![]() ![]() LED current is fixed at about 10-13mA for all voltages. This always needs to be slightly lower than the voltage to be measured, so that fine adjustments can be made with VR1. To save you the (very) tedious calculations, I have prepared a table to use to set the reference voltage (the reference voltage sets the signal level for the 'all LEDs on' condition). 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. Using the circuit in 'dot' more will prolong battery life because The use of raw DC is only a requirement with supply voltages above around 12V to keep the dissipation of the LM3915 within ratings.īattery operation is possible, but be warned that a 9V battery won't last very long. If a low voltage supply is used, RDC and DC+ may simply be joined ![]() ![]() Note: The total supply voltage must be greater than the reference voltage, but theĬircuit will work perfectly with supply voltage down to 5V for a reference voltage of 4V or less. Performance is identical to that of Figure 2 for all intents and purposes. If you wish to use a centre-tapped transformer, use the circuit shown in Figure 3. The supply voltage must not exceed 25V DC or peak.įigure 3 - Power Supply Circuit (Two 15V AC Windings - 15-0-15) A 15-0-15V transformer can also be used, so the circuit can use the same transformer as a preamp (for example). The power supply is very simple, and can easily be hard-wired. The 10 ohm resistor isolates the earth connection to help prevent hum if the same transformer is used to power a preamp (for example).įigure 2 - Power Supply Circuit (Single 15V AC Winding) One 15V output goes to the terminal AC1, the other to AC2. The peak current is about 120mA DC, so a 5VA transformer will be sufficient to power two meter circuits. You can generally use the smallest one available, as average power is quite low. Power comes from a 15V transformer (connected to AC1-AC2). Dot mode uses far less current, but the display is not as visible. If the jumper is omitted, then only the LED corresponding to the current signal level will light. With the jumper installed, the unit operates in bar mode, meaning that LEDs will light in a continuous bar. This is designed for direct connection to high-level preamps or low power speaker output of an amplifier. As shown, full scale sensitivity (with VR1 at maximum) is 4 Volts peak (approximately 2.8 volts RMS). This gives a 9dB overload margin when the unit is calibrated as described below. L1 to L7 will normally be green (normal operating range) and L8 to L10 should be red (indicating overload). This increases the size and cost of the project - especially important if it is to be used in larger numbers as may be the case with a mixer or analyser. 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. DC to the LEDs is almost unfiltered - C1 is included to make sure the IC does not oscillate, and is not a filter cap. The circuit is shown in Figure 1 and as you can see it uses a single IC and a few discrete components. The circuit is completely conventional, and is based on the application notes from National Semiconductor. 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.įigure 1 - Photos of Two Versions of the LED VU Meter There are many other uses for a simple LED VU meter. This project is also an essential part of the expandable analyser to be published soon (or perhaps "eventually"), and one meter circuit is used for each frequency band. Each LED operates with a 3dB difference from the previous one, and a jumper is provided to allow dot or bar mode. This version is based on a National Semiconductor IC, and uses the logarithmic version. The LED meter is simpler and smaller than its analogue counterpart, and is very common in audio equipment. It is quite true that there are many variations of this circuit already on the Net, but for the sake of completeness - and because there are PCBs for this version - here is yet another. Please Note: PCBs are available for this project. ![]()
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