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A 10-LED Bargraph with Want a really flexible bargraph? This 10-LED Bargraph will fill the bill. It can be configured for dot or bar mode, while for audio signal monitoring, extra circuitry can be added to provide for VU or for Peak Program Metering (PPM). It’s a worthy replacement for the now-discontinued LM391X series of bargraph chips. L ED bargraph displays are ubiquitous – you will find them everywhere, in all sorts of electronic equipment. They can be horizontal, vertical, curved, circular or other shapes. They give an immediate visual indication of operating conditions, whether monitoring voltage levels or physical parameters such as temperature, audio signal level or whatever and they can be designed to react rapidly or slowly. While many displays these days are digital read-outs, bargraphs are much better at showing variations in level, especially if those variations happen quickly. This 10-LED Bargraph indicates DC voltage levels in a series of 10 steps but those DC voltages can correspond to any physical measurement, as noted above. The voltage steps to light each subsequent LED can be equal, meaning that the display is linear, or the 64 Silicon Chip steps can be non-linear, for example, giving a logarithmic display. In that case, each step could amount to say a 3dB increment. It’s easy to build this bargraph with a linear, logarithmic or any other scale since the steps are determined by a set of resistors, connected in series. We provide examples of the resistors to use for linear, logarithmic or VU (audio level) scales. Alternatively, you could produce your own custom scale by using a different set of resistors. This project is presented on two PCBs. One is the LED Dot/Bar display PCB and the other is the optional Signal Processing PCB, which is used to convert an AC signal into a suitable DC voltage to drive the bargraph. All the components used on both By JOHN CLARKE Celebrating 30 Years boards are readily available. The main integrated circuits are LM358 dual op amps, two LM324 quad op amps and one LP2951 voltage regulator with most of the remaining components being resistors and capacitors. The LEDs that form the bargraph itself can be surface-mount types that sit directly on the PCB, or standard 3mm LEDs. The LEDs will light up singly in dot mode or in a column of LEDs will light up in bargraph mode. The display mode is selected by bridging pairs of solder pads on the PCB, with nine links (bridges) to solder for each mode. Once you’ve built the unit, it is configured as either a dot or bar display and this can’t easily be changed later. Why not use an LM3914/5/6? No doubt some readers are already thinking, “Why do we need all these comparators when single chip barsiliconchip.com.au really flexible display options graph ICs from National Semiconductor can already do this?” The National Semiconductor LM3914 (linear) LM3915 (logarithmic) and LM3916 (VU) bargraph ICs certainly can do these jobs and they have been very widely used for many years. However, the LM3915 has not been manufactured by National Semiconductor for 15 years and although we are aware that are still dribs and drabs around from some sources, NS advise not to base any new designs on this chip. So we won’t! Its cousin, the LM3916, was discontinued many years ago and is effectively no longer available. The only one that seems to be readily available in large quantities is the LM3914 – but the problem with this is that it can only display a linear scale. And while these three bargraph ICs present an easy single-chip solution for many dot/bargraph applications, they do have limitations when you want to customise the circuit parameters. For example, the LM3914 linear bargraph will always have an overlap in the transition from one LED to the next. That means that at least one LED is always illuminated but it does reduce the precision of the display. In the case of the logarithmic LM3915, the LED step increments are fixed at 3dB, giving a 30dB range. You cannot change the size of the steps to 2dB, or less, for example. And for both chips working in bargraph mode, all the illuminated LEDs are effectively in parallel and that can cause heat dissipation Featur es & specifications problems in the chips; • 10 LEDs – you decide they have limited power which type, colour, etc handling. • Dot or Bar modes Indeed, for many au• DC or AC input voltag dio signal bargraph apes plications, the circuit • Linear, Logarithmic, VU or PPM display we present in this arti• Ru ns from 12V (100mA maxim cle is far more useful. um) This is particularly • Full-scale signal ran ge adjustable from 583mV the case in audio mixto 55V • Uses readily-availab ers where multiple LED le components bargraphs are required, • Suits surface-mount or through-hole LEDs with a resultant high current requirement. In those cases, the LM3914/5/6 series is definitely not ideal. nected as a comparator to drive a LED. Yes, our 10-LED bargraph does use The op amp’s inverting input (-) more components than the single-chip pin 2 is connected to the input signal chip circuits but all the components while the non-inverting input (+) pin 3 are cheap and readily available and is connected to a voltage divider comyou can customise the circuit to suit prising resistors R1 and R2, connected your particular application, somein series between a reference voltage thing that is not easy to do with the (Vref) and ground. chip circuits. Assuming that these resistors are the Finally, these two boards provide same value, the junction of R1 and R2 a useful aid to demonstrate the use is one half of Vref (ie, Vref/2). So pin 3 of op amps as comparators, window of IC2a is held at Vref/2. Now if the incomparators, driving LEDs, along with put signal at pin 2 is lower than Vref/2, signal metering and overall bargraph the output of IC2a will be high. But if design. the input signal at pin 2 is greater than Vref/2, the output of IC2a will be low How it works (at close to 0V). The 10-LED Bargraph circuit comThat means that the op amp will pull prises ten op amps (operational amcurrent through the LED to light it up. plifiers) that are used as comparators. Note that we could use a comparaEach drives one of the LEDs, switching tor (such as the LM339) do this same it on when the input voltage exceeds function but if we wanted to reverse (or drops below) a set threshold. the action of the comparator, to drive To begin, let’s consider Fig.1, which a LED connected between its output shows a single op amp (IC2a) conand the 0V rail, it would not work On the left is the converter PCB which takes an audio signal and processes it into either VU or PPM . . . to be read by the main bargraph display board at right. It can show either a dot graph (ie, one LED alight at a time) or a bar graph (all LEDs alight up to and including the level at that time). siliconchip.com.au Celebrating 30 Years February 2018  65 Fig.1 (above): this shows the operation of a comparator. It compares the input signal with a reference at its non-inverting input and turns on the LED if the input is above the reference. Fig.2 (right): this combines three comparators, each with separate reference voltages at TP1, TP2 and TP3. Each comparator will turn on its respective LED if the input voltage is above its reference voltage. The different LED connections provide for dot or bar modes. since that type of comparator can only “sink” current rather than “source” it. So we use op amps throughout out circuit because their push-pull outputs make them more flexible. So now we want to drive more LEDs. For that, we add more comparators or in this case, op amps. Fig.2 shows a triple comparator setup, with each comparator driving one LED and with its non-inverting input connected to a resistor higher in the series string. The inverting inputs are connected together to monitor the same signal (Vin). Note that while we will refer to comparators in this article, in each case they will actually be op amps. In fact, consider that op amps and comparator ICs contain almost identical circuitry; the main difference, besides the output configuration, is that op amps are compensated for closedloop stability, which makes them slower to react. But for this project, we’re dealing with slowly changing signals so that isn’t a problem. (Op amps are normally configured with external negative feedback while comparators normally have positive feedback [hysteresis]). Fig.2 also shows the LED connections for the dot and bar modes. In bar mode, each LED connects between the positive supply and the op amp output via a series 2.2kΩ resistor. This means each LED will light whenever its comparator output is low. For dot mode, the anode of each LED connects to the next higher op amp 66 Silicon Chip output. So a LED will light when the higher op amp output is high and the lower op amp output is low. For example, for LED1, when Vin is higher than the voltage at TP1 but lower than the voltage at TP2, the output of IC2a will go low and current will flow from the output of IC2b, through LED1 and the 2.2kΩ resistor and then into the output of IC2a. In other words, IC2b is “sourcing” LED1’s current while IC2a is acting as the “current sink”. As stated above, this would not work with a typical (open-collector output) comparator. OK. Now when the voltage at Vin goes above the voltage at TP2 but is still lower than at TP3, IC2b’s output will go low, switching off LED1 but it will sink current through LED2 which ultimately comes from the output of IC3b. Therefore, in dot mode, only one LED will light at any given time. For bargraph mode, the LEDs are reconfigured as shown in Fig.2 (LED1’, LED2’ etc) and so they will light up whenever the associated comparator output goes low, so if LED2’ is lit, LED1’ will be lit and if LED3’ is lit then LED2’ and LED1’ will also be lit. Switching thresholds and dithering LEDs Having said that, it is possible for two LEDs to be alight (or partly alight) when the input signal is close to one of the voltage thresholds, defined by the reference resistor “ladder” (ie, at TP1, TP2, etc). This is due to the fact that the op Celebrating 30 Years amps have inherent noise which can cause them to rapidly switch on and off when the two input voltages are very close together. This can be prevented by using hysteresis and as mentioned above, this involves adding positive feedback between the output of each comparator and its non-inverting input. However, that would require the addition of three resistors to each (op amp) comparator and we have not done that with this 10-LED bargraph circuit since it would mean an additional 30 resistors. That’s a lot of hassle to solve a minor problem. Full circuit description Now let’s have a look at the full circuit of the 10-LED dot/bargraph display in Fig.3. This shows the 10 (op amp) comparators and the 10resistor ladder network providing the reference voltage for each comparator. The resistor network is connected to the output of adjustable voltage regulator REG1, an LP2951. This ensures a stable voltage to the resistor string regardless of variations in the input supply voltage. REG1’s output voltage is adjusted by trimpot VR2 to a precise 10V DC. Note that this bargraph circuit by itself is only suitable with a DC input signal; it will not respond an audio (AC) input signal. In this respect, it is the same as bargraph circuits using the LM3914/15 series chips. (We will get to the additional circuitry which allows that later.) siliconchip.com.au The DC input signal is applied to CON1 and voltage is limited by the clamping diodes D2 and D3, to a range of 0-11.4V, protecting the circuit from excessive voltages. The input 100kΩ resistor limits the current through the D2 and D3 to safe levels. If the input voltage to be monitored swings by more than 10V, it should be attenuated and that can be done by installing link JP1. That places a 10kΩ resistor in circuit which, in conjunction with the input 100kΩ resistor following CON1, attenuates the signal by a factor of 11. Op amp IC1a is configured as a noninverting amplifier with is gain varied by trimpot VR1. Its gain can be varied between unity (one) and six. Note that op amp IC1b (part of the same dual op amp) is not used in the Fig.3: this circuit is an expansion of Fig.2 to show all ten comparators and their LEDs, together with an adjustable input gain stage IC1a. Its gain is varied by trimput VR1. The adjustable regulator, REG1, provides a stable 10V reference supply for the ten comparators. siliconchip.com.au Celebrating 30 Years February 2018  67 circuit and it is disabled by having it pins 1 & 2 connected together and pin 3 connected to 0V (GND), so it won’t oscillate or otherwise misbehave. The circuit is set for dot or bar modes by installing the soldering the appropriate set of PCB copper pads at the output of each op amp comparator, ie, either all the “DOT” pad pairs are joined or all the “BAR” pad pairs are joined. The operation is then as described above, only with ten LEDs rather than three. Handling audio signals If you connected an audio signal up to CON1, half of it would be clipped by D3 and the other half would cause the bargraph to swing up and down rapidly; not really an ideal situation. A better solution is to amplify, rectify and filter the audio signal to produce a DC level corresponding its peak or average amplitude. There are many different ways of doing this, two of which are known as VU Meter or Peak Program Meter (PPM) displays. Further signal processing is required to achieve these responses. All these possibilities are covered by the Signal Processing circuit shown in Fig.4. It consists of a non-inverting amplification stage (IC5a), a precision fullwave signal rectifier (IC6a & IC6b) and a VU response filter stage (IC5b). IC5 & IC6 are LMC6482AIN dual rail-torail op amps. The audio input signal from CON1 is fed via a 100nF capacitor and applied to potentiometer VR3. Instead of being directly grounded, the “cold” side of VR3 is connected to a voltage divider comprising two 10kΩ resistors, with the junction bypassed with a 100µF capacitor. This method of connection allows the incoming signal to swing symmetrically about the half supply point (around 5.7V, ie, 11.4V÷2). Op amp IC5a amplifies the attenuated signal by a factor of 16, giving a gain range of 0-16. Gain is reduced by frequencies above 32kHz due to the 330pF capacitor across the 15kΩ negative feedback resistor. Its low-frequency response rolls off below 16Hz, as set by the 1kΩ resistor and 10µF capacitor between the inverting input (pin 2) and ground (0V). Precision rectification without diodes The output signal from IC5a is fed via a 10µF capacitor to the precision full wave rectifier comprising IC6a and IC6b. Its job is to convert the negative voltage portions of the signal into positive voltages so that we can determine the average signal level (the average of a symmetrical AC waveform is 0V). This precision rectifier is unusual in that it does not use any diodes and nor does it need a negative supply rail. It works because the op amps are rail-torail types. This means that while their inputs and outputs can swing from within a few millivolts from +11.4V (ie, the positive supply rail) to 0V (or actually to -0.3V in the case of the input), if the input signal swings negative, the op amp’s output will swing down to 0V but go no further. So if we apply a sinewave centred about 0V to the input of voltage follower IC6a, its output will precisely follow the input signal for the positive excursion of the signal but the negative excursions will result in a 0V (“clipped”) output. This means that the output signal at pin 1 will be a half-wave rectified sinewave. So that gives us a positive-going signal but only for the positive half of the AC signal. We need the whole thing. This is provided by IC6b and the way it works is very clever. When the input signal at “A” is below 0V, the output of IC6a (at “C”) is 0V as described above and thus the non-inverting input pin 5 of IC6b is at 0V; so it is grounded. It now becomes an inverting amplifier with a gain of -1, as determined by the two 20kΩ resistors at pin 6, one from the output at pin 7 and one from the input signal, at “A”. The third 10kΩ resistor is irrelevant since with an inverting amplifier, both inputs are at 0V and therefore that resistor will have 0V at both ends, so no current will flow through it. It’s effectively out of circuit when the input signal is negative. So IC6b will invert the negative-going signal at point “A” to an identical but inverted positive voltage signal at pin 7 (“E”). But when the input signal swings Fig.4: as an audio signal is AC, this circuit provides both rectification and signal filtering to give either VU and PPM characteristics. Its outputs drive the circuit of Fig.3. 68 Silicon Chip Celebrating 30 Years siliconchip.com.au positive, the output of IC6a at “C” will be identical to the input signal but with half the amplitude, because of the resistive divider at its input (pin 3). Here’s where it gets a bit tricky. Op amps use negative feedback to attempt to keep both their input pins at the same voltage. We have half the input voltage at pin 5 of IC6b, so we would expect to also see half the input voltage at pin 6. The question then is what output voltage from IC6b is required to provide this. We have the full input signal at “A”, which then flows through a 20kΩ resistor to “D”. If we assume that the output of IC6b is identical to the input signal (ie, the signal at “E” is equal to the signal at “A”) then we can consider the two 20kΩ resistors to be in parallel, meaning the current is effectively flowing through a single 10kΩ resistor. This virtual resistor forms a voltage divider with the 10kΩ resistor from “D” to ground, reducing the signal amplitude by half. This matches the signal that’s already present at “C”, hence, this is the condition which will keep both op amp input voltages equal. And that means that for positive voltages at “A”, the output at “E” must be an identical signal. Since we’ve just demonstrated that the output at “E” is identical to the input at “A” for positive voltages and an exact, inverted version for negative voltages, that means that the signal at “E” must be a rectified version of the signal at “A”. We have attempted to illustrate this rectification process with the waveforms at the various circuit points. So there is an sinewave shown at point A and resulting half-wave rectified signal with positive half cycles at points B, C & D. Note the periods for which points B & C and therefore pin 5 is held at 0V. We have shaded the negative-going portions of the signal at point “A”. These portions are effectively ignored by IC6a because it cannot respond to them. But note the complete rectified waveform at point E. See that it includes the shaded portions of the signal which have been inverted and amplified by IC6b. Filtering and processing We now need to filter that rectified signal to recover a DC voltage that’s proportional to either the peak of the incoming signal or the average, or some combination of the two with differing time constants (ie, VU or PPM). VU metering was originally provided by a mechanical meter with particular physical characteristics which determined its response to signals. It is not ideal for indicating transient signals that can cause amplifier clipping or excessive recording levels. However, the display is good for a general guide to signal levels. The electronic VU filter built around op amp IC5b simulates the ballistics of a mechanical VU meter which is relatively slow responding to changes in level. It is specified that upon a step change in the input level, it must reach 99% deflection in 300ms with a 1-1.5% maximum overshoot. This requires a second-order low pass filter with a high-frequency roll-off at 2.1Hz and with a Q of 0.62. IC5b is configured as a Sallen-Key filter with the above characteristics, to produce the VU output at pin 3 of CON3. If you’re recording audio and you’re What do “VU” and a “PPM” stand for – and what do they measure? Just about everyone would have seen (or at least seen a picture of!) a meter on an amplifier or tape recorder labelled “VU” with a scale running from -20 to +3, so it’s a reasonable assumption that it is displaying “VUs”, whatever they are! The VU – which, incidentally, stands for Volume Unit – is arguably the most misunderstood “measurement” (along with the decibel!) in the whole of electronics. Peak Program Meters, or PPMs, probably run a close second. We’ll get to those in a moment. What is a Volume Unit? Even though the VU meters found in a lot of consumer equipment are not particularly accurate (many are there more for show than anything!), the Volume Unit is actually an accurately defined quantity. It was first developed in the USA in 1939 by Bell Labs, along with broadcasters CBS and NBC, to show the “perceived loudness” of an audio signal. It became a US (and later international) standard in 1942. The standard states that a reading of 0VU equals 1.228V RMS at 1000Hz across a 600 ohm resistance. Confused? Don’t be: just remember that the VU meter is normally used to provide a quick visual guide, not give a definitive measurement. Mechanical VU meters are slow to react to changes in level – deliberately so. This is partly due to the inertia of the meter itself (or ballistics) but also due to the circuitry around it; in effect the siliconchip.com.au VU meter integrates the signal, presenting an average level rather than an instantaneous (or peak) level. The whole point of a VU meter is to show a level which the circuit as a whole can handle without overloading (causing distortion). That’s normally a level of 0 (zero) VU (on many VU meters this will also be shown as 100%). Above that (usually marked by a red zone on mechanical VU meters) you run the risk of overload – especially, for example, when you’re recording to an analog tape recorder. That’s why you adjust the level so that the reading seldom, if ever, goes much over 0VU. Incidentally, VU meters and signals with lots of sharp transients (eg, drums) do not work well together – so much so that the VU meter, especially the mechanical variety, has fallen out of favour it recent years. Which is precisely why we are presenting our highly flexible LED version! The Peak Program Meter This is a variation on the VU meter which shows, as its name suggests, the “peak” (or maximum) signal level. Again, this is designed to stop you over-driving a circuit or a recorder. The PPM is often just a single LED which flashes on maximum level. If you set a level where the LED is mostly on, you will undoubtedly get a distorted signal. Sometimes a VU meter will also incorporate a LED (as seen in the photo at left) to give this indication. Celebrating 30 Years February 2018  69 Fig.5: here’s how to assemble the LED display PCB which is shown here with a matching same-size photo. Make sure you connect the bar or dot pads (not both!) on the underside of the PCB. concerned about clipping (ie, the recording level exceeding the capability of the recorder to cleanly reproduce it), you are better off using a Peak Program Meter (PPM) indicator. A PPM meter is built using a filter which ignores very short transients but otherwise has a fast attack and slow decay, so you can better see the peak level. Its response should be 1dB down from the peak level for 10ms tone bursts and 4dB down for 3ms tone bursts. These requirements are met by a filter with an attack time constant of 1.7ms and a 650ms decay rate. Here we use a schottky diode (D4) to charge the 1.047µF capacitance (ie, 1µF and 47nF in parallel) via a 1.6kΩ resistor, which sets the attack time constant. The decay rate is set by the combination of the above capacitance and the parallel 620kΩ discharge resistor. on a PCB coded 04101181 and measuring 58 x 122mm. It fits into an optional UB3 plastic utility box measuring 130 x 68 x 44mm. Follow the overlay diagram of Fig.5 to see how each component is soldered to the PCB. Before construction, decide whether you want a dot or bar display and whether you need a linear, log or VU scale. Use Table 1 to select the values of resistors R1-R10, according to your scale requirement. Fit the resistors first. You can check the colour code for each resistor value by referring to the resistor colour code table but we recommended that you also check each resistor value with a digital multimeter before soldering. Resistors are not polarised but it is a good idea to install them so that their colour codes all run in the same direction. This makes it so much easier to check their values later on. Construction If you want a dot display (ie, only one LED lit at a time), each pair of pads The 10-LED Bargraph is constructed 70 Silicon Chip Dot or Bar mode selection Celebrating 30 Years labelled “DOT” will need to be bridged with solder. There are nine such pairs. The dot links are on the underside of the PCB, between the end of the 2.2kΩ resistor and the LED anode. Conversely, if you want a bargraph (where all LEDs will light on full scale), then bridge the Bar links located near the PCB edge (there are nine of these, too). You may need to use short bits of resistor lead offcuts to bridge the two PCB pads if you find you can’t do it with solder alone. Having done that, install the capacitors. There are two types used in the circuit. One type is MKT polyester and can be recognised by their rectangular prism shape and plastic coating. The second are electrolytic and are cylindrical in shape and have a polarity stripe along one side for the negative lead (the positive lead is also longer than the negative lead). The electrolytic capacitors must be inserted with the correct polarity as shown on the PCB overlay, with the longer lead to the + side and the negative stripe on the opposite side. Electrolytic capacitors will have their value and voltage rating printed on them while MKTs are marked with a code indicating their capacitance, shown in the capacitor codes table. Now install diodes D1, D2 and D3; D1 is a 1N4004 (1A) type while D2 and D3 are 1N4148s (signal diodes). You can then solder a single PC stake at the GND terminal position. This allows you to use an alligator clip lead to connect the negative probe of a meter to the circuit, while the positive lead with a standard needle probe can be used to contact test points TP1-TP10. IC sockets for IC1-IC4 and REG1 should then be installed with the notched end towards pin 1. Scale: R10 R9 R8 R7 R6 R5 R4 R3 R2 R1 Linear 1kΩ 1kΩ 1kΩ 1kΩ 1kΩ 1kΩ 1kΩ 1kΩ 1kΩ 1kΩ Log 6.8kΩ 4.7kΩ 3.3kΩ 2.2kΩ 1.6kΩ 1.2kΩ 820Ω 560Ω 430Ω 1kΩ VU 1.1kΩ 1kΩ 820Ω 750Ω 1.3kΩ 1kΩ 820Ω 910Ω 1.5kΩ 680Ω Table 1 – Values for resistors R1-R10. siliconchip.com.au TO SELECT DOT MODE, SHORT OUT THESE PADS WITH SOLDER (ON ALL LEDS 1-9) TO SELECT BAR MODE, SHORT OUT THESE PADS WITH SOLDER (ON ALL LEDS 1-9) Here’s the area of the main PCB where you select the dot or bar graph mode (right under the LEDs). Simply short out the appropriate pads, as indicated. If you can’t get solder to bridge across the gaps, use short lengths of resistor lead offcuts. Before soldering, check that all the pins have gone through the holes in the PCB and that none are bent under the socket. Terminal blocks CON1 and CON2 must be fitted with the wire entry holes to the nearest edge of the PCB. Trimpots VR1 and VR2 can then be installed. VR1 is a 5kΩ trimpot that may be marked as 503 instead of 5k. Similarly, VR2 may be marked as 504 instead of 500k. Don’t get them mixed up. Now for the LEDs: if using surface mount LEDs, these are soldered in place on the top of the PCB with the anode of each toward the top of the PCB. Use a multimeter set to diode test to check which is the anode and the cathode on each LED. The LED will glow when the red positive lead is on the anode (A) and the black negative lead on the cathode (k). If using leaded LEDs, then the longer lead is the anode. Install these at an equal height above the PCB, which is most easily done using a spacer between the legs to set the height during soldering. Now straighten the IC leads and insert them into their IC sockets, making sure that REG1 is not mixed up with IC1/IC2 and that each is oriented correctly, ie, pin 1 notch/dot lined up with the socket notches, as shown. Signal processing board assembly You only need to build this board if you are feeding an audio signal into the LED Bargraph. The PCB is coded 04101182 and measures 58 x 81mm. It can be stacked below the 10-LED Bargraph on 15mm standoffs if required. The overlay diagram is shown in Fig.6. As before, solder the resistors first, siliconchip.com.au Parts list –10-LED Bar/Dot Graph 1 double-sided PCB, coded 04101181, 58 x 122mm 1 UB3 plastic utility box 130 x 68 x 44mm (optional) 2 14-pin DIL IC sockets 3 8-pin DIL IC sockets 1 2-way PCB-mount screw terminal (5/5.08mm spacing) (CON1) 1 3-way PCB-mount screw terminal (5/5.08mm spacing) (CON2) 1 PC stake 1 5kΩ mini horizontal trimpot (VR1) 1 500kΩ mini horizontal trimpot (VR2) 1 10kΩ 16mm linear potentiometer (for testing purposes) Semiconductors 2 LM358 dual op amps (IC1,IC2) 2 LM324 quad op amps (IC3,IC4) 1 LP2951 adjustable regulator (REG1) 1 1N4004 1A diode (D1) 2 1N4148 small signal diodes (D2,D3) 10 3mm or SMD 1206 LEDs (LED1-LED10) Capacitors 4 10µF 16V PC electrolytic 1 100nF 63V/100V MKT polyester 1 10nF 63V/100V MKT polyester Resistors (all 0.25W, 1%) 1 270kΩ 2 100kΩ For linear scale, add: 10 1kΩ (R1-R10) For log scale, add: 1 6.8kΩ 1 4.7kΩ 1 1.2kΩ 1 820Ω For VU scale, add: 1 1.1kΩ 2 1kΩ 1 910Ω 1 1.5kΩ 1 10kΩ 10 2.2kΩ 1 1kΩ 1 3.3kΩ 1 560Ω 1 2.2kΩ 1 430Ω 1 1.6kΩ 1 1kΩ 2 820Ω 1 680Ω 1 750Ω 1 1.3kΩ Parts for Signal Processing board 1 double-sided PCB, coded 04101182, 58 x 81mm 2 14-pin DIL IC sockets 2 2-way PCB-mount screw terminals (5/5.08mm spacing) (CON3,CON4) 1 3-way PCB-mount screw terminal (5/5.08mm spacing) (CON3) 1 100kΩ mini horizontal trimpot (VR3) Semiconductors 2 LMC6482AIN CMOS dual op amps (IC5,IC6) 1 BAT46 diode (D4) Capacitors 1 100µF 16V PC electrolytic 3 10µF 16V PC electrolytic 3 1µF 63V/100V MKT polyester 1 470nF 63V/100V MKT polyester 1 100nF 63V/100V MKT polyester 1 47nF 63V/100V MKT polyester 1 33nF 63V/100V MKT polyester 1 330pF ceramic Resistors (all 0.25W, 1%) 1 620kΩ 2 100kΩ 3 10kΩ 1 1.6kΩ Celebrating 30 Years 2 62kΩ 2 1kΩ 2 20kΩ 1 15kΩ February 2018  71 Fig.6: same-size PCB overlay and matching photo of the audio signal processor board, which drives the main display PCB in either VU or PPM modes. then the sole diode (D4), then the capacitors. Note that along with the MKT and electrolytic capacitors, this board also uses a ceramic capacitor, which will normally look like a disc and is not polarised. Then fit the IC sockets for IC5 & IC6, as before, making sure the notched end goes towards the pin 1 dot as shown in Fig.6. Follow with trimpot VR3, which may be marked as 104 rather than 100k. Then install terminal blocks CON3 and CON4, again with their wire entry holes towards the closest edge of the PCB. CON3 is made by dovetailing a 3-way and 2-way screw connector together before inserting them into the board and soldering the pins. Finally, insert the two ICs into their sockets, making sure that they are both oriented correctly. for minimum gain from IC1a. Switch on power and the LEDs should all light when the test potentiometer is rotated near fully clockwise and they should all be off when it is fully anticlockwise. LEDs should sequentially light up as the potentiometer is rotated clockwise, one at a time if dot mode was selected or in a bar otherwise. You can check that the reference voltages are correct at test points TP1 to TP10. Table 2 shows the voltages expected at these test points for a 10V reference at TP10. The voltages should be within about 10% of the shown value in the table. As you wind VR2 fully anticlockwise, you will find that the top LED will light with only about half full clockwise rotation. That is because the reference voltage for the LED Bargraph is below 5V and so the output from the potentiometer only needs to be this high for the top LED to light. Similarly, if VR1 is rotated fully clockwise to amplify the potentiometer signal by about a factor of four, the amount of travel required from the potentiometer for a full-scale display will be small. It will be around one-eighth of full rotation in a clockwise direction from an initial fully anticlockwise setting. If you’re using the Signal Processing board and the LED Bargraph board has Testing and setting up Before powering up, check your construction carefully and in particular, check the orientation of the ICs and electrolytic capacitors and diodes. Is it a good idea to test the LED Bargraph PCB by itself first, even if you are going to use the Signal Processing board later. Use a 10kΩ linear potentiometer connected as shown in Fig.7 for testing. Connect the power supply between the +12V and GND inputs but do not switch it on yet. Adjust VR2 so that the voltage between TP10 and GND is 10V and rotate VR1 fully anticlockwise 72 Silicon Chip Fig.7: connections between the audio signal processor PCB (left) and the LED display PCB. Celebrating 30 Years siliconchip.com.au checked out so far, you can now wire the two together as shown in Fig.8. To calibrate it, apply a 250mV RMS audio signal to the signal input and set VR3 fully clockwise. Adjust VR2 for 10V at TP10 and adjust VR1 so the display just lights LED10. You can then apply a line level audio signal to the input to see the display vary. Note that VR3 will need to be adjusted to reduce the line level voltage to a suitable level for monitoring on the bargraph. Line level signals can vary over a wide range, from around 315mV RMS full scale up to 1.228V RMS, with some devices such as CD, DVD and Blu-ray players producing in excess of 2V RMS. To make an accurate VU meter, the 0VU level (LED7) should be set to light with a 1.228V RMS signal applied to the audio signal input. This level can be measured using a multimeter set to read AC Volts and a signal generator set to a frequency that the multimeter will measure accurately. Typically, multimeters will accurately read 50Hz signals but some may measure above 1kHz. Check your meter’s specifications before setting the signal generator frequency. In practice, the sensitivity of the VU meter (or PPM) meter should be adjusted to set the range for the audio SC signal that’s being monitored. Linear TP10 10V TP9 9V LED8, 8V TP8 LED7, 7V TP7 LED6, 6V TP6 LED5, 5V TP5 LED4, 4V TP4 LED3, 3V TP3 LED2, 2V TP2 LED1, 1V TP1 Log 0dB (10V) -3dB (7.08V) -6dB (5.01V) -9dB (3.55V) -12dB (2.51V) -15dB (1.78V) -18dB (1.26V) -21dB (0.89V) -24dB (0.63V) -27dB (0.417V) VU +3dB (10V) +2dB (8.91V) +1dB (7.94V) 0dB (7.08V) -1dB (6.31V) -3dB (5.01V) -5dB (3.98V) -7dB (3.16V) -10dB (2.24V) -20dB (0.71V) Resistor Colour Codes Qty  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  * Value 620kΩ 270kΩ 100kΩ 62kΩ 20kΩ 15kΩ 10kΩ 6.8kΩ 4.7kΩ 3.3kΩ 2.2kΩ 1.6kΩ 1.5kΩ 1.3kΩ 1.2kΩ 1.1kΩ 1kΩ 910Ω 820Ω 750Ω 680Ω 620kΩ 560Ω 430Ω * Quantity depends on configuration – see parts list. 4-Band Code (1%) blue red yellow brown red purple yellow brown brown black yellow brown blue red orange brown red black orange brown brown green orange brown brown black orange brown blue grey red brown yellow purple red brown orange orange red brown red red red brown brown blue red brown brown green red brown brown orange red brown brown red red brown brown brown red brown brown black red brown white brown brown brown grey red brown brown purple green brown brown blue grey brown brown blue red brown brown green blue brown brown yellow orange brown brown 5-Band Code (1%) blue red black orange brown red purple black orange brown brown black black orange brown blue red black red brown red black black red brown brown green black red brown brown black black red brown blue grey black brown brown yellow purple black brown brown orange orange black brown brown red red black brown brown brown blue black brown brown brown green black brown brown brown orange black brown brown brown redblack brown brown brown brown black brown brown brown black black brown brown white brown black black brown grey red black black brown purple green black black brown blue grey black black brown blue red black black brown green blue black black brown yellow orange black black brown Fig.8: test setup connections, using a 10kΩ linear pot, to ensure that all LEDs light up at the right points. Voltages for the various test points are shown at left. Table 2 – Test point voltages/ signal thresholds. siliconchip.com.au Celebrating 30 Years February 2018  73