Thursday, October 10, 2013

Polarity Reverser

There are systems in which it is imperative that the supply voltage of, say, a motor, always has the correct polarity. It is, of course, possible to use a bridge rectifier for this, but if large currents are involved, this is not always possible. This may be because large voltage drops across diodes result in appreciable heat dissipation, or that the peak current exceeds the current rating of a diode. Fortunately, a good, inexpensive mechanical rectifier may be constructed with the aid of a relay. In the diagram, the supply voltage is applied to K1, while the motor that needs a supply with correct polarity is linked to K2. Provided fuse F1 is intact, a positive potential at terminal a of K1 will be applied to the positive terminal of K2. Diode D2 prevents the relay being energized.

Polarity Reverser Circuit DiagramWhen the polarity at K1 is reversed, the relay will be energized via D2. The relay contacts then interchange the connections to the terminals of K2 to ensure that the previous polarity of the supply to the load is retained. Diode D1 is a freewheeling diode for the relay coil. The type of relay to be used depends on the requisite operating voltage and the current through its contacts. Other parts of the circuit are not critical. It stands to reason that the circuit is not suitable for use with a small battery, since the relay coil draws a fairly large current.
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Wednesday, October 9, 2013

Over Voltage Protection

When a sensitive circuit must under no circumstances have too high a supply voltage applied, then some means of disconnecting the supply must be provided. One way to achieve this is to trigger a thyristor to blow a fuse. A less destructive alternative possibility is to use a MOSFET to disconnect the supply. An over-voltage protection IC, the LTC1696 from Linear Technology (www.linear-tech.com), has recently become available, which is suitable for triggering and driving such a device. It operates from a power supply in the range 2.7 V to 27 V and can be connected to the unregulated input of a voltage regulator. Two voltages can be monitored using feedback pins FB1 and FB2, suitably divided down using potential dividers.

The trigger threshold for both FB1 and FB2 is +0.88V. The value of the upper resistor in the potential divider can be calculated using the following formula: R1 = 33 kΩ× [(VLIMIT – 0.88 V)/0.88 V] The value of the capacitor connected to the TIMER/RESET pin sets the delay before the protection is triggered. The charging current for this capacitor depends non-linearly on the amount by which the voltage exceeds the threshold value. The greater the over-voltage, the faster the IC triggers. Once triggered the IC remains in that state until either the input voltage is removed or the internal latch is cleared using the MOSFET connected to the TIMER/RESET input.
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Tuesday, October 8, 2013

Christmas Star


We hope this circuit diagram will help you for your Christmas tree. This is not a difficult circuit but we do not recommend this for kids as it requires two voltages, 230 Volts and 9 Volts.  There are two ICs and one Diac do the major role in this circuits.

When you switch on the L1 bulb gradually increases its brightness. After it comes to the maximum brightness the bulb starts decrease the brightness gradually. This will never stop until you switch off the circuit as the cycle repeats.

You can change the brightness of the bulb by adjusting the VR1, but be careful when the circuit is connected to the live current. Make sure you use the same value and the ratings of the capacitor for the C3. Do not use low voltage of capacitor for it. The brightness of the bulb depends on the C3 capacitor’s charge and discharge.


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Monday, October 7, 2013

220V AC Lamp Toggle Switch

Compact, transformer less circuitry No relays employed

Due to the low current drawing, the circuit can be supplied from 230Vac mains without a transformer. Supply voltage is reduced to 12Vdc by means of C1 reactance, a two diode rectifier cell D1 & D2 and Zener diode D3. IC1A, IC1B, R2, R3 and C3 form a reliable bounce-free toggle switch operated by P1. R4 and C4, wired to pin #6 of IC1B reset the circuit (lamp off) when power supply is applied. IC1C and IC1D wired in parallel act as a buffer, driving the Gate of the Triac through R5.

Circuit diagram:

220v AC Lamp Toggle Switch 220V AC Lamp Toggle Switch Circuit Diagram

Parts:
R1 = 470R
R2 = 10K
R3 = 100K
R4 = 100K
R5 = 1K
C1 = 330nF-400V
C2 = 100uF-25V
C3 = 100nF-63V
C4 = 10uF-25V
D1 = 1N4007
D2 = 1N4007
D3 = BZX79C12
D4 = TIC206M
IC1 = 4011 NAND Gate

Notes:

  • The circuit can be wired permanently to the mains supply as current drain is negligible.
  • Due to transformerless design there is no heat generation.
  • Low Gate-current Triacs are recommended.
  • Obviously, other appliances can be powered in place of a lamp, provided their power dissipation does not exceed about 400W @ 230V
  • 110-120Vac operation is easily obtained by simply changing C1 value to 680nF 250V. No further changes are necessary.
  • In some cases, e.g. when the controlled device is far from the toggle switch, a pilot LED could be necessary for monitoring purposes. If so, disconnect pin #10 of IC1C from pin #11 of IC1D and wire a LED and its 1K series current limiting resistor across pin #10 of IC1C and negative supply.
  • Warning! The circuit is connected to 230Vac mains, so some parts in the circuit board are subjected to lethal potential! Avoid touching the circuit when plugged in and enclose it in a plastic box.
  • P1 will SPST Pushbutton

Source :www.extremecircuits.net

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Sunday, October 6, 2013

Amplifier Timer Circuit Diagram

Turns-off your amplifier when idle for 15 minutes, Fed by amplifier tape-output

This circuit turns-off an amplifier or any other device when a low level audio signal fed to its input is absent for 15 minutes at least. Pushing P1 the device is switched-on feeding any appliance connected to SK1. Input audio signal is boosted and squared by IC2A & IC2B and monitored by LED D4. When D4 illuminates, albeit for a very short peak, IC3 is reset and restarts its counting. Pin 2 of IC3 remains in the low state, the two transistors are on and the relay operates. When, after a 15 minutes delay, no signal appeared at the input, IC3 ends its counting and pin 2 goes high. Q1 & Q2 stop conducting and the relay switches-off. The device is thus completely off as also are the appliances connected to SK1. C5 & R9 reset IC3 at power-on. P2 allows switch-off at any moment.

Parts:

R1,R8___________1K 1/4W Resistors
R2,R3___________4K7 1/4W Resistors
R4_____________22K 1/4W Resistor
R5______________4M7 1/4W Resistor
R6,R9__________10K 1/4W Resistors
R7______________1M5 1/4W Resistor
R10___________100K 1/4W Resistor
R11____________15K 1/4W Resistor
R12____________10M 1/4W Resistor
R13_____________1M 1/4W Resistor
R14_____________8K2 1/4W Resistor
R15_____________1K8 1/4W Resistor
C1____________470µF 25V Electrolytic Capacitor
C2,C3,C6______100nF 63V Polyester Capacitors
C4,C5__________10µF 25V Electrolytic Capacitors
D1_____Diode bridge 100V 1A
D2,D7________1N4002 100V 1A Diodes
D3__________Red LED 5mm.
D4_______Yellow LED 5mm.
D5,D6________1N4148 75V 150mA Diodes
IC1___________78L12 12V 100mA Voltage regulator IC
IC2___________LM358 Low Power Dual Op-amp
IC3____________4060 14 stage ripple counter and oscillator IC
Q1____________BC557 45V 100mA PNP Transistor
Q2____________BC337 45V 800mA NPN Transistor
J1______________RCA audio input socket
P1_____________SPST Mains suited Pushbutton
P2_____________SPST Pushbutton
T1_____________220V Primary, 12V Secondary 3VA Mains transformer
RL1___________10.5V 270 Ohm Relay with SPST 5A 220V switch
PL1____________Male Mains plug
SK1__________Female Mains socket

Notes:
  • Simply connect left or right channel tape output of your amplifier to J1.
  • You can employ two RCA input sockets wired in parallel to allow pick-up audio signals from both stereo channels.
  • The delay time can be varied changing R13 and/or C6 values.
  • Needing to operate a device not supplied by power mains, use a double pole relay switch, connecting the second pole switch in series to the device supply.
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Saturday, October 5, 2013

Logic Probe With Sound Circuit

This logic probe can be selected to operate on TTL or CMOS logic levels, depending on switch S1. A string of resistors associated with switch S1 sets the threshold levels for a window comparator comprising IC1a and IC1b. Depending on whether the level applied to the probe is high or low, the window comparator turns on LED1 (high) or LED2 (low). The 1.2M and 680k resistors set the probe signal to a midrange value when the probe is open-circuit, thereby preventing either LED from being lit.

Circuit diagram:

logic-probe-with-sound-circuit-diagram1 Logic Probe With Sound Circuit digram

If a pulse signal is present, the output of IC1a will toggle the clock input of flipflop IC2a. This drives LED3 which either lights for each pulse or continuously, depending on the setting of switch S2. Finally, the outputs of IC1a & IC1b are connected by diodes D5 & D6 to the base of transistor Q1 which is connected to the Reset input of flipflop IC2b. This has a piezo sounder (not buzzer) connected between its Q and Q-bar outputs so that it produces a sound which echoes the input pulse signal.

Author: Tom Hughes Copyright: Silicon Chip Electronics

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Friday, October 4, 2013

Aviation Intercom Circuit

Before its move offshore, I was lucky enough to be involved in developing the avionics system for the Flightship Ground Effect FS8 craft (see www.pacificseaflight.com/craft.shtml). Although officially classed as a boat, it has wings and can travel at 180km/h some three metres above the water. The communications system was adapted from an aircraft unit and was a particular problem. It was expected to allow speech between the two pilots and radio, as well as receive audible warnings from the onboard computers and feed sound to the onboard data logger. Initially, the system was very noisy due to ground loops and incompatibility problems.

A circuit similar to that shown here was the solution. Although optimised to suit Softcom brand headphones with active noise reduction, it should be suitable for most aviation sets. The plugs indicated are standard aviation types but are insulated from the instrument panel to eliminate earth loops. The inputs from the two pilots microphones are summed and amplified by transistors Q1 & Q2. When one pilot presses his or her transmit key (mounted on the yoke), the transmit relay (RLY1) closes, muting the other pilot’s microphone via the optocoupler (OPTO1).

Circuit diagram:

aviation-intercom-circuit-diagram Aviation Intercom Circuit Diagram

The outputs from the microphone preamp, computer audio transformer (T1) and radio speaker transformer (T2) are summed via 10kΩ resistors and applied to the input of IC1, an LM386 audio amplifier. Note that transformers are used here to avoid creating additional earth loops. The output of the LM386 drives the pilots’ headphones via transformers T3 & T4, which are needed for impedance matching. Each audio source has its own level control (VR1, VR3 & VR4). The main volume control (VR5) is included to allow for ambient noise level. VR2 is used to set the signal level for the data logger.

Author: Gary Smith Copyright: Silicon Chip Electronics

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Thursday, October 3, 2013

Monitor Life Extender

This circuit was designed to protect a computer monitor from overheating. It is recommended to attach this circuit to power users’ monitors! Most computer monitors of the CRT type fail owing to over-heating. After one or two hours of use, the rear of a monitor may become as hot as 45 degrees C, or 20 degrees above ambient temperature. Most heat comes from the VGA gun drivers, the horizontal circuit, vertical circuit and power supply. The best possible way to extract heat and so prolong monitor life (and save the environment) is to add a brushless fan, which is lighter, energy-wiser and more efficient than a normal fan.

In the diagram, diodes D2, D3 and D4 sense the monitor’s temperature. These diodes have a total negative temperature coefficient of 6 mV per degree Celsius. To eliminate noise, shielded wire should be used for the connection of the temperature sensor to the circuit sensor. The +12-V supply voltage is borrowed from the computer’s power supply. Alternatively, a mains adapter with an output of 12 VDC may be used. C1 and C2 are decoupling capacitors to eliminate the ripple developed by switching or oscillation. R1 provides bias current to D1, a 6-V zener diode acting as a reference on the non-inverting pin of opamp IC2.B.

Monitor Life extender circuit schematic

IC1, a ‘precision shunt regulator’ raises the sensor diodes’ voltage to just over 6 V depending on the adjustment of P1. C4 is the decoupling capacitor with the sensor network. Integrator network R4-C5 provides a delay of about 3 seconds, transforming the on/off output signal of IC2.B into an exponentially decreasing or increasing voltage. This voltage is fed to pin 3 of the second opamp, IC2.A. The hard on/off technique would produce a good amount of noise whenever the load is switched, hence an alternative had to be found. IC3, a TLC555, is used as an astable multivibrator with R5 and C6 controlling the charging network that creates a sawtooth voltage with a frequency of about 170 Hz.

This sawtooth is coupled to pin 2 of IC2.A, which compares the two voltages at its input pins and produces a PWM (pulse-width modulated) output voltage. The sawtooth wave is essential to the PWM signal fed to power output driver T1 by way of stopper resistor R6. The power FET will switch the fan on and off fan according to the PWM drive signal. The back emf pulses that occur when T1 switches on and off are clamped by a high-speed diode, D7. Initially, turn P1 to maximum resistance. Blow hot air from a hair dryer onto the sensor-diodes for a minute or so, then get the temperature meter near the sensor diodes and adjust P1 slowly towards the minimum resistance position with a digital meter hooked up on pin 7 of IC2.B. Roughly calibrate the temperature to 40 degrees C. At this temperature, the meter will show approximately 12V. The circuit will draw about 120mA from its 12-V supply.
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Wednesday, October 2, 2013

Rear Light After Glow For Bicycles

This article is of interest only to readers whose bicycle lights are powered by a dynamo. The laws on bicycle lights in the United Kingdom are stricter than in other countries and a dynamo is, therefore, a rarity in this country. From the point of view of traffic safety it is advisable (in UK obligatory) for cyclists to have the rear lamp of their bicycle to light even when they are at standstill. In principle, it is not very difficult to modify the existing rear light with afterglow: all this needs is a large enough energy reservoir. Since the after-glow is required for short periods of time only, a battery is not required: a large value capacitor, say, 1 F, is quite sufficient.

As the diagram shows, in the present circuit, the normal rear light bulb is replaced by two series-connected bright LEDs, D2 and D3. These are clearly visible with a current of only 6 mA (compared with 50 mA of the bulb). The current is set with series resistor R1. The LEDs are shunted by the 1 F capacitor, C1. Since the working voltage of this component is only 5.5 V, it is, in spite of its high value, physically small. An effective regulator is needed to limit the dynamo voltage adequately. Normal regulators cannot be used here, since they do not work at low voltages. Moreover, such a device would discharge the capacitor when the cycle is at standstill.

Rear Light After GlowFortunately, there is a low-drop type that meets the present requirements nicely: the Type LP2950CZ5.0. Of course, the dynamo output voltage needs to be rectified before it can be applied to the regulator. In the present circuit, this is effected by half-wave rectifier D1 and buffer capacitor C2. Diode D1 is a Schottky type to keep any losses low – important for this application, because the ground connection via the bicycle frame usually causes some losses as well. The value of buffer capacitor has been chosen well above requirements to ensure that C1 is charged during the negative half cycles of the dynamo voltage.
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Tuesday, October 1, 2013

Call Acknowledged !

This circuit could be used (depending on your circumstances) by a gentleman to summon his butler, a manager his secretary or as in the author’s case to call the kids down to dinner without having to shout above the level of the CD player/TV/games console in their bedroom. Rather than resorting to a full-blown intercom system, a simpler solution was envisaged and while a buzzer could easily fulfil this function, this circuit has the advantage of providing a visual indication of a call as well as confirming to the caller that the ‘message’ has been received.

This is especially useful in the latter case, as the call may be easily drowned out by the music playing in the headphones. The circuit, which requires no complicated switching, uses a simple two-wire connection between the two stations and utilises the fact that the forward voltage drop of a blue (or white) LED is greater than that of a red, green or yellow one. The circuit is based on a two-transistor multivibrator which is used to pulse a red LED (D3) as well as the buzzer Bz1 on and off at about 1.5 Hz when push button S1 is closed. This frequency may of course be altered if required by changing the values of the capacitors.

Call Acknowledged circuit schematic

The diode D1 in series with the collector of transistor T2 is required to isolate the output from the effects of the buzzer circuitry, which would alter the multivibrator frequency. In principle, the multivibrator could be dispensed with but a pulsed buzzer/flashing led is much more noticeable than a continuous signal especially in noisy conditions. Since the voltage across a red LED is typically about 1.5 V while a blue LED requires at least 2.5 V to 3 V to light, the blue LED will remain off when the call button S1 is pressed. Despite being rated for operation at 3-12 V, most piezo sounders can still produce a piercing sound from the pulsed 1.5-V available across the red LED which should get the attention of even the most preoccupied teenager.

When the recipient presses the acknowledge (push to break) switch S2, the red LED/buzzer are disconnected allowing the blue LED to flash at the sending station indicating to the caller that his call has been received. Alternatively, if a blue LED is not available, a red or green type in series with a forward biased silicon diode to raise its forward voltage above that of the red LED in the receiver could be used instead. The circuit may be powered by a 9-V battery, a mains power supply being unnecessary in view of the low power consumption and infrequency of use of the circuit.
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