VU Meters

The circuit is completely conventional, and is based on the application notes from National Semiconductor. The circuit is shown in Figure 1 and as you can see it uses a single IC and a few discrete components. The extra diode (D3) is included to ensure that the DC to the LEDs is almost unfiltered. C1 is included to make sure the IC does not oscillate, and is not a filter cap. This allows a higher LED current with lower dissipation than would be the case if the DC were fully smoothed, and full smoothing would also require a much larger capacitor.

The LM3915 is a monolithic integrated circuit that senses analog voltage levels and drives ten LEDs providing a logarithmic 3 dB/step analog display. LED current drive is regulated and programmable, eliminating the need for current limiting resistors.

The LM3915 is a monolithic integrated circuit that senses analog voltage levels and drives ten LEDs providing a logarithmic 3 dB/step analog display. LED current drive is regulated and programmable, eliminating the need for current limiting resistors. The IC contains an adjustable voltage reference and an accurate ten-step voltage divider. The input voltage can vary from 12V to 20V, but suggested voltage is 12V.

I like to see lights move to music. This project will indicate the volume level of the audio going to your speakers by lighting up LEDS. The LEDS can be any color so mix them up and really make it look good. The input of the circuit is connected to the speaker output of your audio amplifier. You want to build two identical units to indicate both right and left channels.

Back in the 1970's these were popular and the colored bulbs were typically place in a box with a sheet of "rippled/bubbled" Plastic in front to break up the light patterns. Some units gave very impressive light patterns. Using an LM324, this circuit has 3 active filters for low, medium and high frequencies. Trimpots are initally used to set the 3 filters to equal ranges. A Master control is than used to set for various volume levels.

This circuit uses two quad op-amps to form an eight LED audio level meter. The op-amp used in this particular circuit is the LM324. It is a popular IC and should be available from many parts stores.

This circuit uses two quad op-amps to form an eight LED audio level meter. The op-amp used in this particular circuit is the LM324. It is a popular IC and should be available from many parts stores. The 1K resistors in the circuit are essential so that the LED's turn on at different audio levels. There is no reason why you can't change these resistors, although anything above 5K may cause some of the LED's to never switch on.

The circuit has a flat frequency response from about 20Hz to well over 50Khz. Input sensitivity is 100mV for a full scale deflection on a 100uA meter. Built on two common emitter amplifiers, the first stage has a preset resistor which may be adjusted for a FSD. The last stage is biased to operate at roughly half the supply voltage for maximum ac voltage swing.

Audio levels can be monitored using a small panel meter with this circuit built from discrete components. The circuit has a flat frequency response from about 20Hz to well over 50Khz. Input sensitivity is 100mV for a full scale deflection on a 100uA meter. Built on two common emitter amplifiers, the first stage has a preset resistor which may be adjusted for a FSD.

If two of these circuits are made in the same enclosure for stereo, then there can be a single power supply to run both of them. There should be a resistor in series with the incoming 9V+ lead so as to minimize crosstalk, and reduce the PLOP when it's powered on. Other features can be added to this basic mixer. One or more microphone preamps could be added. A VU level meter or LED bargraph can be added. The microphone preamps could have a transformer to match a low impedance microphone to the preamp input. Etc...