Designing a Compact VU Meter
The VU Meter outlined here serves the purpose of a desktop instrument, perfectly suited for home audio setups. Specifically designed for analog audio media, including radio broadcasting, it maintains a dynamic range within 60 dB limits. One of its unique features is the provision of its independent power supply, catering to its desktop application. It operates with an AC musical signal sourced directly from low impedance loudspeaker connectors, running in parallel with the loudspeakers. With a linear frequency response and utilizing 20 LEDs arranged in a bar graph, it covers this 60 dB dynamic range in 3 dB increments. Its efficient design allows it to fit into small enclosures, such as a compact box or behind a transparent shield, similar to a desktop photo holder made of acrylic material.
Utilizing the LM3915 IC
Central to this design is the LM3915 IC from National Semiconductor, a vital component responsible for sensing voltage levels and driving 10 LEDs. It offers a logarithmic 3 dB/step analog display, effectively covering a 30 dB range. The LED current is programmable and regulated for precise control. The IC includes an adjustable voltage reference source and an accurate 10-step 22-kΩ voltage divider array. Additionally, it features a ground-referenced input buffer amplifier with a ±35V-proof capability. This buffer amplifier is crucial, as it can sense signals down to the ground level, ensuring accurate measurement. Coupled with 10 comparators referenced to the voltage divider, this setup provides a robust and reliable solution for monitoring audio signals.
Enhancing Input Protection and Extending Dynamic Range
To enhance input protection and accommodate a broader range of voltages, an additional resistor is introduced in series with the input, effectively raising input protection to ±100 V. In this setup, two LM3915N ICs, IC1 and IC2, are cascaded to achieve a combined dynamic range of 60 dB. To tailor the LED current on IC2, R5 plays a significant role, while the network comprising R5, R6, P1, and R7 establishes the reference voltage dictating the full-scale input signal level for IC2.
This reference voltage is precisely set at 5.0V in this instance. The full-scale level of IC1 is derived from this reference but shifted 30 dB lower than that of IC2. Accurate presetting of this level is accomplished with the help of P1, with R3 determining the LED current supplied by IC1. R3’s value is adjusted to compensate for IC2’s internal voltage divider, which is connected in parallel with the reference voltage source in IC2. This calibration ensures uniform LED brightness for both IC1 and IC2.
Signal Path and Calibration
The audio signal intended for measurement reaches pins 5 of both IC1 and IC2 through C1, R1, R2, and C4. Within this setup, R1 and R2 form a voltage divider, while C4 is included for RF suppression. With R1 set at 2.7 kΩ as shown in the schematic, full-scale indication is achieved at 6.4 Vrms, corresponding to 10 W across a 4 Ω load. The actual values for R1 and C4 can be adjusted from Table 1 based on your amplifier’s output power. As the VU-meter input is connected in parallel with the loudspeaker, the power (P) and voltage (U) can be expressed as:
P = U^2 / Z
Here, Z represents the loudspeaker impedance in ohms. Each successive LED in the chain indicates 50% power or 70.71% voltage concerning the previous LED. The threshold for LED #1 is merely 7.0 mV, implying that noise, as well as internal buffer and comparator offset voltages, may influence the readout at the lower end of the LED bar graph, particularly for the initial LEDs. To mitigate this, the inclusion of capacitors C4 and C5, appropriate wiring, and a well-designed PCB layout can help maintain a high degree of noise immunity. Should a stereo version of the VU meter be desired, the metering circuits described here can be duplicated for that application.
Stereo Power Supply and Voltage Regulation
The power supply has been configured to accommodate a stereo version of the device. Utilizing a mains adaptor offering an output voltage of approximately 8 Vdc proves to be both cost-effective and secure for powering the circuit. Regulator IC3 effectively reduces the LED voltage to +5.0 V, ensuring IC1 and IC2 operate within safe power dissipation limits. Incorporating a double-pole switch, S1 provides the flexibility to switch between ‘dot’ mode and ‘bar graph’ mode, enhancing the VU meter’s versatility. It’s important to note that while the artwork for the LED VU meter is displayed here, a pre-made printed circuit board is not readily available for this design. Additionally, IC3 does not require heatsinking due to its operational efficiency.
Simple Adjustment and Compatibility
The VU meter necessitates only a straightforward adjustment process. By connecting a Digital Voltmeter (DVM) to pin 6 of IC1 and tuning preset P1. The aim is to achieve a reading of 158 mV (5.0 V / 31.62). This adjustment results in a level of –30 dB relative to the voltage found on pins 7 and 8 of IC2, ensuring accurate and reliable measurements. Lastly, it’s imperative to highlight that this VU meter is not compatible with BTL (Bridge-Tied Load) type audio amplifiers often found in certain car radio receivers. Instead, it should exclusively be used with common-ground type amplifiers for optimal performance and functionality.