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Audio leve meter with LED readout This LED VU meter can be used as an adjunct to the mechanical meters found in older tape decks & in low-cost audio mixers. Its main advantages include a fast response time & a peak hold facility so that you can accurately set recording levels. By BERNIE GILCHRIST When recording audio signals, it's important that input levels be accurately set in order to obtain good results. If the signal level is set too high, the recorded signal will sound distorted due to signal overload. On the other hand, if the input signal level is set too low, the recording will be noisy, will have limited dynamic range and may even suffer from signal dropouts. The traditional way of monitoring signal levels is to use a VU (volume units) meter. In practice, the input signal level is normally adjusted so that peak levels register no more than 56 SILICON CHIP about +3dB. Any more than that and you run the risk of signal overload. Mechanical VU meters have one major disadvantage though - they're not fast enough to respond to fast signal transients. The result is signal clipping and unwanted distortion in your recordings. By comparison, this electronic VU meter can respond to fast transients and also has a peak hold facility to give you sufficient time to observe these signal peaks. Unlike conventional units, this unit displays the signal level using 12 rectangular LEDs arranged in bargraph fashion. The reading is scaled in volume units, which is a logarithmic scale commonly used in audio equipment for displaying dynamic signal levels. Table 1 shows the absolute and relative levels for the LEDs when no input attenuation is used; ie, when the input signal is applied direct to Cl (see Fig.1). The peak hold facility involves the top five LEDs. If the signal peak reaches these LEDs, then the highest LED that lights will be held on long enough for it to be observed, while the other LEDs in the peak hold section turn off. By this means, any peaks that are normally too quick to be observed are captured and displayed. By making a simple circuit modification, two of these VU meters can Below: the LM317 regulator is secured to the underside of the PC board using a screw & nut. The 12 LEDs indicate the signal level. They can be mounted separately & wired back to the main board via a 12-way cable if required. Of TABLE 1 1N4002 +11-40V i CS + 33 3svw-.,. ov°+ C4 + AS 1M 1 sovw- 13 PK OSC -20dB LED1 15 D1 16 .,. -15dB LED2 D2 17 Rt 33k -tOdB LED3 INPUT R2 47k GND°+ 7 VRt 50k .,. .,. D3 18 IN1(·) R4 82k -7d8 LED4 C2 1 SOVW+ 04 19 -SdB LEDS D5 20 ·3dB LED6 IC1 LB1412 LED Input Level Relative Level 1 11mV -20dB 2 20mV -15dB 3 35mV -10dB 4 49mV -?dB 5 62mV -5dB 6 78mV -3dB 7 98mV -1dB 8 110mV 0dB 9 123mV +1dB 10 156mV +3dB 11 196mV +5dB 12 276mV +8dB D6 21 10 ~ AO I -1dB LED7 D7 OdB LEDS R6 4.7k 14 D8 .~. 010 11 D11 I I I 012 + BARGRAPH vu 1 ILED 09 Jf 22 2 3 4 5d8 LE011 5 SdB LE012 K A .,. METER Fig.1: the circuit is based on an LB1412 display driver (ICl ). The audio signal is fed in on pin 7, after which it is amplified & detected before being fed to 12 comparator stages. These stages compare the input level against preset thresholds & operate the appropriate LEDs. share a common peak hold reset li:rie, so that the peak LEDs in each channel switch off at the same time. This enhances the meter display and makes it easier to adjust recording levels in a stereo system. To make the display easy to read, the bottom seven LEDs are all green and these correspond to relative signal levels ranging from -20dB to -ldB. The eighth LED is yellow and corresponds to 0dB, while the top four LEDs are all red and indicate signal levels ranging from +ldB to +BdB. How it works Let's now take a look at the circuitsee Fig.1. It's based on a single LB1412 IC plus a 3-terminal regulator and a handful of minor parts. As can be seen, the audio signal is fed in via an input attenuator consisting of Rl, R2 & VRl. This circuit allows the input sensitivity and impedance to be adjusted (see Table 2). The attenuated signal is then AC-coupled via Cl and R3 to the inverting input of an op amp inside the LB1412 IC. ICl is a complete 12-dot red/green LED level meter on a single chip. It contains two stages of input amplification, 12 comparator stages, an inbuilt oscillator for peak hold reset, two constant current sources, and 12 output driver stages for the LEDs. In addition, there are five RS flipflop stages which function as latches and provide the peak hold logic for the five top LEDs. The gain of the first op amp stage is determined by resistors R3 and R4 and, according to the manufacturer's specifications, must not be made greater than 4. In this circuit, R3 and R4 have been selected to give a gain of 3.7. The output of this first stage is coupled via C2 to the input (pin 9) of the second stage. This stage functions as a half-wave detector with a gain of 2. Its output at pin 10 is filtered by capacitor C3, the value of which determines the attack and decay times of the display. If C3 is made too small, the display will tend to flicker rapidly with low frequency signals. Conversely, if it is made too large, the display will not respond to short peaks or troughs. The 2.2µF capacitor used in this circuit gives attack and decay times of approximately 0.5ms and 250ms respectively. Internal comparators The output of the detector is fed to the 12 comparators (one for each LED). These compare the input level with the preset threshold levels for the display and operate the appropriate LEDs. As indicated above, the peak hold function works by using the internal latches to gate the top five LEDs. This latching circuit holds on the highest LED to be lit until a reset pulse is received. It also ensures that any lower LEDs in the peak hold circuit are FEBRUARY 1993 57 Fig.2: install the parts on the PC board as shown in this wiring diagram, noting especially that IC2 is mounted on the copper side of the board. The LED display section can be separated from the remainder of the circuit by cutting along the dotted line. protection diode and about 2V across the regulator. A relatively large area of copper is provided on the PC board to serve as a heatsink for the LM317. This allows it to operate continuously from a 40V supply without overheating, even with all LEDs turned on. The LM317 has automatic thermal protection but normally only some of the LEDs are on, so even with a 40V supply the power dissipation will be comfortably below its shutdown point. Finally, link J1 (shown dotted) disables the peak hold circuit by pulling the reset line of the internal oscillator to ground. The top five LEDs then function in the same manner as the bottom seven LEDs. Construction turned off (ie, only one of the top five LEDs can be lit at any one time). The reset pulse is supplied by the internal oscillator, the frequency of which is determined by R5 (1MQ) and C4 (1µF). This resets the latches once every cycle; ie, about every 0.9s with the values shown. Because the oscillator is not synchronised to the signal peaks, the actual peak hold time depends on just when the peak occurs within the cycle and may be very short if it occurs just before the reset pulse arrives. In practice though, the varying peak hold time is not all that noticeable since the effect averages out over time. Constant current drivers The two constant current drivers inside IC1 control the current through the bottom seven LEDs. LEDs 1-4 share one constant current driver, while LEDs 5-7 share the other. Resistor R6 sets the current through these drivers to about 11mA. Although the current decreases with an increase in R6, it is not inversely proportional to R6. LEDs 8-12 are individually driven because of their peak hold function, their current being determined by series resistors R7-R11. Because of this, their brightness can vary if the supply voltage to the LB1412 varies but in this circuit, an LM317 voltage regulator provides a fixed supply voltage of 9.6V. Actually, the specified supply voltage range for the LB1412 is 10-16V but tests involving six of these ICs showed that they all worked down to 8.4V without any problems. The LM317 voltage regulator allows input voltages of up to 40V to be used so that the unit can be connected to the positive supply rail of most power amplifiers. The lower supply voltage limit for the circuit is about 11 V, which gives approximately 8.4V across the LB1412 after dropping 0.6V across the All the parts for the LED VU meter are mounted on a small PC board coded ZA-1363 - see Fig.2. You can install the parts in any order but make sure that the IC, diode and electrolytic capacitors are correctly oriented. The LM317 is mounted on the copper side of the PC board and is secured to the board using a screw and nut. If you want the peak hold facility, link J1 should be omitted. Conversely, install J1 to disable the peak hold s ircuit if this feature is not required. As shown in Fig.2, the circuit is configured for a sensitivity range of 600mV to 1.5V and a nominal input impedance of 50kQ (ie, R1 = 33kQ & R2 = 47kQ). This sensitivity range should suit most line level signals; eg, from CD players and tuners. If required, the sensitivity can be increased to suit signals up to 600mV by installing a wire link for R1 and deleting R2. The most difficult part of the construction is getting the ends of the LEDs to line up. One technique that RESISTOR COLOUR CODES 0 0 0 0 0 0 0 0 0 0 58 No. Value 1 1MQ 82kQ 47kQ 33kQ 22kQ 4.7kQ 1.8kQ 560Q 270Q 1 1 1 1 1 1 5 1 SILICON CHIP 4-Band Code (5%) brown black green gold grey red orange gold yellow violet orange gold orange orange orange gold red red orfenge gold yellow vio et red gold brown grey red gold green blue brown gold red violet brown gold 5-Band Code (1%) brown black black yellow brown grey red black red brown yellow violet black red brown orange orange black red brown red red black red brown yellow violet black brown brown brown grey black brown brown green blue black black brown red violet black black brown TABLE 2 R1 (ohms) R2 (ohms) Sensitivity 0 - 110-600mV 15-50k 33k 47k 600mV - 1.5V 50k 100k 47k 1.5V, - 3V 100k 100k 10k 3V-8V 100k 100k 3.3k 8V- 20V 100k 100k 1.2k 20V- 50V 100k can be used is to first carefully solder the two end LEDs so that they are the same distance from the PC board. The remaining LEDs can then be installed and the display pushed against a flat surface to align the ends of LEDs before soldering. It is a good idea to initially solder only one lead of each LED until you are satisfied that they are all correctly aligned. Make sure that the LEDs are correctly oriented - the anode lead is the longer of the two. Use green LEDs for LEDs 1-7, a yellow LED for LED 8, and red LEDs for LEDs 9-12. To allow for different mounting configurations , the display end of the board can be separated from the rest of the circuitry using a fine-bladed hacksaw. The display can then be connected to the main board via flying leads, or directly soldered to it at right angles. If you are soldering the two boards together, lightly solder tack the two end connections first. The angle between the two boards can then be easily adjusted if necessary before the PARTS LIST Input Impedance 1 PC board, code ZA-1363 1 10mm x 3mm-dia. screw, nut & washer 4 PC stakes 1 S0kn miniature vertical trimpot Semiconductors remaining connections are soldered. If two of these VU meters are to be used together in a stereo system, then the following modifications should be included so that the peak hold circuits reset together: (1) disable the p eak hold oscillator on one meter by removing C4 and R5 ; (2) connect pin 13 of ICl in this meter to ground (ie, to OV); and (3) connect the reset pins (11) of the two LB1412 display driver ICs together. The oscillator in one unit will now reset both units simultaneously. Adjustment Trimpot VRl is the only component on the PC board that requires adjustment. If you are using the unit as an adjunct to a mechanical VU meter, adjust VRl so that both meters read the same on a lkHz sinewave signal. If you have access to an audio signal generator, you can calibrate the circuit so that the OVU LED corresponds to a signal level of 775mV at lkHz. SC 1 LB1412 display driver (IC1) 1 LM317T regulator (IC2) 1 1N4002 silicon diode (D1) 7 green LEDs (LED1 -LED?) 1 yellow LED (LED8) 4 red LEDs (LED9-LED12) Capacitors 1 33µF 35VW PC electrolytic 1 4.7µF 25VW PC electrolytic 1 2.2µF 25VW PC electrolytic 2 1µF 50VW PC electrolytic 1 0.1 µF monolithic Resistors (0.25W, 5%) 1 1MQ 1 82kQ 1 47kn 1 33kn 1 22kQ 1 4.7kQ 1 1.8kQ 5 5600 1 270n Where to buy the kit A kit of parts for this project is available from any Dick Smith Electronics store or by mail order from PO Box 321, North Ryde 2113. Phone (02) 888 2105. The price is $24.95 plus $5 p&p. Quote Cat. K-5370 when ordering. 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