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Digital Volume Control POTENTIOMETER By Phil Prosser We got tired of volume control potentiometers going scratchy after just a few years and the very poor balance at low volumes. This drop-in replacement uses a digital IC, so it tracks exceptionally well and will give top performance for decades. The SMD version fits in the space occupied by most regular pots, while the through-hole version does a similar job but is a bit easier to build. W e have had to replace volume controls in our prized equipment too often due to them going ‘noisy’. In the September 2022 issue, a reader wrote in with that exact same problem; we know it affects many people. In one spectacular failure, a ground tab on the volume pot for my work stereo failed, resulting in that channel running flat-out all night, to greet co-workers at full blast the next morning! The gauntlet was thrown down recently when building a remote volume control. The motorised pot literally failed out of the box, the crimped-on tabs being loose (we know others have experienced this too). There must be a better solution! Why aren’t there pot-sized volume digital controls that use some of the excellent electronic volume control ICs with a digital rotary encoder? Well, now there are! The original concept for this project was a straight-out replacement for a volume pot. Our illustrious Editor asked the innocent question: “If you have a PIC in there to control the volume IC, why not have IR remote control as well?”. As it turned out, that was not too difficult to provide. The resulting SMD design is a very modest size at just 25mm wide by 36mm deep. It’s just a little larger than a typical dual-gang log pot, as shown in the photos. While this small size is clearly a boon in many situations, we knew that some readers would baulk at building it. While the board is quite packed, none of the parts are that small. Still, it wasn’t too much work to come up siliconchip.com.au Features ☑ Based on the PGA2311UA stereo digital volume control IC ☑ Two independent channels (expandable up to four, six or more) ☑ Automatically remembers the last volume setting ☑ Volume adjusted by a rotary ☑ ☑ ☑ ☑ ☑ ☑ control on the front panel or universal IR remote control Mute function (remote control only) Soft start at power-up ‘Clickless’ design Controlled by a PIC16F15214 microcontroller and TSOP4136 IR receiver Operates from a preamp power supply from ±8V to ±30V Optional LED indicator showing IR and volume change activity The SMD version of the Digital Potentiometer is a little larger than a dollar coin and just wide enough for the rotary encoder and IR receiver to fit. And for those who don’t want to squint while building it, there’s the larger through-hole version shown below. Specifications ☑ Gain and attenuation range: +31.5dB to -95.5dB in 0.5dB steps ☑ Channel gain match typically within ±0.05dB ☑ 0.0002% distortion ☑ ☑ ☑ ☑ ☑ at 1kHz (using the -UA version of the IC) – see Fig.1 Frequency response: essentially flat from 20Hz to 20kHz Able to drive 600W W loads Input resistance: 10kW W Signal handling: 2.5V RMS maximum input level Output level: up to 2.5V RMS (7.5V peak-to-peak) Australia's electronics magazine This prototype used a TSOP2136, instead of the recommended TSOP4136, which is why the IR receiver is shown mounted on the outer set of pads. Refer to the text on page 36. March 2023  31 Fig.1: THD and THD+N vs frequency plots for the Digital Pot for both channels – nothing to see here, folks! Move along! You’ll get similar or better performance from this design compared to a regular, passive potentiometer. even in these times of IC shortages. The PGA2311 contains a resistor network and analog switch along with switched resistors in the feedback network of the output buffer amplifier, as shown in Fig.2. This allows the device not only to attenuate but also to provide up to 31.5dB gain. Pay attention to this; turning it up too high when you don’t have an input signal leads to a loud surprise! This IC is very quiet, so do not expect to hear hiss or noise to warn you that the volume level is high. While these ICs can provide up to 31.5dB gain, we limited the Electronic Volume Control gain to +10dB. An alternative firmware allows you to run up to +31.5dB, but be warned that this is an awful lot of gain. Circuit details Fig.2: this shows what is inside the PGA2311 (and PGA2310, PGA2320) ICs. The switched resistive attenuator and switched feedback in the output amplifier allow for a wide range of gain and attenuation settings. with a through-hole equivalent design, so that is what I did. I checked its performance and found it to be close enough to the SMD version that nobody would notice an audible difference. So if you have room to fit a larger board, it is certainly an option. Its circuit is identical; it just uses physically larger components on a different PCB that measures 79mm wide and 57mm deep. Performance & IC choice The specifications panel and Fig.1 show the performance of the prototypes. There’s so little noise and distortion that it certainly won’t be audible and will not affect the audio quality of even the best amplifiers. We measured the distortion of five prototypes, and all were in the 0.00020.0003% distortion region, which is 32 Silicon Chip close to the measurement limit of our test equipment. The heart of the project is the PGA2311 Volume Control IC from Texas Instruments. The PGA2320 or PGA2310 can also be used with identical performance, but those versions are much more expensive for reasons we cannot explain, other than they can operate from higher ±15V supply rails compared to ±5V. You need to use one of the PGA2311 chips with a UA suffix to get the specified performance. The obsolete CS3310 will also work just fine, and they are still reasonably easy to find on the grey market (eBay, AliExpress etc). Still, all of those options will give acceptable performance. In short, we are confident that you will find one volume IC or another to fit on your board, Australia's electronics magazine The Digital Pot circuit is shown in Fig.3; there is not much to it. There are a couple of things on the board beyond IC2, the PGA2311 (or equivalent), PIC microcontroller IC1, rotary encoder RE1, IR receiver IRR1 and some power supply components. The audio performance of this project is almost entirely determined by the PGA2311, as there is nothing else in the signal path. The left channel signal is fed in via pin 1 of CON2, goes straight into IC2’s input pin 16, out of its output pin 14 to pin 2 of CON2, for feeding to the amplifier (or whatever is downstream). The other channel is routed similarly, via CON1. We have included input protection with a BAT54S dual schottky diode (or a pair of BAT85s on the through-hole version) from each input pin to the supply rails. This way, if the input is over-driven, the diodes will conduct and help to protect the PGA2311 from damage. We decided not to include DC-­ blocking capacitors on either the input or output. The reason is that four bipolar capacitors would have added probably 20% to the PCB size. We expect these will be in your signal chain already (after all, if you’re replacing a mechanical pot, you won’t be applying DC to it) and the output offset voltage of the PGA2311 is only 0.25mV at 0dB gain. If you have DC in your signal chain, you will need to include a blocking capacitor in series with the Digital Pot – we recommend a 10μF 25V bipolar electrolytic capacitor. You can also siliconchip.com.au use two regular 10μF 25V electrolytics connected in series, negative-to-­ negative or positive-to-positive. Both options will have no noticeable effect on the audio. With the outputs, we are assuming that the Digital Pot will drive short cables to your amplifier or follow on circuitry. The PGA2311 can drive 600W loads and has a short-­circuit current of 50mA, so it is unlikely to misbehave if presented with an unusual load. Still, if you intend to drive long cables with this, add a 100W resistor in series with each output. A convenient place for this would be at your output socket. Controller Power supply If the PGA2311 is the heart of this design, the 8-pin PIC16F15214 is the brain. We discussed the capabilities of this chip in April 2022 (siliconchip. au/Article/15277). The main job of the software running on this PIC is to monitor the rotary encoder and, if it is turned, send a signal to IC2 to adjust the volume appropriately. It also looks for signals from the infrared receiver and, if it receives a valid signal from a remote control, also figures out what command to send to IC2 in response. We’ll have more details on how the software works later. IC2 operates from ±5V supply rails. To allow a wide variety of amplifier/ preamplifier supply rails to be used to run this board, we have onboard 78(L)05 or 79(L)05 regulators. This means you can power the Digital Pot from split supply rails from ±8V to ±30V, which should suit most applications. A typical preamp will have such rails available, and some smaller amplifiers without preamps might too. In keeping with the design concept, the power supply is very simple. The PGA2311 has a typical power supply rejection ratio (PSRR) of 100dB at 250Hz, so any noise that the basic Fig.3: the complete circuit for the Digital Pot; this applies to both the SMD and through-hole versions. Just note that D1a/D1b and D2a/D2b are two dual diodes in the SMD version or four individual diodes in the other. The only extra part not shown here is the optional LED indicator that plugs into CON4. siliconchip.com.au Australia's electronics magazine March 2023  33 linear regulators let through will not realistically affect performance. Of course, you could ‘roll your own’ lownoise ±5V DC supply and delete the regulators as an upgrade. Suppose you want to fit the Digital Pot into a power amplifier with only split supply rails above ±30V. In that case, you could connect 5W zener diodes in series with the two supply rails to drop them into the Digital Pot’s acceptable range. It only draws a few tens of milliamps, so that should work for just about any amplifier. Just ensure the zener polarities are correct (anode to pin 1 of CON3; other cathode to pin 3). If you use the PGA2310 or PGA2320 devices, you also have the option of increasing the analog supply rails as high as ±15V. This will make no difference in the vast majority of applications, but the choice is there. The simplest way of doing this is to drop in 78(L)15 and 79(L)15 regulators for REG2 and REG3, respectively. Don’t change REG1 to a higher voltage type. Firmware The source code and HEX file for this project are available for download from the Silicon Chip website. We can also supply microcontrollers already loaded with the appropriate HEX file. On boot-up, the software configures several registers to set the processor clock speed to 4MHz, much lower than the maximum, and starts a timer for measuring IR signals. It then loads the saved volume level and remote control configuration from flash memory, checks to see if the user wants to change the remote code and, if not, ramps the volume from zero to the last used value over a couple of seconds. It then monitors the rotary encoder and IR input ports for action, and if anything happens, decides if the rotary encoder is being turned up or down or reads the IR stream to see what code was transmitted. The software writes a new volume level value to the PGA2311 IC if required. Then, if there are no changes for about 10 seconds, it saves the new volume level to flash memory. Modulated infrared signals are received by the TSOP4136, which includes an IR detector, 36kHz bandpass filter and output driver. The result is a digital serial stream including intentional signals from your remote control and also ambient light noise. 34 Silicon Chip IR Signal Decoding With Manchester encoding, a logic one is transmitted as a high-to-low transition, while a “0” is a low-to-high transition – see Fig.4. As transmission starts with a one bit, we know that there is a high level, then a low level, at the start of every transmission. Fig.4: the Manchester Encoding scheme used by the RC-5 remote control scheme. This encoding results in no DC component, a well-defined frequency range, and the ability of a receiver to work out the clock rate from the serial data stream. The decoder described here works well and is a good example of a simple state machine. Let’s start by listing what we know: ● A one is encoded as a period of no IR signal for 890μs (nominally), followed by an IR signal for the same time; zero is the reverse. We need to allow for some variation in the transmitter’s clock and thus periods (say ±10%). ● The IR level will never remain the same for much less than the nominally 890μs period, or much more than 1780μs if a zero follows a one or a one follows a zero. ● We are looking for 14 bits of data. The state machine states, shown in Fig.5, are as follows. A Clear any stored value and wait for an IR signal to be present. Set the first bit to one (we know this is true if it is a valid signal) and go to state B. B We are receiving a one. Measure the time until the IR signal stops. If this was too short (say, less than 890μs minus 10%), this is noise; go to state A. if the time was short (closer to 890μs than 1780μs), we have just received another one. Store this and go to state C. if the time was long (closer to 1780μs than 890μs), then we are receiving a zero. Store this and go to state D. if the time was far too long (more than 1780μs plus 10%), this is noise, so go to state A. C We just received an IR pulse starting with a one after having already received a one (there is no IR signal just now). Measure the time until we see an IR signal again. if we see it too soon, this is noise; go to state A. if the time was short, that is to be expected; store the bit and go to state B. if the time was longer than that, this is noise; go to state A. D We just received a zero; there is no IR signal now. Wait until the IR signal starts again. if we see no IR for too short a time, this is noise; go to state A. if we see no IR for a short time, we have just received another zero. Store this and go to state E. if we see no IR for a long time, we are receiving a one. Store this and go to state C. if there was no IR for longer than that, this is noise; go to state A. E We just received an IR pulse for a zero after a zero (there is an IR signal present now). Measure the time until we see no IR signal again. if this is less than a short pulse, this is noise; go to state A. if we see no IR for a short time, that is to be expected; go to state D. if we see no IR for longer than that, this is noise; go to state A. If the software receives all 14 valid bits using the above method, it is considered a valid command and processed, then it returns to state A, ready to receive another command. Otherwise, it throws the data away as it is considered noise. Fig.5 overleaf shows this as a “state diagram”. ↪ ↪ ↪ ↪ ↪ ↪ ↪ ↪ ↪ ↪ ↪ ↪ ↪ Australia's electronics magazine siliconchip.com.au Fig.5: the IR decoder state machine built into the software. This demonstrates how complex logic can be decomposed into a (relatively) simple flow chart and then implemented in logic or software. Writing software can become difficult without breaking the logic down like this. Noise will include ‘signals’ from lights, the sun and other IR remotes in the room. The IR receiver’s internal bandpass filter is not 100% effective at blocking this noise, but it helps a lot by reducing it to a manageable level. While a little old now, Philips RC-5 IR codes are prevalent, and virtually all universal remote controls can generate them. RC-5 IR transmissions each contain 14 bits of data. That’s broken down into five address bits (32 possible values for TV, VCR, DVD, receiver etc) and six command bits (64 possible values). The stream commences with two start bits and a ‘toggle’ bit that inverts with each subsequent command. The data is ‘Manchester encoded’, a clever way of sending a string of ones and zeros on a serial line while embedding a clock signal into it. Our PIC reverses this scheme to decode the serial stream of data from the TSOP4136; more detail on this is provided in the “IR Signal Decoding” panel. The PIC microcontroller untangles all this to extract commands from our remote and change the volume or toggle the mute status. PCB design method We thought it might be interesting to show what we do when designing such a tightly packed board and how we are sure it will all fit. Fig.6 is a 3D rendering of the PCB from Altium during the design phase. Compare this to the actual prototype; it’s pretty close. This depends on us entering the right models for every component, but you only need to do this once. After Note: the headers are swapped in the final version compared to the photos to make it easier to use shielded cable for the audio. Enlarged views of the SMD version of the Digital Volume Control Potentiometer. Note the different IR receiver, as we tested a few common types. siliconchip.com.au Fig.6: this is the 3D rendering we produced using Altium to verify that everything was going to fit. The final result looks remarkably similar. that, we can run interference checks and even get a rendering of what it will look like once assembled. We can spin it around to ensure there are no component collisions (and Altium can warn us if there are). Construction First, you need to choose which board you want to build. The SMD version is suitable for relative beginners as it has a handful of surface mount parts but no really fine pitch components. That said, if you have room for the full-size board, it might save you some squinting to build that. This especially applies to those of us with a few extra ‘miles on the clock’. SMD version The SMD version is built on a double-­ sided PCB coded 01101231 that measures 25.5 × 36.5mm, with the components placed as shown in Fig.7. Components are mounted on both sides to keep the final result compact. You might want to use a small vice or some Blu-Tack to stop the PCB from slipping around on the bench while you work on it. Start by soldering the 10kW resistor on the back of the board. Next, fit the 100nF capacitors. Four are on the board’s back, and two are on the front. Next, fit the PGA2311 IC (or similar) and the PIC16F15124 microcontroller. In both cases, ensure you have identified pin 1 and orientated it as shown in Fig.7 and the PCB silkscreen before tacking one pin. Then check the alignment of the other pins before soldering March 2023  35 them. If they are off, remelt the first solder joint and gently nudge the IC into position. Adding a bit of flux paste along the rows of pins before applying solder is recommended, as it makes the solder flow much better, to form good joints. With flux paste on the pins, you just need to load a little solder on your iron and then touch it to the junction of the pin and pad, and it should flow onto them and form a good joint. With practice, you can even drag the iron down the pins to solder them in rapid succession. After soldering, check carefully to ensure all the joints are good and no pins are bridged to adjacent pins with solder. If they are, add a bit more flux paste and then apply some solder wick to suck up the excess solder and clear the bridge. Now flip the board and solder the two dual BAT54S schottky diodes. These are SOT-23 package devices and the smallest parts you will need to deal with, but luckily, the pins are relatively widely spaced. Once you have these on the board, it is all downhill from there. Next, fit all five 10µF 35V SMD electrolytic capacitors (or, even better, 10µF 35V/50V SMD ceramics). If using electrolytics, orientate them as shown; the base has a chamfer at the positive end. You can use a small amount of solder to wet one pad and tack the capacitor lead to hold it in place before properly soldering both pins. Leave that fine tip on your soldering iron, as while the remaining parts are through-hole types, many of these parts use smaller pads to fit the tracks onto the PCB. Solder in the three voltage regulators next. Be careful to get Fig.8: the through-hole PCB is electrically identical to the SMD version but somewhat larger. For both IC1 and IC2, you can fit a part in a DIP (through-hole) or SOIC SMD package. Be careful which way around you install the regulators and ICs, and note the extra pad next to CON4 so that multiple units can be ‘ganged up’ for four or more channels. the 78(L)05 and 79(L)05 devices in the right spots. Next, mount the power and output connectors. We have chosen different types for these to make it less likely that the power will be inadvertently plugged into the audio connector. It is possible to solder wires directly to the PCB, but connectors provide a more professional finish and make for easier assembly and maintenance. A two-pin header is used as a jumper to isolate the IR receiver in case IC1 needs to be reprogrammed. If you need to program your PIC, install this header but not fit the jumper until after the PIC is programmed. If you are using a pre-programmed PIC, you can insert the jumper before or immediately after soldering it. The final part to install is the IR receiver. There are many similar types on the market, but they have annoying pinout differences. Some have the + and – power supply pins swapped! Check the ones you buy carefully; the specified TSOP4136 devices have GND on the middle pin and fit the inner set of holes on the PCB. TSOP2136 devices have GND on an outer pin, matching the pads nearest the PCB edge. Through-hole version The through-hole version is built on a double-sided PCB that’s coded 01101232 and measures 78.5 × 57mm. The parts layout on this board is shown in Fig.8. This board allows the use of all through-hole parts or, alternatively, you can use the surface-mounting versions of the PIC microcontroller and/ or PGA2311 IC. This makes sourcing parts easier. All the remaining through-hole parts are very common, so we do not envisage any difficulties in sourcing them. The assembly order is essentially the same as for the SMD version, listed above, with a few minor differences besides the different component packages. The main one is that the two dual SMD diodes are replaced with four individual leaded diodes. Also note that the through-hole electrolytics have their positive sides indicated using longer leads, which go towards Fig.7: the SMD version is very compact but is identical in performance and function to the larger through-hole version. If using 10μF ceramic capacitors instead of electrolytic (which we would recommend), they will fit on the same pads but are not polarised. You can use the same type of connector for CON3 as CON1 & CON2 but then there is a risk of accidentally plugging the power cable into the wrong header and doing damage. Choosing an infrared remote control We tested several remotes during development, including the Altronics A1012A. We programmed this for TV codes 0088, 0154, 0169 and others and AUX codes 0734, 0846, 0727 and others. We also tested a “One For All” remote and found it worked with TV code 0556 and RCVR/AMP code 1269. The easiest way to set this up for your remote is to plug the IR activity LED into the program port and watch for the LED lighting when you press buttons on the remote. Flashing indicates that valid IR codes are being received. It’s then just a matter of trying different codes (starting with Philips TVs) until you find one that works. You only need the volume up/down and mute button codes to be correct. 36 Silicon Chip Australia's electronics magazine siliconchip.com.au the + symbols on the board. For the regulators, you might as well use the same 78L05 and 79L05 devices as used on the SMD version; orientate them as per the smaller semi-­cylindrical footprints shown in Fig.8. However, you can also use the 7805/7905 or equivalent TO-220 regulators if you happen to have them on hand; the required orientation of those devices is also shown in Fig.8. Otherwise, follow the same order of assembly as the SMD version, referring to the section above. After that, you can install the four “feet” comprising tapped spacers held into the corner mounting holes with machine screws. These are not only handy during testing; you can use them to mount the more hefty through-hole board to the chassis later. Activity LED An activity LED is a useful thing to have; one that flashes at power-up, when valid infrared commands are received and when the encoder is rotated. To provide for this, the firmware stretches the length of the CS ‘chip select’ signal to the PGA2311 IC. By connecting our LED and resistor between this line and the 5V rail, it will light up whenever commands are sent to that IC. This is a bit cheeky, as we are using the chip select line for two purposes: while the CS line is low to enable the PGA2311’s digital interface, it also drives current through the activity LED to light it. To make the flash visible, we need to extend these pulses from what is required (just a few microseconds) to tens of milliseconds. The wiring for the optional activity LED is shown in Fig.9. It is done by soldering the light-duty figure-up cable to two pins on a female header with three to five pins. You can cut this from a longer header strip. It then plugs onto CON4 and allows you to mount the LED in a visible location, eg, on the front panel of your amplifier using a bezel. Try to keep this lead to a modest length (~10cm), as it helps to prevent noise getting on the CS line. Programming IC1 If you got IC1 from Silicon Chip, it should be pre-programmed and ready to go. If using a blank microcontroller, you will have to program it in-circuit for the SMD version (unless you have an SOIC programming socket). With siliconchip.com.au Parts List – Digital Volume Control ‘Potentiometer’ 1 universal remote control [Altronics A1012A] 1 rotary encoder (RE1) [Altronics S3350 or EN11-VNM1BF15 (Mouser)] 2 3-pin vertical polarised headers, 2.54mm pitch (CON1, CON2) [Altronics P5493] 2 3-way polarised header plugs with pins (for audio signals via CON1, CON2) [Altronics P5473 + 3 x P5470A] 1 3-pin JST style header, 2.54mm pitch (CON3) [Altronics P5743] 1 3-pin JST style plug, 2.54mm pitch (for power via CON3) [2 x Altronics P5743 + 6 x Altronics P5750] 1 2-pin vertical header, 2.54mm pitch, plus jumper shunt (JP1) 1 TSOP4136 or similar IR receiver, SIL-3 (IRR1) [Altronics Z1611A, Jaycar ZD1953, Mouser 782-TSOP4136] Additional components for the SMD version 1 double-sided PCB coded 01101231, 25.5 × 36.5mm 1 6-pin SMD vertical header, 2.54mm pitch (CON4) (optional; for ICSP, activity LED and/or multi-channel use) [Altronics P5435]  1 PIC16F15214-I/SN 8-bit microcontroller programmed with 0110123A.HEX, SOIC-8 (IC1) 1 PGA2311, PGA2310, PGA2320 or CS3310 digital volume control IC, wide SOIC-16 (IC2) 2 78L05 +5V 100mA linear regulators, TO-92 (REG1, REG2) 1 79L05 -5V 100mA linear regulator, TO-92 (REG3) 2 BAT54S 25V 200mA dual series SMD schottky diodes, SOT-23 (D1, D2) [Altronics Y0075] 5 10μF 35V SMD electrolytic capacitors, 5.3×5.3mm [Altronics R9442] OR 5 10μF 35V/50V SMD ceramic capacitors, X5R or X7R, M3216/1206 size 6 100nF 50V X7R SMD ceramic capacitors, M3216 size [Altronics R9935] 1 10kW SMD resistor, M2012/0805 size [Altronics R1148] Additional components for the through-hole version 1 double-sided PCB coded 01101232, 78.5 × 57mm 1 6-pin vertical header, 2.54mm pitch (CON4) (optional; for ICSP, activity LED and/or multi-channel use) 1 8-pin DIL IC socket (optional; for IC1 if DIP version used) 1 PIC16F15214 8-bit microcontroller programmed with 0110123A.HEX, DIP-8 or SOIC-8 (IC1) 1 PGA2311, PGA2310, PGA2320 or CS3310 digital volume control IC, DIP-16 or wide SOIC-16 (IC2) 2 78L05 or 7805 +5V 100mA/1A linear regulators, TO-92 or TO-220 (REG1, REG2) 1 79L05 or 7905 -5V 100mA/1A linear regulator, TO-92 or TO-220 (REG3) 4 BAT85 30V 200mA schottky diodes (D1a/b, D2a/b) [Altronics Z0044] 5 10μF 50V low-ESR radial electrolytic capacitors, 5mm diameter [Altronics R6067] 6 100nF 50V X7R multi-layer ceramic capacitors, 5mm pitch [Altronics R2931] 1 10kW ¼W resistor 4 M3-tapped spacers (for mounting PCB) 8 M3 × 6mm panhead machine screws (for mounting PCB) 4 M3 shakeproof washers (for mounting PCB) Optional parts for activity LED (suits either version) 1 LED with bezel and series current-limiting resistor 1 length of light-duty figure-8 wire, to suit installation 1 3-pin, 4-pin or 5-pin female header, 2.54mm pitch  Most SMD headers, including Altronics Cat P5435, have the pins staggered on either side of the header. The PCB requires the pins to all be on one side. This can generally be achieved by snapping or cutting off a 5-pin or 6-pin length of the header and rotating the even-numbered pins by 180°. Australia's electronics magazine March 2023  37 Fig.9: this circuit shows how to add an IR activity LED. We piggyback off the CS line for the PG2311 IC, which itself re-purposes the incircuit serial programming data line. It can be a 3-, 4- or 5-pin header as long as it’s plugged into CON4 so the correct connections are made; you can adjust the resistor value to suit the LED used. the through-hole version, you can program it in-circuit or off-board before fitting it (or even afterwards if you’re using an IC socket). For programming it in-circuit, remove JP1 and plug a programmer like a PICkit 4 or Snap programmer into CON4 with its pin 1 in the correct position. With the PICkit 4, you can get the programmer to deliver power during programming. For the Snap programmer, it’s probably easiest to apply 12V DC between the +VE and GND pins of power header CON3 during programming. Using MPLAB IPE, select the correct device (PIC16F15214), load the HEX file (available for download from the Silicon Chip website), enable power from the programmer if necessary, then connect to the chip and press the program button. It should only take a couple of seconds to load the firmware, and you will see a success For multi-channel use, a ‘slave’ version of either the through-hole or SMD version can be built using less components. See the panel overleaf for more details. message (or an error message if something goes wrong). Remember to re-fit the shorting block to JP1 after disconnecting the programmer. Changing remote control code The software can decode RC5 signals with any valid TV or “Receiver” address. The software defaults to the TV on first power-up, and if you do not need to change this, there is nothing to do. If you have another Philips TV remote in the room and need to use an alternative code, here is how to set the Digital Pot to use the Philips Receiver codes: To set the remote using a TV code: 1 Remove power from the Digital Pot. 2 Short pins 3 & 5 of CON4. 3 Apply power to the Digital Pot. 4 Wait a couple of seconds Are AliExpress PGA2311 ICs any good? We bought some PGA2311 chips from AliExpress (www.aliexpress.com/ item/1005003043805799.html). We built and measured the performance of a Digital Pot using one of these, and it worked just fine – see Fig.10. At $20.73 for five ICs, this is a rather attractive option! 5 Remove power from the Digital Pot. 6 Remove the short between pins 3 & 5 of CON4. To set it to accept a Philips Receiver remote control code, go through the steps above but instead, put the jumper between pins 3 & 4 of CON4. This procedure is the same for the through-hole or SMD versions, but if you haven’t fitted CON4, you will need to do so. Note that pin 1 of this header on the SMD board is nearest to the PIC microcontroller. Troubleshooting If it isn’t working as expected, check the following: 1 Is there 5V DC ±0.25V on the +5VD (IC2 pins 1, 4 and 8) and +5VA (IC2 pin 12) rails? If not, check for any parts getting hot. Verify that you are providing a minimum of 7V DC to the PCB positive input. Do you have the right parts for REG1 & REG2, in the correct orientations? 2 Are IC1 and IC2 soldered properly? Take a close-up photo if your phone has this function; it is surprising how zoomed-in you can get with some phones. 3 Is there activity on the encoder lines (pins 2 & 3 of IC1)? If you have a ‘scope, probe pins 2 and 3 of IC1 and see if they are at more than 3V, pulsing low as you rotate the encoder. If not, check that you have used a suitable encoder – there are a bewildering variety of rotary encoders; the recommended Altronics and Mouser parts have been tested to work. ↪ ↪ ↪ ↪ Fig.10: despite costing just over $4 each, the board built with the PGA2311UAs we got from AliExpress gave extremely low THD readings, just like the boards built with chips from more reputable vendors. 38 Silicon Chip Australia's electronics magazine siliconchip.com.au 4 Power up the board and monitor the CS line with an oscilloscope (IC1 pin 7). On power-up, the micro writes data to the PGA2311 for a couple of seconds to ramp the volume. If this signal is present, the PIC is running and programmed correctly. If you don’t have an oscilloscope, watch the LED very closely in a darkened room on power-up. After power is applied, you should see the LED light dimly for a second or two. If there is no activity on the CS line, go back and check power and check that your micro is programmed. You can also monitor the SDI and SCLK lines (IC1 pins 5 and 6) for activity. These should be active for the first second or so after power-up and when the encoder is rotated. 5 If the IR remote does not work: Have you installed the shunt on JP1? Have you put the TSOP4136 in the right location? Check the signal on JP1 or pin 2 of the TSOP4136 with an oscilloscope; there should be clear activity when the remote buttons are pressed. Have you programmed the remote with the right code? If using a universal remote, you will likely need to try a few of the configuration numbers for your remote to get it working. Install the activity LED and watch for the LED to flash; this will tell you that the remote is transmitting codes that work. Note that if you need to program your PIC on the board, you will need to remove the shunt from JP1. Many TSOP4136 devices otherwise stop the PIC from being programmed. Remember to reinstall the shunt SC after programming. ↪ ↪ ↪ ↪ ↪ ↪ ↪ ↪ Volume Control Pot Kit There are two kits for this project: • SMD version: SC6623 ($60) • Through-hole: SC6624 ($70) Note that the latter may be supplied with IC2 (the PGA2311) in the wide SOIC-16 package due to the limited availability of through-hole equivalents. The kits include all relevant parts in the parts list except the universal remote control and extra parts for the activity LED. siliconchip.com.au Ganging up multiple boards for more than two channels One really useful feature of this Digital Pot design is that it is easy to run one as a master and one or more as slaves. This allows one volume control or remote to set the level on four, six or more channels. This is great if you are making a home theatre system and want to use your own amplifiers. It is also handy if you want to control multiple channel levels in a multi-room system or need to adjust the level of multiple channels from one control. You can use either the through-hole or SMD versions to do this. The master is fitted with all the parts, while the slave(s) have the microcontroller, rotary encoder, infrared receiver, REG1 and associated parts left off. You need to have the programming header fitted to all the boards, and importantly, it must have six pins rather than five. The extra pin goes into or onto a pad labelled SCLK, right at the end of the programming header, allowing you to extend it by an extra pin. You then run a cable to join all the six-pin programming headers in parallel. That’s all you have to do! But remember to leave off the PIC microcontroller, REG1, IR receiver and encoder on each slave board. Otherwise, they will interfere with the master. To make a six-way ribbon cable that can join the boards, you can use two Altronics P5380 header sockets (or cut two 6-pin sections from a P5390 or similar strip). Wire pins 1-1 through 6-6 together using ribbon cable and insulate the soldered connections using 3mm diameter heatshrink tubing. Mark pin 1 at each end so you don’t accidentally swap them! That could cause damage to one or more boards. We tested this using 200mm of ribbon cable with no problems. This interface does not have fast data, so we expect you can stretch this a little if needed. You could also use a 12-way ribbon cable with IDC connectors as long as you were careful to plug the six-pin header into the same subset of the 12 pins on each connector. That might be easier since crimping IDC headers onto a ribbon cable only takes a few seconds with the right tool. Note that you still need to provide power to all boards (master and slave) since only the 5V digital power rail is carried on the connecting cable. They will generate independent split analog supplies. With the power connections made and the programming headers joined, you just need to connect the audio inputs and outputs to your various channels, ensure JP1 is fitted only on the master board, then power it up and go through the regular testing procedure. Note that the SCLK pin is at opposite ends of programming header CON4 on the SMD and through-hole boards, so you can’t mix the different board types (at least not without re-routing that signal between them). Also note that if you want to connect an IR activity LED to multiple ganged Digital Pots, you will need to split out those two wires from the harness to go to the LED and series resistor. This simple cable allows the master & slave Digital Pot boards to be ganged up to make a four-channel volume control. It can be extended to three boards for six channels and so on. Australia's electronics magazine March 2023  39