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The Tele-com an intercom using analog phones by Greig Sheridan & Ross Herbert Put your old analog telephones to use and build an intercom! Perhaps you have a classic or retro telephone like this red "batphone", or one of the other Bakelite phones with a real bell that generates a fantastic ring sound. Now you can not only hear it again but actually speak to someone at the other end! T echnically, the Tele-com is a ‘private line automatic ringdown unit’, known in the industry as a PLAR. That means that it allows two PSTN telephones to be automatically connected by simply lifting one handset. Colloquially, though, most people would just call it an intercom. Because of this, the device which allows the Tele-com to operate is referred to as the OzPLAR. If you need two-way communication between two nearby locations such as a house and a shed, or a granny flat, or just two rooms in a home, it doesn’t get much more convenient than this. Pick up the phone and the other end rings, then when the other person picks up, you can have a conversation. While the NBN supports analog telephones, we suspect that many people (like us) simply haven’t bothered plugging them in, and now have a box of spare phones. Rather than throw them away, now you can put them to good use. The central OzPLAR unit to which 30 Silicon Chip both telephones are connected (described in this article) performs the following functions; • Provides power to the phones (‘transmission battery feed’). • Detects when a phone is picked up (‘off-hook detection’). • Automatic ringing of an electromagnetic or electronic AC bell. • Ringing uses standard PSTN cadence – Australia/NZ/UK/EU/ USA (long & short) selectable. • The caller hears a ringtone while the called telephone is ringing. • Upon answer, ringing ceases and a speech path is established between the two telephones. • Both telephones must be replaced on-hook after a call before a new connection can be established. • Ring-trip (stopping the ring signal) occurs during either the silent or ringing period, when the called telephone is taken offhook. The design is based entirely on discrete components and logic ICs and has been designed with flexibility in Australia’s electronics magazine mind. The PCB accommodates various alternative parts for the battery feed and the ringing generator. See the features panel for more information. Circuit details The complete circuit of the Telecom is shown in Figs.1 & 2, with Fig.2 having the ring related circuitry (including cadence generation), and Fig.1 the rest. The overall circuit has a few basic jobs: 1. Power the telephones 2. Detect when one is picked up 3. When a call is initiated, cause the called phone to ring and send a ringtone to the calling phone 4. When the other phone is picked up, stop the ringtone and establish voice communications 5. Reset the system when both phones are restored on-hook To achieve this, it consists of multiple interconnected circuit blocks. The left-hand section in Fig.1 is the ‘battery feed’ and loop detect/ring trip circuit, whilst the middle section is the logic engine which detects line status siliconchip.com.au Features of the Tele-com Can be run from 2 x 12V batteries for an off-grid, portable or temporary setup Powered from a 24V DC inline power supply; no mains wiring is involved Support for 48V DC power input (optional) Ring tone is provided to the calling party 20Hz ringing supply for improved ringing of mechanical bells Support for optional bespoke cadence Superior audio performance over longer/ mismatched lines (using an IC-based battery feed) Onboard jumpers (or an external switch) to select AU/NZ/UK, EU or two variations of the USA cadence Choice of inductor-based or solid-state battery feed Crystal-locked source for the cadence generator and ringing inverter requires no adjustments Easy to build using locally-available parts (also readily available overseas) (off-hook/on-hook) and ensures that ringing output occurs only when the first telephone goes off-hook. The far-right section in Fig.1 includes the components required to add an optional polarity reversal on answer (“ROA”) to the calling telephone. Public telephones (PT) connected to Step-by-Step and ARF crossbar switching systems in the now discontinued PSTN used the reversal of the line polarity as the signal to deposit the caller’s money in the coin tin. This option requires 48V operation to work. Off-hook detection & ring trip When a telephone is taken off-hook, current passes through the optocoupler LED associated with the calling telephone (OPTO1 for the one plugged into CON3/4 or OPTO2 for CON5/6). Its output transistor therefore conducts and initiates a series of events to ring the other telephone. The voltage across each optocoupler LED is limited by zener diodes ZD1 & ZD2. At the same time, a low-pass filter siliconchip.com.au This “batphone” is an example of an old analog telephone that could be used with the Tele-com. It’s important to note that not all analog telephones have rotary dials, some have push-button keypads instead; both types will work. Australia’s electronics magazine October 2021  31 Fig.1: the Tele-com circuit, minus the ring and cadence generating circuitry, shown separately in Fig.2. The telephones plug into the sockets at the top and bottom of the left-hand side. The circuitry between them mainly involves supplying current to the phones and ensuring that voice signals pass between them. To the right, we have logic to detect when a phone is picked up and either ring the other phone or ‘answer the call’ if the other has already been picked up. 32 Silicon Chip Australia’s electronics magazine siliconchip.com.au (470W/220μF) bypasses 20Hz ringing signals around the optocoupler LED in the called telephone circuit, to prevent it from conducting during ringing. When the called telephone is taken off-hook to answer, current will flow through the LED in the optocoupler associated with the called telephone, thereby initiating ring trip. Ring trip can take place during the ringing period or the silent period. Initiating a call The following description refers to siliconchip.com.au a call initiated by a telephone connected to CON4 (or CON3) when the board is constructed with the inductorbased battery feed (see below). Note that in this case, the 1μF capacitors in the feed bridge are replaced by links (LK3 & LK4). When the telephone is taken offhook, 24V DC flows through transformer L1 (wired as an inductor) and the 68W resistor, the normally-closed contact of relay RLY1b, the LED in OPTO1, the telephone and back to ground via the normally-closed Australia’s electronics magazine contact of relay RLY1a, the 68W resistor, LK3, and transformer L2 (also wired as an inductor). The off-hook condition detected by OPTO1 results in a high level at the input of schmitt-trigger inverter IC1a. The resulting low output on pin 2 starts the calling process through the combined action of AND gate IC2c and NOR gate IC3a. The Q1 output on pin 1 of J-K master/slave flip-flop IC4a is preset high in the idle state. With both inputs of IC2d now high, its output at pin 11 also goes October 2021  33 high. This feeds into both IC2a and IC2b; however, the low level on IC2b pin 5 prevents RLY1 from operating. Since both inputs of IC2a are high, the output will also be high, which results in RLY2 operating. The RLY2 contacts disconnect the battery feed from the telephone at CON6 (CON5), and instead apply +24V to one leg of the line and the ringing (Vring) signal to the other, causing this telephone to ring. At the same time, the high level at the output of IC2d (pin 11) is inverted by IC1e, sending the Cadence Start line low to enable the crystal oscillator 34 Silicon Chip and the logic controlling the ringing inverter, shown in Fig.2. ‘Cadence’ refers to the timing of the ring bursts and silent periods. 4060 counter IC5 is held in reset at idle, but now commences oscillating. The reset signal is also removed from decade counter IC6, flip-flop IC4b and the cadence generator decade counters IC7 and IC8. Cadence Start is also presented to pin 8 of NOR gate IC10c, which in conjunction with IC7 and IC8, controls the cadence of the AC ringing signal (when set for Australia, producing the traditional ring ring...ring ring... sound). Australia’s electronics magazine The 3.2768MHz crystal oscillator based on X1 has its frequency divided by IC5 to produce 200Hz at its O13 output. This is divided by IC6 to produce the 20Hz alternating signal required for the efficient operation of electromagnetic telephone bells. This signal is also fed to the input of IC1b and IC10a, and in conjunction with the cadence signal at the output of IC1f, enables the ringing inverter. The 20Hz signal at IC6 pin 12 is halved by IC4b to produce the 10Hz clock signal for IC7. The outputs of IC7 go high sequentially, producing a one-second clock signal to feed IC8. siliconchip.com.au Fig.2: the rest of the circuitry which wouldn’t fit on Fig.1. At left is the cadencegenerating circuitry; the outputs of IC7 go high in sequence at 100ms intervals, while those of IC8 go high at one-second intervals. These signals are fed into a series of logic gates depending on the position of jumpers on JP1-JP3 and possibly LK5, resulting in a signal at output pin 10 of IC10c that indicates whether the phone should be ringing or not at any given moment. This is then converted into an AC voltage sufficient to ring a telephone by Mosfets Q6 & Q7 and transformer T1. The outputs of the 4017 decade counters, IC7 and IC8, are encoded in a manner that determines the on-off cadence pattern sent to the ringing inverter – see Fig.3 for details. Regardless of the cadence selection, the instant Cadence Start goes low, the ringing inverter is enabled, and the called telephone commences ringing. When the inputs to NOR gate IC10c are both low, its output is high. This is inverted by IC1f and fed to one input of gates IC10a and IC10b. The second input of these two gates alternates high or low following the 20Hz drive signal, while IC1b ensures that both Mosfet drive signals are complementary (ie, alternately phased). Mosfets Q6 and Q7 alternately switch the 12V DC supply through each primary winding of transformer T1. Due to the step-up ratio, an alternating voltage in the order of 120V peakto-peak is produced in the secondary. PTC thermistor PTC1 provides overcurrent protection, while the 2.2kW resistor provides a degree of clamping of the output voltage, should there be no load connected. While the ringing inverter is operating, the 6.8nF capacitor, normally bypassed by RLY2a, feeds a minute amount of the ringing voltage back to the calling telephone, serving as the ringtone. ► Cadence generation & selection Fig.3: this logic analyser screengrab demonstrates how the cadence generation circuitry works. Ch0 is the Cadence Start line (active-low), Ch1 is the 200Hz square wave at the O13 output of IC5, Ch2 is the 20Hz signal from pin 12 of IC6, Ch3 is the 10Hz signal at TP5 feeding into pin 14 of IC7, and Ch4 is the resulting cadence signal at pin 10 of IC10c (inverted so it is active-high). This shows the AU cadence. siliconchip.com.au Australia’s electronics magazine Jumpers JP1, JP2 and JP3 allow easy selection of the ‘ring-ring-pause’ (400ms on, 200ms off, 400ms on, two seconds off) cadence familiar to Aussies, our Kiwi neighbours and the UK. Other options are for the European cadence (one second on, four seconds off) and the two common versions of the US cadence (two seconds on, four seconds off and one second on, two seconds off), commonly referred to as “US Long” and “US Short” respectively. There are many cadences globally, and they’re documented in the ITU PDF at www.itu.int/ITU-T/inr/forms/ files/tones-0203.pdf Let’s assume the board is set up for AU cadence. When Cadence Start goes low (t=0.0s), the counter in IC6 is released from its reset state and commences counting. At that same instant, the reset signal is removed from IC4b, IC7 & IC8 in readiness for clock ticks to arrive. October 2021  35 ► Having just been released from reset, output O0 of IC7 is high. Pin 12 of NOR gate IC9 is thus high, so its output is low. O0 of IC8 is also high. This feeds to pins 12 and 13 of IC10d via JP2 pins 2 & 3, and thus pin 11 of IC10d is low. For a brief period, the inputs of NOR gate IC3d are both low, so its output is high. IC1d again inverts this to a low signal and this is fed via JP1 pins 1 & 2 to pin 9 of IC10c. The ringing inverter is enabled and it generates the 20Hz alternating voltage to ring the telephone. 100ms later, counter IC7 increments, sending O1 high, then on to O2 & O3. The ringing generation is maintained by linking these outputs to IC9’s inputs, resulting in a continuous on-period of 400ms. Outputs O4 & O5 of IC7 are not connected, so for those two 100ms ticks, IC9 has all low levels on its inputs, its NOR output goes high, so the ringing inverter is disabled for 200ms. For the final 400ms of the first one second of cadence, IC7 outputs O6-O9 are clocked sequentially high, and the ringing inverter is enabled again. At t=1.0s, IC7 resets and IC8 increments, sending its O0 output low. IC10d now prevents further signals from IC7 and IC9 from enabling the ringing inverter for the remaining period of the selected cadence pattern up until the instant output O3 of IC8 goes high, at t=3.0s. This signal, via JP3 pins 2 & 3 and diode D5, resets IC7 & IC8 and the cadence pattern repeats. The US and EU cadences are simpler, as IC9 and its related logic are no longer in play. JP2 instead directs either O0 or O1 of IC8 via IC10d and JP1 to the ringing inverter’s drive logic, thereby enabling the inverter which produces ringing for either one second (EU, US-S), or two seconds (US-L). The silent period for both AU and US-S cadence is terminated after three seconds, when output O3 of IC8 goes high, as explained earlier. The silent period for the EU cadence is terminated after five seconds, via JP3 pins 1 & 2 and diode D5. The silent period for US-L cadence is terminated after six seconds, when output O6 of IC8 goes high, via diode D4. Bespoke cadence creation is beyond the scope of this article, but any combination of 100ms on/off times can be created by mating the required O outputs of IC7 with up to eight inputs of IC9. This is via the pins of JP1-JP3, CON7, CON8 & LK5 as described at https://greiginsydney. com/ozplar-customisation/#bespoke Called party answers (ring trip) The 20Hz ringing voltage is superimposed upon the 24V DC supply. This ever-present DC allows the LED in the optocoupler associated with CON6 (or CON5) to conduct when the handset is lifted to answer a call. That’s regardless of whether it happens during a ringing or silent period. When ringing is present, the LED is prevented from conducting by the low-frequency filter formed by the two 470W resistors and the 220μF NP capacitor. The 10MW resistor provides a slight ‘off’ bias to the base of the optocoupler transistor, while the 56pF capacitor minimises noise pickup in the base connection. The 330kW resistor acts as the emitter load for the optocoupler output transistor. When answered, the optocoupler transistor turns on, and the resulting low at the output of inverter IC1c pin 6 causes NOR gate IC3a pin 3 to go high, thereby resetting flip-flop IC4a, causing its Q1 output to go low and RLY2 to release. The low on IC4a Q1 also causes the Cadence Start line to This is the finished Tele-com PCB without the optional IC-based battery feed, 48V power input components or “polarity reversal on answer” feature. 36 Silicon Chip Australia’s electronics magazine siliconchip.com.au go high, holding all the counters reset and disabling the ringing inverter. The release of RLY2 restores the change-over contacts to normal, thus connecting the called telephone to the battery feed and establishing a speech path between the two telephones. One party clears If we assume the telephone at CON4 (or CON3) hangs up first, the output of OPTO1 goes low and pin 2 of IC1a goes high. IC3a’s output goes low, removing the reset on IC4a, but the flip-flop’s outputs remain unchanged in the absence of any other stimulus. If this telephone again goes off-hook before the other telephone hangs up, the reset on IC4a is once more asserted, but again there is no change of state in its outputs, so the speech path remains connected. Fig.4: this shows the simplest way to power two telephones. Two high impedance inductors allow DC current to supply the transmitter while blocking AC signals through the low resistance of the battery. However, the proportion of the available current to each telephone is dependent upon the length of both lines and a very long line may reduce the current to an unworkable level. Both parties clear If the telephone at CON6 (or CON5) hangs up after the other telephone goes on-hook, both of the inputs to IC2c become high, causing its output to go high, setting the flip-flop in IC4a and restoring all circuitry to the idle state in readiness for the next call. Indicator LED The bi-colour LED (LED1) displays the various phases of a call. At idle, driver transistors Q3 (red) and Q4 (green) are both off, preventing both LEDs from illuminating, despite Q5 being on at this time. When a telephone is being called, both Q3 & Q4 are fully on while Q5 switches alternately on and off in response to the 20Hz LED drive signal, resulting in both red and green LEDs following the ring cadence. When a call is in progress, both telephones are off-hook. The green LED is illuminated due to the high on the output of IC3c forcing Q4 to conduct, while the low on the output of IC3b holds the red LED off. These two gates toggle when only one party has hung up, resulting in a steady red LED to indicate a possible fault condition – see the troubleshooting section below. Feeding power to the phones The Tele-com can be configured to use an inductor-based battery feed, as shown in Fig.4, where a 24V DC supply is fed to both legs of the line siliconchip.com.au Fig.5: this is a more complicated battery feed scheme known as a Stone Bridge which uses virtual inductors to feed DC current to each telephone independently, with capacitors coupling speech signals between them. It can handle very long lines over 1km in length. The virtual inductors are contained in a special IC available via eBay or suppliers of obsolete components via inductors L1 and L2. Since the total available current must be shared between both telephones, the current to each telephone is dependent mainly upon line length, ie, the shortest line gets the most current. The two 1μF capacitors shown on the circuit diagram are omitted and replaced by links in this case. Tests show that good speech is possible with line lengths up to 500m or more in this configuration – quite adequate for most situations. Provision has also been made to replace the inductor-based battery feed with an electronic battery feed using special 8-pin ICs – see Fig.5. The use of two such devices allows the implementation of what’s known as a Stone Bridge, such that the transmitter current supply to the two telephones is separate and determined only by individual line lengths. In Fig.5, the electronic battery feed ICs are depicted as individual inductors designated IC13 and IC14. The electronic battery feed device was designed by AT&T with the part Australia’s electronics magazine number LB1011. It is now obsolete and available only from electronics surplus component suppliers (eg, via eBay). It simulates two separate inductors having very high impedances at voice frequencies. When IC13 and IC14 are installed in place of inductors L1 and L2, the two 1μF capacitors need to be fitted to the board. These capacitors provide speech coupling between the two telephones connected to CON4 (or CON3) and CON6 (or CON5). In this configuration, the maximum current in each telephone circuit is approximately 36mA, so line lengths of several kilometres are possible. Optional reversal on answer To allow this Tele-com to work with with public (coin) telephones that require a line reversal on answer, the polarity of the line to CON6 (or CON5) can be made to reverse when the telephone at CON4 (or CON3) answers a call. This means that the public telephone must be connected to CON6 (or CON5). October 2021  37 Parts List – Tele-com 1 double-sided PCB coded 12110211, 200.5 x 143mm 1 PacTec LH96-200 ABS instrument case or equivalent, 260x180x65mm [Altronics H0476, RS 291-4169, Mouser 616-74213-510-039] 1 set of front & rear 3D-printed panels (size to suit case, see www.thingiverse.com/thing:4922521) 1 24V DC 2A power supply [Altronics M8970D, WES SMP2500-24RLP + ACL104-075] 1 3VA 12+12V PCB-mount mains transformer (T1) [Altronics M7024A ➊] 2 600W:600W isolation transformers ➋ (L1, L2) [Altronics M1000 or Triad TY-305P/306P/400P] 2 Omron G5V-2-H1DC12 12V DC coil relays or equivalent (RLY1, RLY2) [Altronics S4150] 1 3.2768MHz crystal resonator (X1) 1 RXEF030 300mA hold current PTC thermistor (PTC1) [element14 1175861, Mouser 650-RXEF030, Digi-Key RXEF030-ND] 1 10kW 9-pin, 8-element SIL resistor network (RN1; only needed for bespoke cadence) [element14 9356819, Digi-Key 4609X-101-103LF-ND] 1 PCB-mount barrel socket, 2.1/2.5mm inner diameter (CON1) [element14 1854512, RS 805-1699] 3 right-angle two-way pluggable headers (CON2, CON3, CON5) [Jaycar HM3102 + HM3122, Altronics P2592 + P2512, element14 2527811 + 2527762] 2 PCB-mounting 6P6C “RJ12” sockets (CON4, CON6) [Altronics P1425, Jaycar PS1474, Wurth 615006138421] 2 1-pin headers (can be snapped from a longer strip) (CON7, CON8; only needed for bespoke cadence) 3 3-pin headers with shorting blocks (JP1-JP3) 1 2x10-pin header or header socket (LK5; only needed for bespoke cadence) 5 PCB pins (optional; for test points TP1-TP5) 12 M3 x 6mm panhead machine screws 6 6mm-long M3-tapped spacers 6 6mm-long 6G self-tapping screws (PacTec case only) 3 300mm-long 4mm-wide cable ties 5 14-pin DIL IC sockets (optional) 5 16-pin DIL IC sockets (optional) 1 12-pin snappable IC socket strip (optional, for OPTO1-2) ➊ alternatives include RS 504-464, element14 1712727 (Vigortronix VTX-120-003-612), Mouser 553-FS24-100 (Triad FS24-100) & 838-3FD-324 (Tamura 3FD-324), RapidOnline 88-3883 (Vigortronix VTX-120-3803-412) Semiconductors 1 40106B or 74C14 hex inverter IC, DIP-14 (IC1) 1 4081B quad 2-input AND gate IC, DIP-14 (IC2) 2 4001B quad 2-input NOR gate ICs, DIP-14 (IC3, IC10) 1 4027B dual J-K flip-flop IC, DIP-16 (IC4) 1 4060B 14-stage ripple-carry binary counter IC, DIP-16 (IC5) 3 4017B decade counter/divider ICs, DIP-16 (IC6-IC8) 1 4078B 8-input OR/NOR gate IC, DIP-14 (IC9) 2 4N35 optocouplers, DIP-6 (OPTO1, OPTO2) 1 Switchmode 12V 1A regulator ➌ (Pololu D24V10F12 or Aug20; siliconchip.com.au/Article/14533) (REG3) 3 BC547 100mA NPN transistors (Q1-Q3) 2 BC557 100mA PNP transistors (Q4, Q5) 2 IRFZ44N 55V, 49A N-channel Mosfets (Q6, Q7) 1 3-pin bicolour/tricolour (red/green) common cathode 5mm LED (LED1) [Jaycar ZD0252] 38 Silicon Chip 2 3.3V ±5% 1W zener diodes (eg, 1N4728A) (ZD1, ZD2) 1 MBR10100 100V 10A schottky diode, TO-220 (note: not dual [CT] version) (D1) 2 1N4004 400V 1A diodes (D2, D3) 3 1N4148 or equivalent small signal diodes (D4-D6) Capacitors 2 220μF 10V non-polarised (NP/BP) electrolytic [Altronics R6600A or Mouser 667-ECE-A1AN221U] 2 100μF 63V electrolytic 1 1μF 100V MKT 3 100nF X7R ceramic 2 6.8nF 63V MKT 2 56pF 50V NP0/C0G ceramic disc 2 18pF 50V NP0/C0G ceramic disc Resistors (all ¼W 5% metal film unless otherwise stated) 3 10MW 1 2.2kW 3W 5% 2 330W 2 330kW 2 1.5kW 4 68W ➌ 6 10kW 4 470W 2 15W Additional parts for IC-based battery feed (exclude parts marked ➋ above) 2 AT&T/Lucent LB1011 battery feed ICs, DIP-8 (IC13, IC14) [eBay or one of the suppliers listed at www. digipart.com/part/LB1011AB] 2 8-pin DIL IC sockets (optional) 2 1μF 250V MKT capacitors 2 470nF 63V MKT capacitors 2 1kW ¼W 5% resistors 4 180W ¼W 5% resistors ➌ Additional parts for reversal on answer 1 Omron G5V-2-H1 12V DC coil telecom relay or equivalent (RLY3) [Altronics S4150] 1 16-pin DIL IC socket 1 4027B dual J-K flip-flop IC, DIP-16 (IC12) 1 BC547 100mA NPN transistor (Q8) 1 1N4004 400V 1A diode (D7) 1 10kW ¼W 5% resistor Additional parts for 48V DC supply (exclude parts marked ➌ above) 1 Traco TMR 6-4812 48V DC to 12V DC converter (REG1) [Mouser 495-TMR-6-4812] OR 1 Mean Well SKMW06G-12 48V DC to 12V DC converter (REG2) [Mouser 709-SKMW06G-12] 4 390W ½W 5% metal film resistors 4 150W ¼W 5% resistors Resistor Colour Codes Australia’s electronics magazine siliconchip.com.au The Tele-com is recommended to be built into the PacTec LH96-200 enclosure as shown (which can be purchased from RS Components or Mouser). However, mounting holes for the larger Altronics H0476 case are also provided on the PCB. Two flip-flops (IC12a and IC12b) are interconnected to provide this function. With both telephones onhook, both flip-flops are held reset. When either phone goes off-hook, the reset signal is removed. If the telephone connected to CON6 (CON5) is the caller, the output of IC2b presents a high to pin 7 of IC12a, setting this flip-flop. The high on the Q1 output is tied to the J2 input of IC12b, and with J2 high and K2 low, an answer signal from IC4a pin2 will toggle IC12b and set output Q2 high. NPN transistor Q8 then switches on and RLY3 operates, reversing the line polarity of CON6 (CON5). Should the telephone connected to CON4 (CON3) initiate a call, pin 7 of IC12a will not be set, and the J2 input to IC12b will remain low; therefore, the outputs of this flip-flop will not change state when the answer signal from IC4a pin 2 is applied to pin 13 of IC12b. RLY3 will remain in the unoperated condition and the line polarity will not be reversed. Flip-flops IC12a and IC12b will reset siliconchip.com.au only when both telephones are restored on-hook, causing RLY3 to release. Power supply The power supply takes an incoming +24V DC through reverse-polarity protection diode D1, and REG3 supplies +12VDC to power the logic, relays and the ringing inverter. A linear 7812 regulator was tried during the design phase, and replaced with a switchmode equivalent due to excessive heat dissipation, particularly when ringing. For an application where a higher ringing duty cycle is anticipated, or the Tele-com is to be powered from batteries, a switch-mode equivalent should be used instead (eg, our August 2020 design; see siliconchip.com.au/ Article/14533). If a 48V DC supply is to be used, REG3 is omitted and instead, a MeanWell (REG2) or Traco (REG1) DC-DC converter is fitted to accept the higher input voltage and step it down to +12V. Construction The Tele-com project is built on a Australia’s electronics magazine double-sided PCB coded 12110121 that measures 200.5 x 143mm. Start by giving the PCB a quick visual inspection for any obvious damage (although that is quite unusual). Use the PCB overlay diagram, Fig.6, as a reference during construction but note that there are a few different options that affect which components are fitted. If you are planning to build the Telecom with a custom cadence, you will need to cut some tracks on the underside of the board below LK5, separating the rows of pads on either side. Take care when cutting these tracks, as there is very little separation between the two rows of pads. If you plan to add the Reversal on Answer relay RLY3, there are two tracks noted with the word “cut” on the underside of the board – they are also indicated on the component overlay as two short lines joining two of the centre pads below RLY3. In both cases, if cutting, check with a continuity tester to ensure that the tracks have been completely separated before continuing. October 2021  39 The six mounting holes in the board fit mounting posts in the PacTec LH96200 enclosure. If you’re using that case, you can jump to the board assembly. If you’re building into the Altronics H0476 instead, there are two holes near the rear (connector) edge that align with two mounting posts under the board. They’re marked on the component overlay (Fig.6) with “#” marks. Temporarily screw the board to these, as this will align the board correctly within the box, then use the mounting holes in the four corners as a template to drill holes that will support the board. Remove the temporary screws and continue with the assembly. Breaking with tradition, mount the connectors first and ensure these all align and project through the rear panel. The pads for the power and screw connectors have been drilled oversize to provide a little extra wriggle room. Continue with the resistors and other low-profile components like the axial diodes and the crystal. If you’re building it with the inductor-based battery feed, don’t forget to replace the 1μF capacitors to the right-hand side of the transformers with links. Also, if you’re building for a 48V supply, note that the resistors marked on the overlay with an asterisk have different values for 48V. See the parts list for details. You can then install the SIL resistor array if you will be using the custom cadence feature, with its dot at the end shown in Fig.6 and on the PCB silkscreen. Now add the capacitors, starting with the smallest ceramic types and working your way up to the bigger ones. Confirm the polarity of the two electrolytics at the top right of the board and double-check that you have non-polarised electros adjacent to the telephone connectors. Now is also a good time to fit the PTC thermistor. The LED should be soldered at full extension onto the board if it’s to go into the PacTec case; however, you’ll need to add some short flying leads for it to reach the panel in the Altronics case. Add the remaining active components (ICs, regulators, optos and transistors), plus the TO-220 package diode, ensuring all the ICs have pin 1 on the right-hand side, and the TO-220 40 Silicon Chip devices all face left (with their metal tabs to the right). The use of IC sockets is recommended (including the optos), but check that +12V and GND (0V) are present on the correct pins before inserting ICs in their sockets. The optional test point PCB stakes and jumpers can be fitted next, then the relays, which must be orientated as shown in Fig.6. If you need LK5 and haven’t already fitted it, do so now, along with the headers for jumpers JP1-JP3. Follow with the switchmode DC-DC converter (REG1 or REG2) if you will be using a 48V supply. Finally, fit the transformers one by one. Place them, then wrap a cable tie around them firmly before soldering their pins. Take extra care if you’re using Tamura or Triad transformers for T1, as these can go into the board either way, but only one way is correct. Their ‘mains’ winding faces the rear panel connectors. The formers of both have pin numbers moulded into them, with the “1-2-3-4” side being the mains side. Troubleshooting There isn’t much to testing it. Plug in a couple of known-good telephones, apply the appropriate DC voltage and check that it works as expected. If you encounter problems, the nature of the fault should tell you which part of the circuit requires attention, but always start by confirming that the “Vin” voltage (24/48V) and 12V rails are present. You can sometimes isolate faults by touching the top of each IC, where any heat detected indicates a faulty device (CMOS ICs generally don’t produce significant heat unless they are faulty). If you’ve done this before, you probably know to apply a little saliva to your fingertip first to prevent burning yourself. No sidetone You should only connect knowngood telephones to the Tele-com. You should hear ‘sidetone’ if they are working correctly – some amount of your own voice is audible in the receiver. The easiest way to check for sidetone is to gently blow into the transmitter – you should hear the resulting hiss in the receiver. If sidetone is absent in either telephone, start by checking that power is switched on and 24V (48V) is present Australia’s electronics magazine on the board test pins. If the fault is not in the telephones, then check the wiring. If one is working and the other not, follow the circuit with your multimeter and compare between the two channels until the fault reveals itself. Don’t forget to swap the phones as a first check! No ringing First, check that jumpers JP1-JP3 are correctly set for one of the ring cadence patterns – follow the silkscreen legend on the board adjacent to these jumpers to select the desired cadence. If there’s no ringing when the first telephone goes off-hook, check the LED. If the LED is not lit at all, first make sure that it is a common-cathode device and driver transistors Q3, Q4 & Q5 are fitted in their correct positions. Briefly short pins 4 & 5 of OPTO1 or OPTO2. If that brings it to life, there’s most likely a problem with the optocoupler or the components on the LED side of this device. Check that the 3.3V zener cathodes are both facing ‘up’, towards the rear panel. If one of the relays operates when a telephone goes off-hook, that confirms that the main logic engine is functioning correctly. If neither relay operates, this narrows your focus to IC2-IC4 or the 12V rail. If the LED is flashing, this confirms the oscillator and cadence components are working OK, suggesting you should check the Mosfets and transformer. TP4 should have a pulsing 120V (approximately) alternating voltage on it, according to the selected cadence. Check also that the centre tap on the secondary of the transformer has +12V applied. If the LED is lit but not flashing, check with an oscilloscope, logic probe, or the frequency range on your multimeter that TP5 (near the LED) is fluctuating at 10Hz. If 10Hz is present, focus on IC7, IC8, the jumpers LK5, JP1 & JP2, diodes D4, D5 & D6, and the 10kW resistor immediately adjacent to these diodes. If TP5 is not fluctuating at 10Hz, focus on the 3.2768MHz crystal, its loading caps, IC5, IC6 & IC4b. Cadence problems An unexpected cadence indicates an incorrect placement or missing jumper on LK5 or JP1-JP3. Try changing siliconchip.com.au Fig.6: assembly of the Tele-com is straightforward, but there are quite a few different options, some of which involve fitting different parts. So you won’t necessarily install everything shown here. It’s best to work out what you will or won’t be mounting, and the components that might change in value, before you start. As you build the board, be careful to ensure that all the ICs, diodes, LED, optocouplers, transformers, transistors and polarised electrolytic capacitors are orientated correctly, as shown here. If using a 48V DC supply the four 180W resistors in the centre red box, and marked with an asterisk, are replaced with 390W resistors, while the 68W resistors marked with an asterisk become 150W. the jumpers to select an alternative cadence. If correct operation can be achieved when set to the EU or US cadences but not AU/NZ/UK, check that IC7 and IC9 are correctly seated. Check also that RN1 is not reversed and has the correct internal configuration, and one end pin is common. If you’ve cut the tracks under LK5 siliconchip.com.au in anticipation of using a custom cadence, make sure you have inserted links to replace the track segments which have been cut. If problems remain, confirm that TP5 is pulsing at exactly 10Hz, re-check the board for any solder shorting adjacent IC pins and repeat the ‘touch test’ on the tops of the ICs. Australia’s electronics magazine Red LED on idle If both telephones are on-hook and the LED is solid red, there’s most probably a fault on the line or with one of the telephones, causing one not to be correctly seen as on-hook. Unplug each phone in turn to see if the LED extinguishes. If it does, the fault is in the wiring or telephone itself. SC October 2021  41