BarGraph Ccts

Description: The LED meter is simpler and smaller than its analog counterpart and is commonly found in audio equipment. This version utilizes a National Semiconductor integrated circuit (IC) and employs a logarithmic scale. Each LED operates with a 3 dB difference from the preceding one, and a jumper is included to switch between dot or bar mode. This project is also a crucial component of an expandable analyzer that will be published soon. One meter circuit is allocated for each frequency band. There are numerous other applications for a basic LED VU meter, including its use as power meters on amplifiers, in mixers (including a high-quality mixer described in the project pages), preamps, and any application where monitoring the signal level is essential. The circuit is entirely conventional and is based on the application notes from National Semiconductor. The schematic, shown in Figure 1, features a single IC and a few discrete components. It includes two rectifier circuits, ensuring that the DC supplied to the LEDs is nearly unfiltered. This design allows for higher LED current with reduced dissipation compared to fully smoothed DC, which would necessitate a larger capacitor, thereby increasing the size and cost of the project—particularly important for the expandable analyzer, which will require at least ten meter circuits. LEDs L1 to L8 are typically green (indicating normal operating range), while L9 and L10 are red (indicating overload), providing a 6 dB overload margin when the unit is calibrated as described. Full-scale sensitivity (with VR1 at maximum) is set at 12 Volts peak (approximately 8.5 volts RMS). This design is intended for direct connection to the speaker output of an amplifier but can also be adapted for use with preamps if sensitivity adjustments are made. Power is supplied by a 15-0-15 transformer. The smallest transformer available can generally be used, as average power consumption is relatively low. The peak current is approximately 120 mA DC, so a 5 VA transformer is adequate to power two meter circuits. One 15V winding connects to terminal AC1, the other to AC2, while the center tap connects to the common (Com). A 10-ohm resistor isolates the earth connection to mitigate hum, particularly when the same transformer powers a preamp. The sensitivity calculation is somewhat complex, complicated further by the fact that the same resistors that alter the reference voltage also influence the LED current. As designed, LED current is approximately 12 mA. To simplify the calculations, a table has been prepared to aid in setting the reference voltage, which must always be slightly lower than the voltage to be measured, allowing for fine adjustments with VR1. LED current remains fixed at about 10-13 mA across all voltages. The circuit only detects the positive signal (i.e., it is half-wave only). In most instances, this is not an issue, as audio waveforms are typically asymmetrical, but the overall signal tends to balance over time. If this characteristic is undesirable, a simple rectifier circuit using a dual op-amp (a low-cost option is sufficient) is illustrated in Figure 2 and can be added between the signal source and the input. This is not a "precision" rectifier and will introduce a minor error into the signal, potentially reducing the sensitivity of low-level signals. Consequently, the lowest couple of LEDs may not be exactly 3 dB apart, but this error is negligible for monitoring purposes. If this rectifier is implemented, a fixed 100 kΩ resistor should replace VR1 (from Pin 5 to ground) in Figure 1, with the signal entering the IC via R1 as shown. The VR1 in the signal rectifier will then be used to adjust the gain rather than the meter circuit. Resistors R3 and R4 should adhere to the specified values.

The LED meter circuit operates effectively by utilizing a logarithmic response to audio signals, providing an intuitive visual representation of signal levels. The choice of a logarithmic scale allows for better differentiation of signal levels, particularly in audio applications where the dynamic range can be significant. The design uses a minimal number of components to maintain cost-effectiveness and simplicity, making it suitable for a variety of applications, from basic audio monitoring to more complex setups involving mixers and amplifiers.

The circuit's design philosophy emphasizes efficiency and practicality. By employing a half-wave rectification method, the circuit minimizes the need for large filtering capacitors, thus reducing the overall footprint and cost. This is particularly advantageous in projects requiring multiple LED meter circuits, as in the case of the expandable analyzer, where space and budget constraints are critical.

Calibration of the LED meter is straightforward, with full-scale sensitivity adjustable via the variable resistor (VR1). This flexibility allows the user to tailor the meter's response to specific applications or signal sources. The inclusion of overload indicators (L9 and L10) ensures that the user is alerted to potential signal clipping, which is vital in preventing distortion and damage to subsequent audio equipment.

In summary, the LED meter circuit is a versatile and efficient solution for monitoring audio signal levels, with various applications across the audio spectrum. Its design is rooted in practicality, making it accessible for both hobbyists and professionals in the field of audio engineering.The LED meter is simpler and smaller than it`s analogue counterpart, and is very common in audio equipment. This version is based on a National Semiconductor IC, and uses the logarithmic version. Each LED operates with a 3dB difference from the previous one, and a jumper is provided to allow dot or bar mode.

This project is also an essential part of the expandable analyser to be published soon, and one meter circuit is used for each frequency band. There are many other uses for a simple LED VU meter. They are ideal as power meters on amplifiers, can be used with mixers (including the high quality mixer described in the project pages), preamps and any other application where it is important to know the signal level. 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. There are two rectifier circuits so that the DC to the LEDs is almost unfiltered. 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.

This increases the size and cost of the project - especially important if it is to be used for the expandable analyser, since there will be at least 10 meter circuits needed. L1 to L8 will normally be green (normal operating range) and L9 and L10 should be red (indicating overload).

This gives a 6dB overload margin when the unit is calibrated as described below. As shown, full scale sensitivity (with VR1 at maximum) is 12 Volts peak (approximately 8. 5 volts RMS). This is designed for direct connection to the speaker output of an amplifier, but is still suitable for use with preamps if the sensitivity is changed. Power comes from a 15-0-15 transformer. You can generally use the smallest one available, as average power is quite low. The peak current is about 120mA DC, so a 5VA transformer will be sufficient to power two meter circuits.

One 15V winding goes to the terminal AC1, the other goes to AC2 and the centre tap is connected to Com (Common). The 10 ohm resistor isolates the earth connection to help prevent hum if the same transformer is used to power a preamp (for example).

The formula for sensitivity is somewhat complex, and is further complicated by the fact that the same resistors that change the reference voltage also affect the LED current. As shown, LED current is about 12mA. To save you the (very) tedious calculations, I have prepared a table to use to set the reference voltage.

This always needs to be slightly lower than the voltage to be measured, so that fine adjustments can be made with VR1. LED current is fixed at about 10-13mA for all voltages. The circuit only senses the positive signal (i. e. it is half-wave only). In most cases this is not a problem, because although audio waveforms are asymmetrical, the overall signal usually balances out over a period of time.

If this is not desirable, a simple rectifier circuit using a dual opamp (a cheap one is quite OK) is shown in Figure 2, and can be added between the signal source and the input. This is not a "precision" rectifier, and as such will introduce a small error into the signal, causing the sensitivity of low level signals to be reduced.

The lowest couple of LEDs will therefore not be exactly 3dB apart, but for monitoring purposes this error can be completely ignored. If this is to be used, substitute a fixed 100k resistor for VR1 (from Pin 5 to ground) in Figure 1, and bring the signal into the IC via R1 as shown.

VR1 in the signal rectifier will be used to change the gain rather than the meter circuit. R3 and R4 should use the values shown