IR–S/PDIF Receiver Circuit

This simple circuit proves to achieve surprisingly good results when used with the IR–S/PDIF transmitter described elsewhere in this site. The IR receiver consists of nothing more than a photodiode, a FET and three inverter gates used as amplifier. The FET is used as an input amplifier and filter, due to its low parasitic capacitance. This allows R1 to have a relatively high resistance, which increases the sensitivity of the receiver. The bandwidth is primarily determined by photo-diode D1, and with a value of 2k2 for R1, it is always greater than 20 MHz. The operating current of the FET is intentionally set rather high (around 10 mA) using R2, which also serves to ensure adequate bandwidth. The voltage across R2 is approximately 0.28–0.29 V. The combination of L1 and R3 forms a high-pass filter that allows signals above 1 MHz to pass. L1 is a standard noise-suppression choke. From this filter, the signal is fed to two inverters configured as amplifiers. The third and final inverter (IC1c) generates a logic-level signal. This 74HCU04 provides so much gain that there is a large risk of oscillation, particularly when the final stage is loaded with a 75-Ω coaxial cable.
In case of problems (which will depend heavily on the construction), it may be beneficial to add a separate, decoupled buffer stage for the output, which will also allow the proper output impedance (75 Ω) to be maintained in order to prevent any reflections. When building the circuit, make sure that the currents from IC1 do not flow through the ground path for T1. If necessary, use two separate ground planes and local decoupling. Furthermore, the circuit must be regarded as a high-frequency design, so it’s a good idea to provide the best possible screening between the input and the output. With the component values shown in the schematic, the range is around 1.2 metres without anything extra, which is not especially large. However, the range can easily be extended by using a small positive lens (as is commonly done with standard IRDA modules). In our experiments, we used an inexpensive magnifying glass, and once we got the photodiode positioned at the focus after a bit of adjustment.
Circuit diagram :
IR–S PDIF Receiver Circuit Diagram
We were able to achieve a range of 9 metres using the same transmitter (with a sampling frequency of 44.1 kHz). This does require the transmitter and receiver to be physically well aligned to each other. As you can see, a bit of experimenting certainly pays off here! It may also be possible to try other types of photodiode. The HDSL-5420 indicated in the schematic has a dome lens, but there is a similar model with a flat-top case (HDSL-5400). It has an acceptance angle of 110°, and with the same level of illumination, it generates nearly four times as much current.
The current consumption of the circuit is 43 mA with no signal and approximately 26 mA with a signal (fs = 44.1 kHz) That is rather high for battery operation, but it can handled quite readily using a pair of rechargeable NiMH cells. Incidentally, the circuit will also work at 4.5 V and even 3 V. If a logic-level output is needed, C3 at the output can be replaced by a jumper. Finally, there is one other thing worth mentioning. With the HSDL-5400 that we had to play with, the cathode marking (a dark-blue line on the side below one lead) was on the wrong side (!). So if you want to be sure that the diode is fitted properly, it’s a good idea to measure the DC voltage across R1, which should be practically zero.
Author: T. Giesberts - Copyright: Elektor Electronics
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Dual Power Supply 78xx-79xx

Many times the hobbyist wants to have a simple, dual power supply for a project. Existing powersupplies may be too big either in power output or physical size. Just a simple Dual Power Supply is required.For most non-critical applications the best and simplest choice for a voltage regulator is the 3-terminal type.The 3 terminals are input, ground and output.
The 78xx & 79xx series can provide up to 1A load current and it have onchip circuitry to prevent damage in the event of over heating or excessive current. That is, the chip simply shuts down rather than blowing out. These regulators are inexpensive, easy to use, and they make it practical to design a system with many PCBs in which an unregulated supply is brought in and regulation is done locally on each circuit board.
Circuit diagram:
Dual_Power_Supply_Schematic Circuit diagram
This Dual Power Supply project provides a dual power supply. With the appropriate choice of transformer and 3-terminal voltageregulator pairs you can easily build a small power supply delivering up to one amp at +/- 5V, +/- 9V, +/- 12V, +/-15V or +/-18V. You have to provide the centre tapped transformer and the 3-terminal pair of regulators you want:7805 & 7905, 7809 & 7909, 7812 & 7912, 7815 & 7915or 7818 & 7918.
Note that the + and - regulators do not have to be matched: you can for example, use a +5v and -9V pair. However,the positive regulator must be a 78xx regulator, and the negative a 79xx one. We have built in plenty of safety into this project so it should give many years of continuous service.  The user must choose the pair he needs for his particular application.
Parts :
Dual_Power_Supply_Parts list
This Dual Power Supply design uses a full wave bridge rectifier coupled with a centre-tapped transformer. A transformer with a power output rated at at least 7VA should be used. The 7VA rating means that the maximum current which can be delivered without overheating will be around 390mA for the 9V+9V tap; 290mA for the 12V+12V and 230mA for the 15V+15V. If the transformer is rated by output RMS-current then the value should be divided by 1.2 to get the current which can be supplied. For example, in this case a 1A RMS can deliver 1/(1.2) or 830mA.
We use an epoxy-packaged 4 amp bridge rectifier with at least a peak reverse voltage of 200V. (Note the part numbers of bridge rectifiers are not standardised so the number are different from different manufacturers.) For safety the diode voltage rating should be at least three to four times that of the transformers secondary voltage. The current rating of the diodes should be twice the maximum load current that will be drawn.
Filter Capacitor
The purpose of the filter capacitor is to smooth out the ripple in the rectified AC voltage. Theresidual amount of ripple is determined by the value of the filer capacitor: the larger the value the smaller the ripple.The 2,200uF is a suitable value for all the voltages generated using this project. The other consideration inchoosing the correct capacitor is its voltage rating. The working voltage of the capacitor has to be greater than thepeak output voltage of the rectifier. For an 18V supply the peak output voltage is 1.4 x 18V, or 25V. So we havechosen a 35V rated capacitor.
The unregulated input voltage must always be higher than the regulators output voltage by at least 3V inorder for it to work. If the input/output voltage difference is greater than 3V then the excess potential must bedissipated as heat. Without a heatsink 3 terminal regulators candissipate about 2 watts. A simple calculation of the voltage differential times the current drawn will give the watts tobe dissipated. Over 2 watts a heatsink must be provided. If not then the regulator will automatically turn off if theinternal temperature reaches 150oC. For safety it is always best to use a small heatsink even if you do not think youwill need one.
C4 & C5 improve the regulators ability to react to sudden changes in load current and to preventuncontrolled oscillations.
The monoblok capacitor C2 & C6 across the output provides high frequency decoupling which keepsthe impedence low at high frequencies.
Two LED's are provided to show when the output regulated power is on-line. You do not have to use theLED's if you do not want to. However, the LED on the negative side of the circuit does provide a minimum load tothe 79xx regulator which we found necessary during testing. The negative 3-pin regulators did not like a zeroloadsituation. We have provided a 470R/0.5W resistors as the current limiting resistors for the LED's.
Diode Protection
These protect mainly against any back emf which may come back into the power supply when itsupplies power to inductive loads. They also provide additional short circuit protection in the case that thepositive output is connected by accident to the negative output. If this happened the usual current limiting shutdownin each regulator may not work as intended. The diodes will short circuit in this case and protect the 2 regulators.
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USB Converter

Does this sound familiar: you buy a small piece of equipment, such as a programming & debugging interface for a microcontroller, and you have to use a clunky AC wall adapter to supply it with power? It’s even worse when you’re travelling and there’s no mains socket anywhere in sight. Of course, you can use the USB bus directly as a power source if the supply voltage is 5 V. If you need a higher voltage, you can use the USB converter described here. This small switch-mode step-up converter can generate an output voltage of up to 15 V with a maximum output current of 150 mA.
USB Converter
The LM3578 is a general-purpose switchmode voltage converter. Figure 1 shows its internal block diagram. Here we use it as a step-up converter. The circuit diagram in Figure 2 shows the necessary components. Voltage conversion is achieved by switching on the internal transistor until it is switched off by the comparator or the current-limiting circuit. The collector current flows through coil L1, which stores energy in the form of a magnetic field. When the internal transistor is switched off, the current continues flowing through L1 to the load via diode D1. However, the voltage across the coil reverses when this happens, so it is added to the input voltage. The resulting output voltage thus consists of the sum of the input voltage and the induced voltage across the coil.

USB Converter Circuit
The output voltage depends on the load current and the duty cycle of the internal transistor. Voltage divider R5/R6 feeds back a portion of the output voltage to the comparator in the IC in order to regulate the output voltage. C5 determines the clock frequency, which is approximately 55 kHz. Network R4, C2 and C3 provides loop compensation. The current-sense resistor for the current-limiting circuit is formed by three 1-Ω resistors in parallel (R1, R2 and R3), since SMD resistors with values less than 1 Ω are hard to find. The output voltage ripple is determined by the values and internal resistances of capacitors C11, C8, C7 and C6.
  USB Converter Circuit Diagram

The total effective resistance is reduced by using several capacitors, and this also keeps the construction height of the board low. L2, C1, C9 and C10 act as an input filter. Ensure that the DC resistance of coil L2 is no more than 0.5 Ω. Use a Type B PCB-mount USB connector for connection to the USB bus.  A terminal strip with a pitch of 5.08 mm can be used for the output voltage connector. Of course, you can also solder a cable directly to the board. Two additional holes are provided in the circuit board for this purpose. As we haven’t been able to invent a device that produces more energy than it consumes, you should bear in mind that the input current of the circuit is higher than the output current. As a general rule, you can assume that the input current is equal to the product of the output current and the output voltage divided by the input R5 and R6 for other output voltages:
6V: R5 = 47k, R6 = 9,1k
12V: R5 = 110k, R6 = 10k
15V: R5 = 130k, R6 = 9,1k
voltage and divided again by 0.8. Specifically, with an output current of 100 mA at 9 V, the input current on the USB bus is approximately 225 mA. Finally, Figure 3 shows a small PCB layout for the circuit. All of the components except the connector and the terminal strip are SMDs.
USB Converter pcb
Parts List:
(for UO = 9 V)
R1,R2,R3 = 1Ω
R4 = 220kΩ
R5 = 82kΩ
R6 = 10kΩ
(SMD 1206)
C1 = 100nF
C2 = 2nF2
C3 = 22pF
C4 = 100nF
C5 = 1nF5
(tantalum SMD 7343)
C6 = 68μF 20V
C7 = 68μF 20V
C8 = 68μF 20V
C9 = 47μF 16V
C10 = 47μF 16V
C11 = 68μF 20V
L1 = 820μH (SMD CD105)
L2 = 47μH (SMD 2220)
D1 = SK34SMD (Schottky)
IC1 = LM3578AM (SMD SO8)
K1 = 2-way PCB terminal block, lead pitch 5mm
K2 = USB-B connector

PCB layout, free download from Elektor website, 070119-1.pdf
Author : Jörg Schnyder  copyright : Elektor
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RC (Remote Control) Switch

It is sometimes necessary for an RC (remote control) model to contain some kind of switching functionality. Some things that come to mind are lights on a model boat, or the folding away of the undercarriage of an aeroplane, etc. A standard solution employs a servo, which then actually operates the switch. Separate modules are also available, which may or may not contain a relay. A device with such functionality is eminently suitable for building yourself. The schematic shows that it can be easily realised with a few standard components.
Picture of the project:
The servo signal, which consists of pulses from 1 to 2 ms duration, depending on the desired position, enters the circuit via pin 1 of connector K1. Two buffers from IC2 provide the necessary buffering after which the signal is differentiated by C2. This has the effect that at each rising edge a negative start signal is presented to pin 2 of IC1. D1 and R4 make sure that at the falling edge the voltage at pin 2 of IC2 does not become too high. IC1 (TLC555) is an old faithful in a CMOS version. A standard version (such as the NE555) works just as well, but this IC draws an unnecessarily high current, while we strive to keep the current consumption as low as possible in the model. The aforementioned 555 is configured as a one-shot. The pulse-duration depends on the combination of R2/C1. Lowering the voltage on pin 5 also affects the time. This results in reducing the length of the pulse. In this circuit the pulse at the output of IC will last just over 1.5 ms when T1 does not conduct.
Circuit diagram:
When T1 does conduct, the duration will be a little shorter than 1.5 ms. We will explain the purpose of this a little later on. Via IC2.C, the fixed-length pulse is, presented to the clock input of a D-flip-flop. As a consequence, the flip-flip will remember the state of the input (servo signal). The result is that when the servo-pulse is longer than the pulse form the 555, output Q will be high, otherwise the output will be low. It is possible, in practice, that the servo signal is nearly the same length as the output from the 555. A small amount of variation in the servo signal could therefore easily cause the output to ‘chatter’, that is, the output could be high at one time and low the next. To prevent this chatter there is feedback in the form of R1, R3 and T1. This circuit makes sure that when the flip-flip has decided that the servo-pulse is longer than the 555’s pulse (and signals this by making output Q high), the pulse duration from the 555 is made a little shorter. The length of the servo-signal will now have to be reduced by a reasonable amount before the servo-pulse becomes shorter than the 555’s pulse.
Parts and PCB layout:
The moment this happens, T1 will stop conducting and the mono-stable time will become a little longer. The servo-pulse will now have to be longer by a reasonable amount before the flip-flip changes back again. This principle is called hysteresis. Jumper JP1 lets you choose between the normal or inverted output signals. Buffers IC2.D through to IC2.F together with R5 drive output transistor T2, which in turn drives the output. Note that the load may draw a maximum current of 100 mA. Diode D2 has been added so that inductive loads can be switched as well (for example, electrically operated pneu-matic valves).
Resistors: R1 = 470k
R2 = 150k
R3 = 47k
R4 = 100k
R5 = 4k7
C1 = 10nF
C2 = 1nF
C3,C4 = 100nF
D1 = BAT85 or similar Schottky diode
D2 = 1N4148
IC1 = CMOS 555 (e.g., TLC555 or ICM7555)
IC2 = 4049
IC3 = 4013
T1,T2 = BC547B
Miscellaneous: JP1 = jumper with 3-way pinheader
K1 = servo cable
K2 = 2-way pinheader or 2 solder pins
Author: Paul Goossens - Copyright: Elektor Electronics
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LCD Module in 4-bit Mode

In many projects use is made of alphanumeric LCDs that are driven internally by Hitachi’s industry-standard HD44780 controller. These displays can be driven either in 4-bit or 8-bit mode. In the first case only the high nibble (D4 to D7) of the display’s data bus is used. The four unused connections still deserve some closer attention. The data lines can be used as either inputs or outputs for the display. It is well known that an unloaded output is fine, but that a floating high-impedance input can cause problems. So what should you do with the four unused data lines when the display is used in 4-bit mode? This question arose when a circuit was submitted to us where D0-D3 where tied directly to GND (the same applies if it was to +5 V) to stop the problem of floating inputs.
The LCD module was driven directly by a microcontroller, which was on a development board for testing various programs and I/O functions. There was a switch present for turning off the enable of the display when it wasn’t being used, but this could be forgotten during some experiments. When the R/Wline of the display is permanently tied to GND (data only goes from the microcontroller to the display) then the remaining lines can safely be connected to the supply (+ve or GND). In this application however, the R/Wline was also controlled by the microcontroller. When the display is initialised correctly then nothing much should go wrong. The data sheet for the HD44780 is not very clear as to what happens with the low nibble during initialisation.
Circuit diagram :
LCD module_in_4-bit_Mode_Circuit_Diagramw
After the power-on reset the display will always be in 8-bit mode. A simple experiment (see the accompanying circuit) reveals that it is safer to use pull-down resistors to GND for the four low data lines. The data lines of the display are configured as outputs in this circuit (R/Wis high) and the ‘enable’ is toggled (which can still happen, even though it is not the intention to communicate with the display). Note that in practice the RS line will also be driven by an I/O pin, and in our circuit the R/W line as well. All data lines become high and it’s not certain if (and if so, for how long) the display can survive with four shorted data lines. The moral of the story is: in 4-bit mode you should always tie D0-D3 via resistors to ground or positive.
Author: L. Lemmens - Copyright: Elektor Electronics
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Direction Sensitive Light Barrier

With two light barriers closely positioned one after the other it is possible to establish in which direction they have been crossed. If, for example, you place it at the entrance of the toilet then you can use it to control the lights: on when entering and off when leaving the room. The circuit for this has many similarities with the modulated light barrier appearing else-where in this Summer Circuits issue. There are two ways to position the light barriers, namely a completely duplicated installation in opposing directions (this to prevent mutual interference) and a version with one IR transmitter and two receivers.
Block diagram:
Both types of installation are shown here, which one is most suitable depends on the actual application. When used in a doorway, one transmitter is sufficient if the receivers are placed about 5 cm apart. With a wider passage, an installation with two separate IR-transmitters is a better solution. This circuit has a range of several meters, even if the sun shines directly on the receiver! We use the exact same IR-transmitter(s) as for the modulated light barrier. For the installation with two separate IR-transmitters it is sufficient to duplicate R6, T1, D1, C3 and R7 from the circuit of the modulated light barrier.
Circuit diagram:
Output OUT (pin 3) of IC2 can drive two of these IR-drivers without any difficulty. The receivers are slightly different than those of the modulated light barrier and the circuit is the same for both types of installation. We again use the TSOP1736, which is sensitive to IR-light that is modulated at a frequency of 36 kHz. D2, R8 and C4 ensure that the received pulses from IC3 at the output of IC5a result in a ‘1’ when the beam is not interrupted. When the beam is interrupted this output will become a ‘0’ within about 1 ms. In the same way IC5b generates a ‘0’ when IC4 stops receiving IR-light. The 4013 CMOS-IC used here contains two D-flipflops, of which we use only one. The instant that light barrier 2 (IC4) is unblocked again, is used to clock the state of light barrier 1 (IC3) through to output Q1. This signal drives the relay via T2, which operates the light in the room. The circuit therefore turns the light on or off the moment that light barrier 1 is uninterrupted.
Author: Heino Peters - Copyright: Elektor Electronics Magazine
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Laser-Guided Door Opener

This automatic door opener can be made using readily available components. The electromagnetic relay at the output of this gadget can be used to control the DC/AC door-opener motor/solenoid of an electromechanical door opener assembly, with slight intervention in its electrical wiring. A laser diode (LED1) is used here as the light transmitter. Alternatively, you can use any available laser pointer. The combination of resistor R1 and diode D1 protects the laser diode from over-current flow. By varying multi-turn trimpot VR1, you can adjust the sensitivity. (Note that ambient light reflections may slightly degrade the performance of this unit.) Initially, when the laser beam is falling on photo-transistor T1, it conducts to reverse-bias transistor T3 and the input to the first gate (N1) of IC1 (CD4001) is low. The high output at pin 3 of gate N1 forward biases the LED-driver transistor (T4) and the green standby LED (LED2) lights up continuously.
Circuit diagram :
Laser-Guided Door Opener Circuit Diagram
The rest of the circuit remains in standby state. When someone interrupts the laser beam, photo-transistor T1 stops conducting and transistor T3 becomes forward-biased. This makes the output of gate N1 go low. Thus LED-driver transistor T4 becomes reverse-biased and LED2 stops glowing. At the same time, the low output of gate N1 makes the output of N2 high. Instantly, this high level at pin 4 of gate N2 triggers the monostable multivibrator built around the remaining two gates of IC1 (N3 and N4). Values of resistor R8 and capacitor C1 determine the time period of the monostable. The second monostable built around IC2 (CD4538) is enabled by the high-going pulse at its input pin 12 through the output of gate N4 of the first monostable when the laser beam is interrupted. As a result, relay RL1 energizes and the door-opener motor starts operating. LED3 glows to indicate that the door-opener motor is getting the supply.
At the same time, piezo-buzzer PZ1 sounds an alert. Transistor T5, whose base is connected to Q output (pin 10) of IC2, is used for driving the relay. Transistor T6, whose base is connected to Q output of IC2, is used for driving the intermittent piezo-buzzer. ‘On’ time of relay RL1 can be adjusted by varying trimpot VR2. Resistor R9, variable resistor VR2 and capacitor C3 decide the time period of the second monostable and through it on time of RL1. The circuit works off 12V DC power supply. Assemble it on a general-purpose PCB. After construction, mount the laser diode and the photo-transistor on opposite sides of the door-frame and align them such that the light beam from the laser diode falls on the photo-transistor directly. The motor connected to the pole of relay contacts is the one used in electromechanical door-opener assembly. If you want to use a DC motor, replace mains AC connection with a DC power supply.
Author : T.K. Hareendran - Copyright :
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