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Last month, we introduced our new Infrared Stereo Headphone Link, which allows you to settle back and enjoy your favourite music, the TV or other audio source without being tied down by a cord. This month, we complete the project with the receiver section. Infrared stereo headphone link PART 2 – THE RECEIVER Fig.1 (page 55) shows the circuit of the infrared stereo receiver. As with the transmitter, it follows some of the circuit techniques used in stereo FM transmission and reception. A photodiode, PD1, detects the IR pulse stream from the transmitter and produces DC current pulses which are fed directly to op amp IC1a, a current-to-voltage converter. IC1a is AC-coupled to op amp IC1b which has a gain of 6.8. The amplified signal is fed to IC2, an LM311 comparator. Its output is an 88kHz 6V peak-to-peak square wave when there is no audio modulation from the transmitter and an 88kHz PWM stream when audio modulation By RICK WALTERS October 1996  53 TL062 for IC1 and a TL064 for IC6 because of their low current consumption, an important factor in a battery operated circuit. The more common TL072 and TL074 types can be used instead but the current consumption is appreciably higher. Left channel identification There are two PC boards in the infrared receiver but only one is visible in this photo. The IR receiver LED is underneath the top board, at the focal point of the lens assembly on the lefthand side. is transmitted. This waveform contains all the audio information that was encoded by the transmitter; all we have to do is decode it. This is done in two steps. First, we recover the signal in mono. To produce the mono signal, all we need to do is to connect a low pass filter at the output of comparator IC2. To produce the stereo left and right channels though, we need to de­ multiplex the audio by switching it to the left and right channels in sequence, using a square wave with the same frequency and phase as the multi­plexing frequency in the transmitter. Phase locked loop This is where the phase locked loop, IC4, comes into the picture. IC4 can be considered to be a square wave oscillator which is “locked” to an incoming reference frequency. Its output frequency will be the same as the transmitter’s but 90° out of phase and the filter components are selected so that it will not follow the modulation. To ensure an exact symmetrical square wave we run IC4 at double the received frequency; ie, at 176kHz. This is divided by two, using flipflop IC3a, to give an 88kHz square wave. This is divided by two again, by flipflop IC3b. In this case, the 18kΩ resistor and 100pF capacitor delay the clock signal to IC3b by 90° to ensure that 54  Silicon Chip its outputs are in phase with the incoming signal. The 44kHz outputs of IC3b, at pins 12 and 13, are used to switch the signal from IC2 to the right and left channels alternately using IC5, an HC4066 CMOS switch. This switching process is called “de­multiplexing” and is the reverse of the multi­­ plexing process in the trans­­mitter. The left and right channel signals appear at pins 1 and 11 of IC5, respectively. As there is a large amount of high frequency noise on the recovered audio signals, heavy filtering is required. To this end, we use a 4-pole filter which gives an attenuation rate of 24dB per octave, above 10kHz. For the right channel the first filter consists of the two 10kΩ resistors and .0012µF and .0018µF capacitors around op amp IC6a. The second filter involves the .0015µF capacitors in a similar configuration around IC7a, Q1 & Q2. There is identical filtering for the left channel, involving op amps IC6b, IC7b, Q3 & Q4. To compensate for the high frequency pre-emphasis which was applied in the transmitter we use the 1kΩ series resistor and the .022µF capacitor across each volume control to attenuate the upper frequency response. This is de-emphasis. By the way, we have specified a To establish which is the left channel we take the signal from the output of the left channel filter, IC6b pin 7, and feed it to two cascaded 10Hz bandpass filters, comprising op amps IC6d and IC6c. The 10Hz signal, if it is present in the left channel, will be amplified, clamped to ground by D1 and then will charge the 0.1µF capacitor at the gate of FET Q5, via D2. This will hold Q5 turned on and its drain will be almost at 0V. If the left channel signal has been switched to the right amplifier there will be no 10Hz signal present in IC6b’s output, thus the 10MΩ resistor will discharge the 0.1µF capacitor on Q5’s gate and the FET will turn off. This will allow the drain of Q5 to rise to the battery voltage (+6V) via the 47kΩ resistor. Because the set input of flipflop IC3b is connected to this point, the flipflop will be held set. This holds pin 13 of IC3b high, switching the input signal permanently to the left amplifier, thus allowing the 10Hz signal to be fed to the bandpass filters. The FET will turn on as described previously and the flipflop will begin to toggle again to give correct de­ multiplex operation. Due to the low frequency of the synchronising signal (10Hz) and consequently, the long time constants in the FET gate circuit, it may switch several times before it gets the phase right. Loss of infrared signal Since the audio signal is sent via an infrared beam, what happens when the beam is interrupted? When the PLL, IC4, is locked to the incoming frequency, the signal at pin 1 is normally high with a brief negative transition every Fig.1 (right): the circuit of the infrared stereo receiver uses seven ICs, four transistors and two FETs. Its operation is explained in the text. October 1996  55 PARTS LIST – RECEIVER 1 PC board, code 01109962, 120 x 60mm 1 PC board, code 01109963, 45 x 42mm 1 plastic box, 130 x 68 x 41mm, Jaycar HB6013 or equivalent 1 lens assembly, Oatley Electronics OLP1 or equiv. 1 pushbutton switch, Jaycar SP0710 or equivalent (S1) 2 AA battery holders, Jaycar PH9202 or equivalent 2 216 snap-on battery connectors 1 3.5mm stereo socket, Jaycar PS0132 or equivalent 1 red knob, Altronics H6001 or equivalent 1 green knob, Altronics H6005 or equivalent 11 PC stakes 2 6PK x 6mm self tapping screws 2 10kΩ 16mm diameter PC mount log pots, Jaycar RP3610 or equiv. (VR1,VR2) Semiconductors 1 TL062 dual op amp (IC1) 1 LM311 comparator (IC2) cycle; when it is not locked this output is low. The resistor and capacitor at pin 1 filter the negative spikes, feeding a steady voltage to pins 12 and 13 of IC5. If this voltage is high, the audio is switched through to IC6a and IC6b but if it is low, the switches are open. Therefore, if the transmitted light source is obstructed for any reason the audio signal to the headphones will be “killed” and there will be no extraneous noises produced. 56  Silicon Chip 1 4013 dual D flipflop (IC3) 1 74HC4046 phase lock loop (IC4) 1 74HC4066 quad CMOS switch (IC5) 1 TL064 quad op amp (IC6) 1 LM833 audio amplifier (IC7) 2 BC337 NPN transistors (Q1,Q3) 2 BC327 PNP transistors (Q2,Q4) 2 BS170 FETs (Q5,Q6) 1 LT536 or equiv. photodiode (PD1) 2 1N914 silicon diodes (D1,2) MKT polyester or ceramic 3 .0012µF 63VW MKT polyester or ceramic 1 .001µF 63VW MKT polyester or ceramic 2 120pF 63VW MKT polyester or ceramic 1 100pF 63VW MKT polyester or ceramic Note: if ceramic capacitors are used, they should be within ±10% tolerance. Capacitors 2 470µF 16VW electrolytic (for 8Ω headphones) 1 330µF 16VW electrolytic 4 100µF 16VW electrolytic 2 4.7µF 16VW electrolytic 2 0.47µF 16VW electrolytic 4 0.15µF 63VW MKT polyester 1 0.1µF 50VW monolithic 6 0.1µF 63VW MKT polyester 2 .022µF 63VW MKT polyester 1 .01µF 63VW MKT polyester 2 .0018µF 63VW MKT polyester or ceramic 4 .0015µF 63VW Resistors (0.25W, 1%) 2 10MΩ 1 18kΩ 2 2.7MΩ 2 12kΩ 3 1.2MΩ 17 10kΩ 1 680kΩ 4 8.2kΩ 1 470kΩ 1 6.8kΩ 1 120kΩ 4 1kΩ 1 100kΩ 1 820Ω 2 68kΩ 1 220Ω 1 47kΩ 2 47Ω 1 39kΩ You will notice in the photos that the end of the receiver case has a tube mounted on it. This is a lens assembly and its job is to focus the received IR radiation onto the photodiode. It gives a significant increase in the distance that the transmitter and receiver can be separated. Miscellaneous Hookup wire, machine screws and nuts, solder. Accordingly, IC7a drives a pair of complementary emitter followers Q1 and Q2, which are connected within the negative feedback loop to keep distortion low. The inputs of the LM833 have to be Below: opening out the top PC board reveals the method of construction, We have used an LM833 dual op with the batteries and lower board amp but this does not have sufficient clearly shown. Use no more wire output to drive all headphones.­ between the boards than is necessary to allow them to come apart. Headphone drive Fig.2: the parts overlay for the two receiver PC boards. It should be followed closely during assembly. In particular, check that all polarised components (semiconductors, diodes and capacitors) are inserted the correct way around. biased to half the supply voltage to ensure a symmetrical output swing. To do this and also to simplify things, the “earthy” ends of the volume pots are taken to this potential; ie, +3V. Automatic switch-off As the receiver is battery operated there will be a tendency to take the headphones off and walk away “for a minute or two” leaving the unit running. When you come back, in a week’s time for example, the batteries could be very flat. To avoid this embarrassment, we have an automatic off switch comprising Mosfet Q6 and a few other components. Hence, there is an ON button but no OFF switch. When the ON button is pressed, the 330µF capacitor connected from gate to source of Q6 is charged to +6V via the 220Ω resistor. This turns Q6 on, applying power to the receiver. The 330µF capacitor will then gradually discharge via the 10MΩ resistor until the voltage is insufficient to keep the FET switched on. At this stage, a few squawks will come through the headphones to alert you to the imminent switch-off. Pushing the ON button again will let you listen for another half hour or so, the idea being to press it at the beginning of each program. modating IC7, Q1, Q2, Q3 & Q4. The larger board, coded 01109962, holds the remainder of the circuitry. Note that neither PC board has holes for mounting screws or pillars. Instead, the headphone drive board is secured in place by soldering two PC stakes at the corners to the metal cases of the volume controls which mount on one end of the plastic case. The main board has the corners cut out to clear the integral pillars of the case. It is neat fit into the case and is sandwiched between the lid and a piece of foam rubber. We’ll cover more of the details as we go. Fig.2 shows the component overlays Case assembly Receiver assembly Now let’s start constructing the receiver. This has the two boards shoe-horned into a plastic case. The smaller board, coded 01109963, is the head­phone driver board, accom- for the two receiver PC boards. Let’s start with the audio amplifier board. First, fit the nine PC stakes, the resistors and the IC into the board and solder them. This done, insert and solder the four MKT capacitors, the four transistors and lastly, the four electrolytics. If you are going to use 32Ω headphones (as supplied with Walkman-type cassette players) all the time, then we suggest fitting 100µF output coupling capacitors to the board. However, if you expect to use conventional 8Ω headphones, you will need to use 470µF output coupling capacitors, otherwise the bass response will be deficient. The larger board has six links which should be fitted first, followed by resistors and diodes, IC sockets (if you use them) and then the capacitors. We don’t recommend using PC stakes in this board except as test points for the left and right audio outputs, as it is more convenient to bring the wires from the copper side of the board to the volume controls and the audio amplifier. The ICs, being CMOS devices, should be plugged into the sockets or soldered in last. The photodiode is mounted on the copper side of the PC board with full lead length. Make sure that the chamfer is on the left side when viewed from the front. The infrared beam is focused on the photodiode by this lens assembly to significantly increase the range. As already noted, the headphone drive board is secured by soldering two corner PC stakes to the cases of the two volume controls. These volume controls must be the 16mm dia­meter type otherwise they will not fit together in the confines of the case. October 1996  57 Fig.3: this wiring diagram shows how the wiring is run from the underside of the larger board to the top of the smaller board. Note that the photodiode, PD1, is soldered to the underside of the larger board. Make sure that this device is oriented correctly. points on the main board. Fig.3 shows the full details of the wiring between the main PC board and the headphone drive board. Follow the wiring diagram of Fig.3 carefully. It is probably easier to solder the wires onto the volume control lugs before you mount them in the case, as you will have easier access to the terminals. Testing As well as drilling holes in one end of the case for the pots, you will need holes in the side for the pushbutton ON switch and the 3.5mm stereo head­phone socket. Finally, you will need to drill holes in the other end of the case to take the lens assembly which was 58  Silicon Chip mentioned above. It is supplied by Oatley Electronics (OLP1). Note that the lens assembly should not be fitted until the transmitter and receiver have been tested. Two double-AA cell holders provide the 6V battery supply. These are wired in series and then to the +6V and 0V If you are very careful with your assembly and check everything closely there is no reason why it won’t work first up. If it doesn’t, you will have to decide which of the units is not functioning properly. If you have access to an oscilloscope this is easily checked out. If no scope is available, testing is a little harder. Starting with the transmitter, normally the first things to measure after you apply power are the rail voltages. In this project, if these check out at +15V and -15V and the regulator tabs don’t burn your finger, it’s a good start. If you have a multimeter with a frequency response to above 100kHz, you will be able to check for the presence of signal at pin 3 of IC1 (176kHz), pins 1, 2, 12 & 13 of IC2 (88kHz, 44kHz), pin 7 of IC5 and IC6, and the collector of Q1 (set the meter to AC volts). If all these points have signals you can feel reasonably sure that there are no problems with the transmitter. RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 2 2 3 1 1 1 1 2 1 1 1 2 17 4 1 4 1 1 2 Value 10MΩ 2.7MΩ 1.2MΩ 680kΩ 470kΩ 120kΩ 100kΩ 68kΩ 47kΩ 39kΩ 18kΩ 12kΩ 10kΩ 8.2kΩ 6.8kΩ 1kΩ 820Ω 220Ω 47Ω The audio can be followed through from the inputs to pin 2 of IC5 or IC6 with high impedance headphones or a signal tracer. If a signal is missing you must check around that area until you find the cause of the trouble. To work on the receiver you must have the transmitter turned on and pointing in the direction of the receiver. Press the ON button on the receiver and measure the battery current. It should be around 17mA. To test for the presence of an 88kHz carrier, set your multimeter to AC volts and check pin 7 of IC2, pin 3 & pin 4 of IC4 and pin 1, pin 12 & pin 13 of IC3. If the sound only comes through the left channel it means that the FET Q5 is turned off. Check the soldering 4-Band Code (1%) brown black blue brown red violet green brown brown red green brown blue grey yellow brown yellow violet yellow brown brown red yellow brown brown black yellow brown blue grey orange brown yellow violet orange brown orange white orange brown brown grey orange brown brown red orange brown brown black orange brown grey red red brown blue grey red brown brown black red brown grey red brown brown red red brown brown yellow violet black brown RESISTOR COLOUR CODES No. ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ Value IEC Code EIA Code 0.15uF   150n   154 0.1uF   100n   104 .022uF   22n   223 .0018uF   18n   183 .0015uF   15n   153 .0012uF   12n   123 .001uF   10n   103 120pF   120p   121 100pF   100p   101 around the bandpass filter and also ensure that diodes D1 and D2 are inserted correctly. If you want to install an on/off 5-Band Code (1%) brown black black green brown red violet black yellow brown brown red black yellow brown blue grey black orange brown yellow violet black orange brown brown red black orange brown brown black black orange brown blue grey black red brown yellow violet black red brown orange white black red brown brown grey black red brown brown red black red brown brown black black red brown grey red black brown brown blue grey black brown brown brown black black brown brown grey red black black brown red red black black brown yellow violet black gold brown switch to replace the FET switch, omit Q6, the 330µF capacitor and the 10MΩ and 220Ω resistors. Connect one end of the switch to the battery minus (Q6 source) and the other side of the switch to 0V (Q6 drain). Finally, after testing is complete, the main board can be assembled into the receiver case. Before this is done, the photodiode must have its leads bent so that its face is square in the hole in the end of the case; its face should be flush with the outside surface of the case, as this is the focal point of the lens assembly. The lens assembly can then be secured in position with two self-tapping screws. Finally, fit the lid of the case SC and the job is done. Fig.4: check your PC board against this full-size etching pattern before installing any parts. October 1996  59