 50dB because it has automatic gain control. Its input is a microphone and its LEDs do what you want.

Attached Images

Sound Level Indicator schematic.PNG (64.3 KB, 14 views)

__________________
Uncle $crooge

on1aag

Why would someone put a AGC circuit in a VU meter ?

on1aag.

audioguru

The range of a single LM3915 is only 30dB. I used to have a VU meter with two cascaded LM3915 ICs with a range of 60dB which was too much.

My circuit with AGC detects a pin dropping on the floor of the next room or people quietly talking, and also shows pretty loud music.

__________________
Uncle $crooge