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The prototype remote control module complete with optional 27mm motorised potentiometer. A standard low-cost 16mm version can also be fitted. By PETER SMITH Studio Series Remote Control Module Wor k s W i t h A n y Un i v er s al Remo t e Con t r ol! If you’ve built our preamp described in November 2005, then this project is a must-have addition. It allows you to control your preamp’s volume level and select the music source using any universal infrared remote. As a bonus, we’ve added support for an audiophile-grade potentiometer for those who want the best. L ET’S FACE IT – any sound system is incomplete without at least a remote volume control. We described an excellent unit back in October 2002, based on a motorised potentiometer. However, while that project would work well with the Studio Series Preamp, it lacks any means of controlling the preamp’s source relays. And that’s a problem – you don’t want to abandon your comfy chair just to select a different music source, do you? The volume control features of this unit are virtually identical to our 36  Silicon Chip previous project. Again, it’s based on a motorised potentiometer. Press the “Volume Up” and “Volume Down” buttons on your remote and the pot rotates right and left. It takes about nine seconds for the pot to travel from one end to the other using these controls. For finer adjustment, the “Channel Up” and “Channel Down” buttons can be used instead; these cause the pot shaft to rotate only about 1° for each press. Automatic muting is another handy feature. A press of the “Mute” button and the pot rotates to its minimum position. Hit the button again and it returns to its original position. Don’t want the volume to return all the Fig.1: the complete circuit diagram for the control module. An AT90S2313 microcontroller (IC1) decodes data from the infrared receiver (IC3) and drives the motorised potentiometer accordingly. Five outputs from port B drive the relay circuits on the preamp to provide remote control of the music source as well. siliconchip.com.au siliconchip.com.au April 2006  37 Fig.2: the infrared receiver module contains a lot more than just a PIN (photo) diode. This block diagram of the internals reveals an amplifier, discrimination and demodulation circuits, all integrated in the 3-pin package. After the 38kHz carrier is removed, the data appears on the “OUT” pin ready for handling by the micro. Fig.3: when the pot reaches full travel, a clutch begins to slip, loading the motor and increasing the supply current. The muting function uses a comparator in the microcontroller (IC1) to detect this current increase and switch off the motor. This simplified diagram shows how the comparator is connected. way? Easy – just hit one of the volume control buttons when the volume has reached the level that you desire. Selecting any of the preamp’s signal sources is just as easy. All you need to do is press the associated numeric button on the remote. For example, to select the “Tuner” input, you’d press “3” and for “CD” you’d press “5”. Finally, this new design can be fitted with either a low-cost 16mm motorised pot or a more expensive, highquality 27mm unit. The advantages of the 27mm units include longer life, lower noise and better tracking than their cheaper counterparts. How it works As can be seen from the circuit diagram (Fig.1), the design is based on an AT90S2313 microcontroller from 38  Silicon Chip Atmel. This device includes 2k bytes of code (FLASH) memory, 28 bytes of RAM and 128 bytes of EEPROM and has featured in a number of our past designs. The microcontroller is supported by a power supply and several interface circuits, which are responsible for driving the motor, receiving infrared signals and controlling the preamp’s relays. Let’s look at each section in a little more detail. Looking first at the power supply portions of the circuit, the module expects a regulated 5V (±5%) supply on CON1. A large 3A diode (D1) across the input terminals provides rudimentary reverse-polarity protection for the board. If the power leads are accidentally reversed, D1 conducts and pulls the power supply rail down to about 1V or so. Assuming you see the smoke signals and react quickly, damage to the board should be minimal, although D1 may not survive and should be checked for a short circuit before reapplying power. The supply to the micro (IC1) is derived from the +5V rail via a 100mH choke (RFC1), which acts as a simple filter to reduce RF emissions. This is an important consideration for our sensitive audio circuitry. Separate low-pass filtering is needed for the infrared receiver module (IC3) to keep digitally-generated noise out of its sensitive front-end circuitry. A 100W resistor in series with IC3’s supply pin and a 100mF capacitor to deck do the job. An under-voltage sensor (IC2) monitors the supply rail and generates a reset signal for the micro whenever it drops below 4.3V. This function is often referred to as “brown-out” detection and it ensures that the micro doesn’t behave erratically during supply rail transitions. Incidentally, this design uses an MC34164-5 sensor, rather than the MC34064-5 device found in several of our past designs. The MC34164-5 has a lower threshold voltage than the latter, needed here to allow for worst-case supply regulation during motor operation. Infrared receiver Infrared pulses from the remote control are detected by IC3. In addition to a sensitive photodiode, this device contains an amplifier and other logic necessary to receive and extract the incoming digital data, which is modulated on a 38kHz carrier (see Fig.2). The demodulated data is pumped into the microcontroller on pin 2. Under program control, it is then reconstituted into byte-wide format using the Philips RC5 protocol specification. Once deciphered, the results can be used to determine which button has been pressed on the remote and the appropriate action taken. H-bridge drive Average pot motor current ranges from about 40mA to 100mA, depending on the model used. Start-up current is higher still and so the drive requirements easily exceed the maximum sink and source capabilities of the microcontroller’s port outputs. This siliconchip.com.au Par t s Lis t Fig.4: although we think that manual selection switches are unnecessary, we’ve made provision for them for those who prefer front-panel controls. One push-button switch is required for each source – here’s how to connect them to CON3 on the control module. Keep all wiring as short as possible and make sure that the ground connection is insulated from chassis earth. necessitates the use of four smallsignal transistors (Q1-Q4) as buffers and drivers, arranged in a “H-bridge” configuration so that the motor can be driven in either direction. The transistors operate in pairs. To drive the motor in one direction, port bit PD5 is driven low and PD3 high. This biases Q1 & Q4 into conduction and creates a current path from the 5V rail, through Q1, the motor and Q4 to ground (via R1). To spin the motor in the reverse direction, the opposing transistor pair (Q3 & Q2) is switched on instead. To do this, port bit PD2 is brought low and PD4 high. Motor hash is reduced using two 100nF capacitors, one of which is soldered directly across its terminals. A ferrite bead in line with the motor’s supply leads also helps by blocking high-frequency noise components. Current sensing Once the pot’s wiper reaches its fully clockwise or anti-clockwise position, a friction type clutch in the gearbox begins to slip. This prevents the motor from stalling, while also allowing the user to manually rotate the pot shaft when necessary. The muting function depends on the microcontroller’s ability to detect when the wiper is “on the stops”. For the Altronics model, typical motor current is 40mA, increasing to about siliconchip.com.au 1 PC board, code 01104061, 72mm x 150mm 1 2-way 5mm/5.08mm terminal block (CON1) 2 3-way 5mm/5.08mm terminal blocks (CON4, CON5) 1 10-way shrouded (boxed) header (CON2) (Altronics P-5010) 1 6-way 2.54mm header (CON3) (Altronics P-5496) 1 2-way 2.54mm header (CON6) (Altronics P-5492) 1 2-way 2.54mm plug (CON6) (Altronics P-5472) 1 4-way 2.54mm SIL header (JP1,JP2) 2 jumper shunts 1 8mm ferrite sleeve (Altronics L-4910A) 1 100mH choke (RFC1) 1 20-pin gold-plated IC socket 4 M3 x 10mm tapped spacers 4 M3 x 6mm pan-head screws 1 5kW miniature horizontal trimpot (VR2) 1 20kW log motorised pot (VR1) (Altronics R-2000) –or1 10kW log motorised pot (Alps RK27 series) (see text) Semiconductors 1 AT90S2313-4 or -10 microcontroller (IC1) programmed with MPOT.HEX 1 MC34164P-5 or MC33164P-5 under-voltage sensing IC (IC2) (Farnell 791-908) 1 infrared receiver module (TSOP4838 or equivalent) (IC3) (Altronics Z-1611, Farnell 491-3190) 50mA when driving the clutch. This handy side effect is put to good use by including a small current sense resistor (R1) in series with the motor driver’s ground circuit. If R1 is 10W, 0.4V will be dropped across it during normal rotation and 0.5V when driving the clutch. A lowpass filter comprising an 18kW resistor and 100nF capacitor remove much of the motor noise, after which the signal is fed into pin 12 (AIN0) of the microcontroller. Internally, this pin is connected to the non-inverting input of a voltage comparator (see Fig.3), while the inverting input is connected 1 4MHz crystal, HC49S package (Y1) (Altronics V-1219) 2 BC327 PNP transistors (Q1, Q3) 2 BC337 NPN transistors (Q2, Q4) 1 1N5404 diode (D1) 6 3mm red LEDs Capacitors 4 100mF 16V PC electrolytic 1 1mF 16V PC electrolytic 4 100nF 50V metallised polyester (MKT) 1 100nF 50V multilayer ceramic 2 22pF 50V ceramic disc Resistors (0.25W, 1%) 1 18kW 2 330W 1 16kW 1 100W 3 10kW 1 10W 9 1kW 1 6.8W 5% Additional items 2-core shielded audio cable for pot wiring Medium-duty hook-up wire for power supply & motor wiring 2 x 10-way IDC cable-mount sockets (Altronics P-5310) 10-way IDC ribbon cable 2 small cable ties Power supply modification 1 1N5338B 5.1V, 5W zener diode (Altronics Z-0405, Farnell 933-120) Note: the program file (MPOT. HEX) will be available for download from the SILICON CHIP website for those that wish to program their own microcontrollers. to an external voltage reference on pin 13 (AIN1). The voltage reference is made adjustable with trimpot VR1, which forms a simple voltage divider with a 16kW resistor. When the sense voltage exceeds the reference voltage set by trimpot VR1, the comparator’s output swings high, generating a program interrupt. The interrupt handling code then switches off the active transistor pair to stop the motor. In use, the trimpot is adjusted so that the comparator doesn’t trip during normal pot travel. However, when the clutch is slipping, the increase in April 2006  39 fore which signal source is selected. Optionally, push-button switches can also be wired to each port line via CON3, enabling manual source selection (see Fig.4). To facilitate this function, the microcontroller pulls its inactive port lines high and continually monitors them for a low level (button press). When a button is pressed, the chosen relay is immediately activated. Just a few milliseconds later, the microcontroller senses the low level and returns the currently active output high while driving the new output low, in effect “latching” the user’s button press. Before you begin, check that the holes in the PC board are large enough to accept the motorised pot. The footprint for the standard (Altronics) and optional (Alps) pots is quite different, so two sets of holes have been provided. Some boards will have slots for the front row of pins, allowing either type of pot to fit without modification, whereas others may have two rows of closely spaced small holes instead. If you find that the required row of holes is too small to accept the pot’s pins, then they’ll need to be drilled out to about 1.2mm. That done, set the pot aside and following the usual practice, begin by installing all of the lowest profile components. The two wire links and the resistors are a good place to start. Use the overlay diagram (Fig.5) as a guide to component placement. All other components can then be installed as you see fit, but leave out the microcontroller (IC1), infrared receiver (IC3), LEDs and motorised pot for now; we’ll come back to these shortly. Be sure to mount the five 100mF capacitors and the diode (D1) around the right way and check that the keyed side of CON2 is oriented towards IC1. Also, be particularly careful not to mix up the two transistor types, or indeed the under-voltage sensor (IC2), as they’re all housed in identical TO-92 packages! Note that the crystal (Y1) must be mounted vertically and with minimum lead length. Once in place, connect its metal can to ground by soldering a short length of tinned copper wire between the can and the ground pad underneath (see photo). After installing the motorised pot, solder a 100nF capacitor directly across the motor terminals (see photo). Next, solder a pair of medium-duty wires to the terminals and pass these through a ferrite sleeve before terminating in a 2-way plug to mate with CON6. Alternatively, the wires can be soldered directly to the PC board without the header & plug, if desired. Use a small cable tie or two to hold the ferrite sleeve close to the motor side of the wiring. Assembly Front panel stuff Assembly is relatively straightforward, with all components mounted on a single PC board coded 01104061. The remote control module is designed to be mounted directly behind the front panel of a low-profile case. Universal Infrared Remote Controls The remote control module is designed to work with most universal (“onefor-all”) infrared remotes. It recognises the RC5 protocol that was originally developed by Philips, so the remote must be programmed for a Philips (or compatible) appliance before use. Most universal remotes are provided with a long list of supported appliances and matching codes. To set the remote to work with a particular piece of gear, it’s usually just a matter of entering the code listed for the manufacturer (in this case, Philips), as detailed in the instructions. You’ll also note that different codes are provided for TV, CD, SAT, and so on. This allows two or more appliances from the same manufacturer to be operated in the same room and even from the same handpiece. This multiple addressing capability can be useful in our application, too. Normally, we’d program the remote to control a TV, as this works with the control module. But what if you already have a Philips TV (or a Chinese model that uses the RC5 protocol)? Well, in this case, you’d simply use a CD or SAT code instead – the control model can handle any or these! Let’s look at an example. To set the AIFA Y2E remote (see below) to control a Philips TV, you’d first press and hold “SET” and then press “TV”. This puts the remote in programming mode, as indicated by the red LED, which should remain illuminated. Now release both keys and punch in one of the listed Philips TV codes. For this project, code 191 works well. The red LED should now go out and the remote is ready for use. All universal remotes can be programmed in a similar manner but when in doubt, read the instructions! If the first code listed doesn’t work with the control module, then try another. Once the remote has been programmed, the control module must be set up to recognise the particular equipment address that you’ve chosen (TV, CD, SAT, etc). Details on how to do this are in the setup and testing section. Although this project should work with any universal remote, we’ve tested the following popular models: AIFA Y2E (Altronics A-1013), AIFA RA7 (Altronics A-1009) and BC3000 (Jaycar AR-1710). For all these models, the setup codes are as follows: TV = 191, CD = 651 (but not for BC3000 remote), SAT1 = 424 and SAT2 = 425. Note that the “mute” button doesn’t work for all codes and in the case of the AIFA Y2E, is missing anyway! In these cases, you may be able to use the “12” or “20+” buttons instead. motor current causes a proportional increase in voltage at the comparator’s non-inverting input, causing its output to switch high. Preamp control Source switching on the Studio Series Preamp (see SILICON CHIP, October 2005) is performed by miniature 5V relays, which are in turn switched by PNP transistors. On the control module, five outputs from the micro (PB3-PB7) are used to drive the preamp’s transistors and select between the various signal sources. These outputs are routed to CON2, where they’re connected to the preamp via ribbon cable. Each port line is protected with a 1kW series resistor, while LED1-LED5 indicate which line is low and there40  Silicon Chip siliconchip.com.au Fig.5: follow this diagram when assembling your board. Take care not to mix up the two transistor types and the under-voltage sensor (IC2), as they’re all in TO-92 packages. Mount the diode (D1) a few millimetres above the board surface for a little extra insurance in case of a wiring error! Below is the fully-assembled unit with the Alps pot. Table 1: Resistor Colour Codes o o o o o o o o o siliconchip.com.au No. 1 1 3 9 2 1 1 1 Value 18kW 16kW 10kW 1kW 330W 100W 10W 6.8W 5% 4-Band Code (1%) brown grey orange brown brown blue orange brown brown black orange brown brown black red brown orange orange brown brown brown black brown brown brown black black brown blue grey gold gold 5-Band Code (1%) brown grey black red brown brown blue black red brown brown black black red brown brown black black brown brown orange orange black black brown brown black black black brown brown black black gold brown not applicable April 2006  41 Some constructors will prefer the lower cost 16mm motorised pot, as shown installed here. the 7805 regulator (see photo), and its body spaced about 3mm above the board surface. The two PC board holes may need to be drilled out to 1.2mm to accept the larger diameter leads. Wiring To this end, the infrared receiver, LEDs and pot all mount along one edge of the PC board so that they will protrude through the front panel. If necessary, trial fit the module into the chosen case to gauge the required lead length and bend for the LEDs and infrared receiver. If you’re drilling the case yourself, then note that the hole for the infrared receiver should be The 100nF polyester capacitor is soldered directly across the terminals of the pot motor, as shown here. This close-up view shows how a wire link is used to connect the crystal case to a ground pad. 42  Silicon Chip slightly larger than the “bump” in the package to ensure operation over the widest possible area. Before drilling the four mounting holes for the module, note that the front boss (face) of the pot should make firm contact with the rear of the front panel. This is very important, as it prevents stress being placed on the pot assembly when the nut is tightened and the pot is manually operated. If necessary, fit one or more additional washers over the pot shaft to bring it in contact with the panel when the board is positioned flush against the rear. Note that a number of other mounting options are possible, depending on your requirements. For example, the pot could be mounted a short distance from the board, with the shielded audio cable terminated directly at its pins rather than at CON4 & CON5. If a different power source is to be used, it must have a well-regulated output of 5V ±5%. A plugpack or other poorly regulated source is unacceptable and may cause erratic operation or even component failure! The chosen supply should also power the 5V relay circuit on the preamp board, or at least share a common ground with it. Use mediumduty multi-strand cable for the supply wiring and twist the two wires together to reduce noise and improve appearance. We suggest using black for ground (0V) and some other colour for +5V – preferably a different colour to that used for the ±15V wiring! The power input connector (CON1) can then be marked using the same felt-tipped pen colour to reduce the chances of cabling mistakes. Next, hook the 10-way headers on the preamp and control module together using a length of 10-way IDC cable. The plugs and sockets are keyed, so as long as you take care to create a one-to-one connection when crimping on the IDC plugs, all should be well! Finally, it is very important that the motor housing is connected to chassis earth. We suggest running a separate wire from the point marked “EARTH” on the PC board to the main earth point, rather than relying on the pot to make contact with the metalwork. Note that the motor housing is not connected to the ground (GND) rail on the control module to avoid creating an earth loop. Power supply upgrade Setup & testing Power for the control module can be sourced from the low-noise power supply module described as part of the Studio Series Preamp in October 2005. Unfortunately, the module’s peak current requirements are a little higher than we’d anticipated, so a minor modification is required to the power supply before it can be used here. The modification is quite straightforward and simply involves replacing the 100W 5W resistor (R1) with a 5.1V, 5W zener diode. The banded (cathode) end of the zener must point away from To successfully complete the following instructions, you’ll need a universal remote control that you have programmed for use with a Philips brand appliance. Refer to the panel titled “Universal Infrared Remote Con­ trols” before proceeding. OK, let’s check the supply rails. Apply power and measure the voltage between pins 10 & 20 of IC1’s socket. Your meter should read 5V ±5% – if not, switch off immediately and look for cabling faults and the like. Assuming all is well, power off and siliconchip.com.au High-Quality Pot Upgrade In anticipation of this project, several readers suggested that we present a design with a digital, rather than analog (ie, motorised pot) volume control. Why digital? Well, apparently they used our previous design (published in June 2002) so much that the pot wore out within a year! So we considered the possibilities of a digital design. It appeared that the best performance could be realised by using a digitally controlled analog gain/attenuation block. As luck would have it, Burr-Brown (TI) offers a single-chip device that integrates all of the necessary elements and introduces very low distortion. That seemed like the right solution to the audio part of the design (ignoring the additional distortion) but elsewhere it starts to get complicated! For a start, we’d need some method of indicating the volume settings to the user. We’d also need a means of adjusting the volume. In our opinion, simple “up” and “down” buttons don’t cut the mustard; you just can’t beat a rotary dial for volume! So at a minimum, we’d need a “high-spec” digital/analog volume control IC, a liquid crystal display insert IC1 in its socket, making sure that the notched (pin 1) end is oriented as shown on the overlay diagram (Fig.5). Now insert a jumper shunt on JP1 to place the module in setup mode and power up again. The five red LEDs should flash in sequence the moment power is applied to indicate setup mode. Now point your remote at the infrared receiver (IC3) and press one of the numeric keys (1-9) twice. On the first press, the “acknowledge” LED should flash once, whereas on the second press, it should flash five times. This indicates that the micro has successfully determined the equipment address and stored it in EEPROM for future use. This completes the microcontroller setup, so power the module down and remove JP1. Pot’n around We’ll test the motorised pot next, so be sure to insert a jumper on JP2 if siliconchip.com.au (or large LED bargraph), a rotary encoder and a microcontroller. Unfortunately, the whole shooting match would be too expensive for most constructors, particularly if it were not made available as a kit. Anyway, we believe we’ve struck a good compromise. Once again, our design uses a motorised potentio­ meter but we’ve included provision for either the low-cost Altronics pot or a higher quality RK27 series Alps pot. These 27mm Japanese-made pots have a rated minimum life of 15,000 rotations and a maximum gang error of 2dB over the -60dB to 0db range. Only two small changes need to be made to the board to support either type of pot. To use the Altronics pot, use a 10W value for the current sense resistor (R1) and leave out jumper JP2. For the Alps pot, fit a 6.8W resistor instead and install a jumper shunt on JP2. That’s it – with one caveat, as follows. No mute? During prototype development, we were unable to get the muting facility to work reliably with the Alps pot. We found that the motor current tended to vary from pass to pass, perhaps suggesting a peculiarity with the gearbox design. It may also have been peculiar to our batch of pots – we can’t be absolutely sure! Regardless, this made it impossible to adjust VR1 for reliable cut-off when hitting the end stop. In the end, we went ahead with support for the Alps pot anyway, as we believe that most constructors who would be willing fork out for this expensive option would also be willing to forgo the muting function, for which they may have little (if any) use. Note that at time of publication, we were unable to find an Australian distributor who is offering the Alps RK27 pots in one-off quantities. However, they are available from a variety of overseas Internet sites. Be sure to get a 10kW type with a “15A” resistance taper and check that the shaft style and length suits your particular application. For detailed technical information on the RK27 series, check out the product catalog on the Alps website at www.alps.com. Replace the 100W 5W resistor on the power supply board with a 5.1V 5W zener diode, as shown here. Note the orientation of the cathode (banded) end of the zener. you’ve fitted an Alps pot. Conversely, if you’re using the standard Altronics pot, this jumper must not be installed. Exercise the pot by moving it manually over its full range of motion several times. This helps to break in the clutch April 2006  43 Fig.6: check your board against this is the full-size etching pattern before installing any of the parts. before we continue with the adjustment procedure. Next, rotate trimpot VR1 fully clockwise and power up. You should now be able to use the volume up/down and channel up/down buttons on the remote to move the pot in both directions. If it moves the wrong way, simply reverse the leads to the motor. Now set the pot to its mid position and hit the “mute” button (“12” on the AIFA Y2E). The pot will rotate anti-clockwise for 12 seconds and as soon as it hits the stops, the clutch will start to slip. While this is happening, rotate trimpot VR1 slowly in an anti-clockwise direction until the motor cuts out. Now drive the pot clockwise for a second or so and press the “mute” but- RC5 Infrared Protocol – A Primer Every time you press a button on your remote, a message comprised of the key code and equipment address is composed, encoded and then modulated before being transmitted using a high-brightness infrared LED. In the RC5 coding scheme, each message is composed of a 14-bit serial stream. A message consists of four parts: • Start part – 1.5 bits (2 x logic “1”) • Control part – 1 bit • System part – 5 bits • Command part – 6 bits The start bits give the receiver time to “lock on” to the incoming data. The control bit, also called the toggle bit, is simply a flag to indicate whether the following code is new or repeated. If a new key is pressed, the control bit toggles (changes state) from its previous value, otherwise it remains the same. The system bits represent the equipment address (TV, CD, VCR, etc), while the command bits are the code for the actual key pressed. On the physical level, data is transmitted using bi-phase (also known as Manchester) encoding. A logic one is represented by a zero-to-one transition at 1/2 bit time, whereas a logic zero is represented by a one-to-zero transition. One bit time is approx. 1.778ms, so a complete message is 24.889ms long, with messages repeated at a minimum of 114ms intervals. To reduce interference from other light sources, data is transmitted on a 38kHz carrier. 44  Silicon Chip ton again. This time, the motor should stop as soon as the pot reaches its minimum position. If it stops prematurely or fails to stop at all (ie, the motor runs for the full 12 seconds), try redoing the adjustment. Once the adjustment is correct, pressing the mute button a second time will result in the pot being returned to its original position. It’s important to note that if the cutout function fails to operate when the pot reaches its minimum position, the motor will continue to run for 12 seconds (the full-travel period). Pressing the mute button a second time will have no effect, as the program has no record of the original shaft position! Wrap up Well that’s about it. All that’s left to do is to connect the two sections of the motorised pot to the preamp using shielded audio cable. Each side of the pot is brought out to a 3-way terminal block (CON4 & CON5) on the PC board to make hook-up relatively easy. The cable on the lefthand side can be routed through the large hole just to the rear of CON4. As shown on Fig.5, the centre terminal (GND) connects to the cable shields; do not connect the shield to chassis ground! Refer to the preamp project for more details. In an upcoming article, we’ll show you how to assemble the preamp, headphone amplifier, remote control module and power supply into a very nice slimline case! In the mean time, SC happy listening! siliconchip.com.au