MP3 FM Transmitter Circuit Diagram

Heres a simple VHF FM transmitter that could be used to play audio files from an MP3 player or computer on a standard VHF FM radio. The circuit use no coils that have to be wound. This FM transmitter can be used to listen to your own music throughout your home. When this FM transmitter used in the car, there is no need for a separate input to the car stereo to play back the music files from your MP3 player.

Project image :

USB FM transmitter

To keep the circuit simple as well as compact, it was decided to use a chip made by Maxim Integrated Products, the MAX2606 [1]. This IC from the MAX2605-MAX2609 series has been specifically designed for low-noise RF applications with a fixed frequency. The VCO (Voltage Controlled Oscillator) in this IC uses a Colpitts oscillator circuit. The variable-capacitance (varicap) diode and feedback capacitors
for the tuning have also been integrated on this chip, so that you only need an external inductor to fix the central oscillator frequency.

t is possible to fine-tune the frequency by varying the voltage to the varicap. Not much is demanded of the inductor, a type with a relatively low Q factor (35 to 40) is sufficient according to Maxim. The supply voltage to the IC should be between 2.7 and 5.5 V, the current consumption is between 2 and 4 mA. With values like these it seemed a good idea to supply the circuit with power from a USB port.

Circuit diagram:

USB FM transmitter schematics Circuit diagram

 Parts List
Resistors (all SMD 0805)
R1,R2 = 22kΩ
R3 = 4kΩ7
R4,R5 = 1kΩ
R6 = 270Ω
P1 = 10kΩ preset, SMD (TS53YJ103MR10 Vishay Sfernice, Farnell # 1557933)
P2 = 100kΩ preset, SMD(TS53YJ104MR10 Vishay Sfernice, Farnell # 1557934)
Capacitors (all SMD 0805)
C1,C2,C5 = 4μF7 10V
C3,C8 = 100nF
C4,C7 = 2nF2
C6 = 470nF
Inductors
L1 = 390nF, SMD 1206 (LQH31HNR39K03L Murata, Farnell # 1515418)
L2 = 2200Ω @ 100MHz, SMD, common-mode choke, 1206 type(DLW31SN222SQ2L Murata, Farnell #1515599)
Semiconductors
IC1 = MAX2606EUT+, SMD SOT23-6 (Maxim Integrated Products)
Miscellaneous
K1 = 3.5mm stereo audio jack SMD (SJ1-3513-SMT
CUI Inc, DIGI-Key # CP1-3513SJCT-ND)
K2 = 5-pin header (only required in combination with 090305-I pre-emphasis circuit)
K3 = USB connector type A, SMD (2410 07 Lumberg, Farnell # 1308875)

A common-mode choke is connected in series with the USB connections in order to avoid interference between the circuit and the PC supply. There is not much else to the circuit. The stereo signal connected to K1 is combined via R1 and R2 and is then passed via volume control P1 to the Tune input of IC1, where it causes the carrier wave to be frequency modulated. Filter R6/C7 is used to restrict the bandwidth of the audio signal. The setting of the frequency (across the whole VHF FM broadcast band) is done with P2, which is connected to the 5 V supply voltage.

The PCB designed uses resistors and capacitors with 0805 SMD packaging. The size of the board is only 41.2 x 17.9 mm, which is practically dongle-sized. For the aerial an almost straight copper track has been placed at the edge of the board. In practice we achieved a range of about 6 metres (18 feet) with this. There is also room for a 5-way SIL header on the board. Here we find the inputs to the 3.5 mm jack plug, the input to P1 and the supply voltage. The latter permits the circuit to be powered independently from the mains supply, via for example three AA batteries or a Lithium button cell. Inductor L1 in the prototype is a type made by Murata that has a fairly high Q factor: minimum 60 at 100 MHz.

PCB Layout :

 USB FM transmitter Layout PCB

Take care when you solder filter choke L2, since the connections on both sides are very close together. The supply voltage is connected to this, so make sure that you don’t short out the USB supply! Use a resistance meter to check that there is no short between the two supply connectors before connecting the circuit to a USB port on a computer or to the batteries.

P1 has the opposite effect to what you would expect (clockwise reduces the volume), because this made the board layout much easier. The deviation and audio bandwidth varies with the setting of P1. The maximum sensitivity of the audio input is fairly large. With P1 set to its maximum level, a stereo input of 10 mVrms is sufficient for the sound on the radio to remain clear. This also depends on the setting of the VCO. With a higher tuning voltage the input signal may be almost twice as large (see VCO tuning curve in the data sheet). Above that level some audible distortion becomes apparent. If the attenuation can’t be easily set by P1, you can increase the values of R1 and R2 without any problems. 

Measurements with an RF analyzer showed that the third harmonic had a strong presence in the transmitted spectrum (about 10 dB below the fundamental frequency). This should really have been much lower. With a low-impedance source connected to both inputs the bandwidth varies from 13.1 kHz (P1 at maximum) to 57 kHz (with the wiper of P1 set to 1/10). In this circuit the pre-emphasis of the input is missing. Radios in Europe have a built-in de-emphasis network of 50 μs (75 μs in the US). The sound from the radio will therefore sound noticeably muffled. To correct this, and also to stop a stereo receiver from mistakenly reacting to a 19 kHz component in the audio signal, an enhancement circuit Is published elsewhere in this issue (Pre-emphasis for FM Transmitter, also with a PCB). Author: Mathieu Coustans, Elektor Magazine, 2009

Notice. The use of a VHF FM transmitter, even a low power device like the one described here, is subject to radio regulations and may not be legal in all countries.

http://www.ecircuitslab.com/2011/07/mp3-fm-transmitter-circuit-diagram.html

Wire Loop Game

In the ‘Wire Loop Game’, a test of dexterity,  the player has to pass a metal hoop along a  twisted piece of wire without letting the hoop  touch the wire. Usually all the associated electronics does is ring a bell to indicate when the  player has lost. The version described here has  a few extra features to make things a bit more  exciting, adding a time limit to the game and a ticking sound during play. 

Two 555 timer ICs are used to provide these  functions. IC1 is configured as a monostable which controls the time allowed for the  game, adjustable using potentiometer P1. IC2  is a multivibrator to provide the ticking and Two 555 timer ICs are used to provide these  functions. IC1 is configured as a monostable which controls the time allowed for the  game, adjustable using potentiometer P1. IC2  is a multivibrator to provide the ticking and he continuous buzz that indicates when the  player has touched the wire with the hoop. 

Circuit diagram :

Wire Loop Game Circuit Diagram

When the monostable is in its steady state,  the output of IC1 (pin 3) is low. T1 acts as  an inverter, and D2 is thus forward biased.  R8 and R4 are therefore effectively in parallel, with the result that IC2 produces a low audible tone. The value of R4 is considerably  greater than that of R8, and so the frequency  of the buzz generated by IC2 is chiefly deter-mined by the value of R8.

When the monostable is triggered, the high  level at the output of IC1 is again inverted  by T1. D2 is reverse biased and so R8 is effectively removed from the circuit. The frequency of IC2 is now largely determined by  the value of R4. The ratio of R4 to R5 and the  value of C4 affect the mark and space periods for the multivibrator: for a satisfactory  ticking sound short pulses with long gaps  between work well. 

Whether a sound is produced also depends  on the voltage on pin 4 of IC2. When the 9 V  supply is connected the monostable is initially inactive and there is no voltage across  C1. Pin 4 (reset) on IC2 is thus low and no tone  is produced. IC1 is activated by a brief press of  S1, which generates a low-going trigger signal  on pin 2 to start the game. C1 now charges via  D1 and IC2 is allowed to oscillate, generating  the ticking sound. 

The pulse width of the monostable sets the  game duration, and can be adjusted using  P1. If the allowed time expires, or if the reset  input to IC1 is taken low (which happens when the hoop touches the wire), the monostable  returns to the quiescent state. This causes IC2  to generate the low buzz sound. D1 is now  reverse biased and C1 discharges through the  relatively high-valued resistor R9. After a few  seconds the voltage across C1 falls sufficiently  that the buzz stops and the circuit is ready for  the next player. 

The circuit can be built first on a breadboard,  so that the component values can easily be changed to suit particular preferences for  game duration and buzz pitch. When suitable  values have been selected the circuit can be  built more permanently on a printed circuit  board. The author used a sheet of plywood  to form a base for the game, the twisted wire  being fixed to the board and wired to the electronics mounted below it. 

Author: Andreas Binner

SW Converter for AM Radio

Apart from chucking it in the bin, what can you do with old AM car radio or clock radio in your junkbox? How about turning it into a crystal controlled, stable, short wave radio receiver, for a minimum investment in time and money? Read on. The heart of the circuit shown here is an IC which goes by the name NE602, NE612 or SA612. It is a double balanced mixer that includes an oscillator that can be crystal-controlled, free running or even driven externally from a PLL, etc. It was originally designed for mobile telephones and is probably available in junked car phones from the tip. The NE602/612 contains a differential input amplifier (called a Gilbert Cell), an oscillator/buffer, a temperature compensated bias network and a power regulator. Typical frequency response is in excess of 500 MHz for the input and 100 MHz for the oscillator.

Circuit diagram :

SW Converter for AM Radio Circuit Diagram

 

Supply current is 2.4 mA and the absolute maximum supply voltage is 9 V. Input and output impedances are approx. 1.5 kΩ. As you can see from the circuit diagram, the input from the aerial is passed through a 10.7 MHz IF (intermediate frequency) transformer. This gives isolation from the aerial and reduces the effect of strong local AM radio breakthrough.The transformer can be salvaged from a dead FM radio or stereo or even the FM section of an old clock radio. (The AM section is what we want to use anyway so ratting a bit from the FM section saves cost). A number of 10.7 MHz IF coils from Toko and other far-Eastern manufacturers may be used, including the 94AES30465N and 94ANS30466N, but obtaining these as new parts may be more costly than a complete radio rescued from the tip. There is usually a small capacitor under the IFT coil, between the pins. If so, remove it by crushing it with a pair of pliers and ripping out the remains. The capacitor is not needed as we add an external one according to the band wanted. The input signal is fed into the balanced input of the IC.

 

The crystal is connected to pin 6. It oscillates at its fundamental frequency and is mixed with the input signal giving a number of outputs. The mixer output signal appears on pins 4 and 5. Here, only pin 5 is used for the output. By the way, the inputs and outputs are internally biased with pull-up resistors, so there is no need to tie the unused pins to ground or power. The 220 pF capacitor gives isolation to any DC into the AM radio aerial input. Note also that the same circuit can be used to extend the range of an existing short wave radio receiver in exactly the same manner. The AM radio is used as a tuneable intermediate frequency amplifier, with a tuning range of about 1.6 MHz. You can try different values for C1 to get resonance at the NE602 input: 150 pF for up to 5 MHz, 47 pF for up to 8 MHz, and no capacitor for up to 10 MHz. In practice however 33 pF should do for all ranges. Almost any crystal can be used. The author tried many types from FT-243 WW2 surplus ones to 27 MHz, 3rd overtone CB crystals. Every crystal tried worked. TV sub-carrier crystals work well, as do large oven types. Several crystals can be connected through a switch, giving a convenient way of switching bands. Keep the leads to the switch as short as possible though to prevent radiation of the crystal oscillator. There are many ways to build the circuit. You could make it into an external metal box that can be connected to several radio’s, depending on your location. For instance, if you are a traveller, make it in a small box with an internal 9-volt battery, and leave enough wire on the output to wrap a few dozen turns around the clock radio in your Hotel room.

This will give you your short-wave reception on the go. It is also possible to build the converter right into the car radio. Any sort of construction method can be used, from a small piece of perforated board that I used, to a more elaborate printed circuit board and even just lash all the small components underneath the IC socket. A small switch may be used to change from AM to short-wave. Connect the circuit to the car radio with screened cable to prevent or lessen the effect of strong station breakthrough. To couple the output of the converter to a radio without an external AM aerial input, wind several turns of wire around the internal ferrite rod aerial. As suggested before, winding a dozen or so turns around the plastic radio case will also couple the converter to the radio. This will work at the expense of increased AM signal breakthrough. Connect the positive power lead to the switch on the radio so that it switches the converter on and off as well.

The short-wave aerial can be 2 to 3 meters of wire strung around the room, but better results will be obtained with a outdoor aerial. The test aerial was about 100 meters long and 10 meters high. At night there is a lot of activity on the short waves after dark. Find a weak station around 1 MHz on the AM dial and adjust the core of the IFT for minimum volume from the broadcast station. That’s the only adjustment. SSB signals can be heard, but as no beat frequency oscillator is fitted, you hear the “duck talk” of the signal. The 10 kHz bandwidth of the radio means that on the ham bands, signals do overlap, but it also makes the broadcast stations sound better as most of them do broadcast with reasonable quality audio. Digital tuned AM radios are usually not suitable for the circuit as presented, because the tuning steps are 9 or 10 kHz apart and we want much smaller steps. The old manually tuned types of car radio are what you want. The idea of the circuit is not to get too complicated, but to just enjoy listening on a simple, stable, cheap, short wave receiver. Experiment and enjoy!

Author : P. Laughton, VK2XAN – Copyright: Elektor Electronics

Minimalist Dip Meter

In days gone by a radio amateur always had a dip meter close to hand in his ‘shack’. Now that people can afford oscilloscopes, the poor old dip meter has lost its importance and is  frequently no longer to be seen. Actually this is a shame because many tasks are much easier to carry out with a dip meter. Anyone who’s interested (perhaps the second time around) can easily build one rapidly with this very simple but adequate circuit. The interesting question is namely what do you actually need from a dip meter? 

Minimalist Dip Meter Circuit Diagram

• A visual display of the dip? Nope, the ‘scope can handle that task.
• A large frequency scale? Not necessary, as you can connect a frequency counter for this.
• A selection of coils? We don’t need these because we can use a jumper to change range (no coils to lose any more!).

The sensor coil L1 has ten turns and is wound  using an AA-size battery as a former. This coil will allow us to over the range from 6 MHz to 30 MHz. With jumper JP1 open an additional fixed inductance of 10 μH comes into circuit. The frequency measurement range is then from 2.5 MHz to 10 MHz. The switch may be replaced by a jumper. 

To take measurements you hold a resonant circuit close to the sensor coil. Tune the rotary capacitor C1 slowly to and fro in order to find the resonant frequency, at which the oscillator amplitude decreases somewhat. The frequency can then be read directly off the oscilloscope.

To obtain a very accurate measurement you can additionally connect your frequency counter to the second output.

http://www.ecircuitslab.com/2012/05/minimalist-dip-meter.html

The Gentle Touch Circuit Diagram

Consumer appliances these days hardly ever have a proper mains switch. Instead, appliances are turned on and off at the touch of a button on the remote control, just like any other function. This circuit shows how a device (as long as it does not draw too high a current) can be switched on and off using a pushbutton. The approach requires that a microcontroller is already available in the circuit, and a spare input port pin and a spare output port pin are required, along with a little software. When power is applied T1 initially remains turned off. When the button is pressed the gate of T1 is taken to ground and the p-channel power MOSFET conducts. The microcontroller circuit is now supplied with power. Within a short period the microcontroller must take output PB1 high. This turns on n-channel MOSFET T1 which in turn keeps T1 turned on after the push-button is released.

Now the microcontroller must poll the state of the push-button on its input port (PB0) at regular intervals. Immediately after switch-on it will detect that the button is pressed (a low level on the input port pin), and it must wait for the button to be released. When the button is next pressed the device must switch itself of f: to do this the firmware running in the microcontroller must set the output port pin to a low level. When the button is subsequently released T1 will now turn off and the supply voltage will be removed from the circuit.

The circuit itself draws no current in the off state, and for (rechargeable) battery-powered appliances it is therefore best to put the switch before the voltage regulator. For mains-powered devices the switch can also be fitted before the voltage regulator (after the rectifier and smoothing capacitor). Since there is no mains switch there will still be a small standby current draw in this case due to the transformer. Be careful not to exceed the maximum gate-source voltage specification for T1: the IRFD9024 device suggested can withstand up to 20 V. At lower voltages R2 can be replaced by a wire link; otherwise suitable values for the voltage divider formed by R1 and R2 must be selected.

Circuit diagram:

 The Gentle Touch Circuit Diagram

The author has set up a small website for this project at http://reweb.fh-weingarten.de/elektor, which gives source code examples (which include dealing with pushbutton contact bounce) for AVR microcontrollers suitable for use with AVR Studio and GNU C. Downloads are also available at http://www.elektor.com.

SW Converter for Digital AM Car Radio

This circuit is purposely presented with many loose ends (not literally, of course) to stimulate experimenting with RF circuitry at a small outlay. Looking at the circuit diagram you may recognize a modified version of the SW Converter for AM Radios described elsewhere in this issue. The modifications were necessary to make the circuit compatible with a digital rather than analogue AM car radio. The main difference between digital AM radios and their all-analogue predecessors is that tuning is in 9 kHz (some-times 4.5 kHz steps) in compliance with the international frequency allocation for the band. Obviously, that particular step size, desirable as it may be on MW, is a stumbling block if you want to use a digital AM receiver in combination with a frequency step-up converter for SW, where chaos reigns and there is no fixed step size. The first attempt was to make the crystal oscillator variable by about 5 kHz each way.

 

Circuit diagram :

SW Converter for Digital AM Car Radio Circuit Diagram

 

Unfortunately, despite serious efforts, the crystal could not be pulled more than 1 or 2 kHz so another solution had to be found. After studying the NE/SA602/612 datasheet, it was found that a variable LC based oscillator was the best alternative. The circuit worked after winding a resonant LC circuit and adding a 0.1 µF series capacitor to block the DC component on pin 6 of the NE602 (612). When the tuning was found to be a bit sharp with the original capacitor, a simple bandspread (or fine tuning) feature was added by shunting the LC resonant circuit with a lightly loaded 365 pF tuning capacitor (C10) which, like the main tuning counterpart, C8, was ratted from an old transistor radio. The tuning coil, L1, consists of 8 to 10 turns of 0.6-0.8mm dia. enamelled copper wire (ECW) on a 6-8 mm dia. former without a core. With this coil, frequency coverage will be from about 4 MHz to 12 MHz or so. Details on Tr1 may be found in the referring article.

 

Note that no tuning capacitor is used on the secondary — the input stray capacitance of the NE602 (612) does the trick. A BFO (beat frequency oscillator) was added to enable SSB (single sideband) signals to be received. The BFO built around T1 is simple, has a heap of output and is stable enough to hold an SSB signal for a few minutes without adjustment. The BFO frequency is tuned with C3. Tr2 is a ready-made 455 kHz IF transformer whose internal capacitor was first crushed and then removed with pliers. When S2 is closed the BFO output signal is simply superimposed on the NE602 (612) IF output to the MW radio. The converter should be built into a metal box for shielding. If you find that the BFO gives too much output, disconnect it as suggested in the circuit diagram and let stray coupling do the work. Sensitivity, even on a 1-metre length of car radio aerial, is quite amazing. Bearing in mind that most of the major international SW broadcasting stations like Radio NHK Japan, Moscow, BBC etc.) generate enough power to make sure that you will hear them, it is still quite exciting to hear such signals for the first time on your car radio. 

http://www.ecircuitslab.com/2012/02/sw-converter-for-digital-am-car-radio.html

Audio Controlled Mains Switch

It is often useful for audio or video equipment to be switched off automatically after there has been no input signal for a while. The function of the on-off switch in such equipment is then taken over by switch S2 in the accompanying diagram. It remains, however, possible to  switch off manually by means of Si. Automatic  switch-off occurs after there has been no input  signal for about 2 minutes: this delay makes it possible for a new record or cassette to be placed in the  relevant machine.

 

The audio input to the proposed circuit may be  taken from the output of the relevant TV set, amplifier, or whatever. The input earth is held at + 6 V  with respect to the circuit earth by potential divider  Ri-R2-R3-R4. The two 741s function as comparators: the output of ICi goes high when the in- put signal is greater than + 50 mV, whereas the out- put of IC2 goes high when the input signal  becomes more negative than -50 mV. Resistors  R6, R7, and R8 form an OR gate that drives transistor Ti. If the output of either ICi or IC2 is logic  1, Ti conducts.

 

Circuit diagram :

Audio Controlled Mains Switch Circuit Diagram

The 555  operates as a retrigger able monostable,  whose period is determined by Rio and Ci. The  device is triggered when its pin 2 is earthed by the  closing of S2. Its output, pin 3, then remains high  for 1 to 2 minutes, depending on the leakage cur- rent of the 555. 

The monostable resets itself as soon  as the potential across Ci exceeds a certain value.  As long as there is an input signal to the circuit, Ti conducts and Ci remains uncharged. As soon as  the audio signal ceases, Ti switches off, and Ci  charges until the potential across it is sufficient to  reset the 555. The monostable may also be reset by  closing Si, which connects pin 6 of the 555 to + 12 V.

 

When IC3 is reset, Ci is discharged via its pin 7. Resistor Rrn serves as protection, because without it Ti could short-circuit the supply lines. When the output of IC3 goes high, T2 conducts,  the relay is energized, and the relay contacts switch on the mains voltage as appropriate. To counter the induced potential when the relay contacts close, which could damage T2, diode Di has been connected in parallel with the relay coil. 

http://www.ecircuitslab.com

Digital Fan Regulator

The circuit presented here can be used to control the speed of  fans using induction motor. The speed control is nonlinear, i.e. in steps. The current step number is displayed on a 7-segment display. Speed can be varied over a wide range because the circuit can alter the voltage applied to the fan motor from 130V to 230V RMS in a maximum of seven steps.  The triac used in the final stage is fired at different angles to get different voltage outputs by applying short-dura-tion current pulses at its gate. For this pur-pose a UJT relax-ation oscillator is used that outputs sawtooth waveform. This waveform is coupled to the gate of the triac through an optocoupler (MOC3011) that has a triac driver output stage. 

Pedestal voltage control is used for varying the firing angle of the triac. The power supply for the relaxation oscillator is derived from the rectified mains via 10-kilo-ohm, 10W series dropping/limit-ing resistor R2.  The pedestal voltage is derived from the non-filtered DC through optocoupler 4N33. The conductivity of the Darlington pair transistors inside this optocoupler is varied for getting the pedestal voltage. For this, the positive sup-ply to the LED inside the optocoupler is connected via different values of resistors using a multiplexer (CD4051). 

Circuitdiagram:

Digital Fan Regulator Circuit Diagram

The value of resistance selected by the multiplexer depends upon the control in-put from BCD up-/down-counter CD4510 (IC5), which, in turn, controls forward bi-asing of the transistor inside optocoupler 4N33. The same BCD outputs from IC5 are also connected to the BCD-to-7-seg-ment decoder to display the step number on a 7-segment display.  NAND gates N3 and N4 are config-ured as an astable multivibrator to produce rectangular clock pulses for IC5, while NAND gates N1 and N2 generate the active-low count enable (CE) input using either of push-to-on switches S1 or S2 for count up or count down operation, respectively, of the BCD counter. 

Optocoupler 4N33 electrically isolates the high-voltage section and the digital section and thus prevents the user from shock hazard when using switches S1 and S2. BCD-to-7-segment decoder CD4543 is used for driving both common-cathode and common-anode 7-segment displays. If phase input pin 6 is ‘high’ the decoder works as a common-anode decoder, and if phase input pin 6 is ‘low’ it acts as a common-cathode decoder.  Optocoupler 4N33 may still conduct slightly even when the display is zero, i.e. pin 13 (X0, at ground level) is switched  output pin 3. To avoid this problem, adjust preset VR1 as required using a plastic-handled screwdriver to get no output at zero reading in the display. 

http://www.ecircuitslab.com/2011/10/digital-fan-regulator.html

Automatic TV Lighting Switch

The author is the happy owner of a television set with built-in Ambilight lighting in the living room. Unfortunately, the television set in  the bedroom lacks this feature. To make up for this, the author attached a small lamp to the wall to provide background lighting, This makes  watching television a good deal more enjoyable, but it ’s  not the ideal solution. Although the TV set can be  switched off with the remote  control, you still have to get out of bed to switch off the lamp.

Circuit diagram :

Automatic TV Lighting Switch Circuit Diagram

Consequently, the author devised this automatic lighting switch that switches the background light on and off along with the T V set. The entire circuit is fitted in series with the mains cable of the TV set, so there’s no need to tinker with the set. It works as follows: R1 senses  the current drawn by the TV  set. It has a maximum value  of 50 mA in standby mode,  rising  to around   500 m A  when  the  set  is  operating. The voltage across R1 is limited by D5 during negative  half- cycles  and  by  D1– D4  during positive half-cycles.  T he  voltage  across  these  four diodes charges capacitor C1 via D6 during positive  half-cycles. This voltage drives the internal LED of solid-state switch TRI1 via R2, which causes the internal triac to conduct and pass the mains voltage to the lamp.   Diode D7 is not absolutely necessary, but  it is recommended because the LED in the  solid-state switch is not especially robust  and cannot handle reverse polarisation. Fuse  F1 protects the solid-state switch against  overloads. T he  value  of  use d  here  (10 Ω)  for  resistor R1 works nicely with an 82-cm (32 inch)  LCD screen.

With smaller sets having lower  power consumption, the value of R1 can be  increased to 22 or 33 Ω, in which case you  should use a 3-watt type. Avoid using an  excessively high resistance, as otherwise TRI1 will switch on when the TV set is in standby mode.  Some TV sets have a half-wave rectifier in the  power supply, which places an unbalanced  load on the AC power outlet. If the set only  draws current on negative half-cycles, the cir-cuit won’t work properly. In countries with  reversible AC power plugs you can correct  the problem by simply reversing the plug. Compared with normal triacs, optically cou-pled solid-state relays have poor resistance  to high switch-on currents (inrush currents).

For this reason, you should be careful with  older-model TV sets with picture tubes (due  to demagnetisation circuits). If the relay fails,  it usually fails shorted, with the result that the TV background light remains on all the time. If you build this circuit on a piece of perf-board, you must remove all the copper next  to conductors and components carrying  mains voltage. Use PCB terminal blocks with a spacing of 7.5 mm. This way the separation between the connections on the solder  side will also be 3 mm. If you fit the entire  arrangement as a Class II device, all parts of  the circuit at mains potential must have a  separation of at least 6 mm from any metal  enclosure or electrically conductive exterior  parts that can be touched.

Author :Piet Germing - Copyright : Elektor