Thursday, October 10, 2013
Liquid Crystal Display LCD Tester
The space between the sheets of glass is filled with a liquid that, stimulated by an electric voltage, alters the polarization of the incident light. In this way, segments may appear light or dark and give rise to the display of lines or shapes. A segment may be tested by applying an alternating voltage of a few volts across it. Note that the application of a direct voltage will damage the display irreversibly: the resulting current will remove the tracks. The alternating voltage should contain not even a tiny direct voltage component. An alternating current also removes part of the tracks when the current flows in one direction, but restores it when the current flows in the opposite direction.
The tester described here consists of a square-wave generator that produces an absolutely symmetrical alternating voltage without any d.c. component. Most logic oscillators are incapable of producing a squarewave signal: they generate rectangular waveforms whose duty cycle hovers around the 50%. The 4047 used in the tester has a binary scaler at its output that guarantees symmetry. The oscillator frequency is about 1 kHz. It may be powered from a 3–9 V source. Normally, this will be a battery, but a variable power supply has advantages. It shows at which voltage the display works satisfactorily and also that there is a clear relationship between the level of the voltage and the angle at which the display is clearly legible.
The tester draws a current not exceeding 1 mA. The test voltage must at all times be connected between the common terminal, that is, the back plane, and one of the segments. If it is not known which of the terminals is the back plane, connect one probe of the tester to a segment and the other successively to all the other terminals until the segment becomes visible. Note, however, that there are LCDs with more than one back plane. Therefore, if a segment does not become visible, investigate whether the display has a second back plane terminal.
Wednesday, October 9, 2013
Mains Slave Switcher II
Tuesday, October 8, 2013
Build A Synchronous Clock
The quartz clocks which have dominated time-keeping for the past 20 years or so have one problem: their errors, although slight, are cumulative. After running for several months the errors can be significant. Sometimes you can correct these if you can slightly tweak the crystal frequency but otherwise you are forced to reset the clock at regular intervals. By contrast, mains-powered synchronous clocks are kept accurate by the 50Hz mains distribution system and they are very reliable, except of course, when a blackout occurs. This circuit converts a quartz clock to synchronous mains operation, so that you can have at least one clock in your home which shows the time. First, you need to obtain a quartz clock movement and disassemble it down to the PC board. For instructions on how to do this, see the article on a "Fast Clock For Railway Modellers" in the December 1996 issue of SILICON CHIP. Then isolate the two wires to the clock coil and solder two light duty insulated hookup wires to them (eg, two strands of rainbow cable). Drill a small hole in the clock case and pass the wires through them. Then reassemble the clock case.
Circuit diagram:
A Synchronous Clock Circuit Diagram
To test the movement, touch the wires to the terminals of an AA cell, then reverse the wires and touch the cell terminals again. The clock second hand should advance on each connection. The circuit is driven by a low voltage AC plug pack. Its AC output is fed to two bridge rectifiers: BR1 provides the DC supply while BR2 provides positive-going pulses at 100Hz to IC1a, a 4093 NAND Schmitt trigger. IC1a squares up the 100Hz pulses and feeds them to the clock input of the cascaded 4017 decade counters. The output at pin 12 of IC3 is 1Hz. This is fed to IC4, a 4013 D-type flipflop, which is connected so that its two outputs at pins 12 & 13 each go positive for one second at a time. As these pulses are too long to drive the clock movement directly, the outputs are each fed to 4093 NAND gates IC1b & IC1c where they are gated with the pin 3 signal to IC4. This results in short pulses from pins 3 & 10 of IC1 which drives the clock via limiting resistor R1. The value of R1 should be selected on test, allowing just enough current to reliably drive the clock movement.
Author: A. J. Lowe - Copyright: Silicon Chip
Monday, October 7, 2013
Antenna Tuning Unit ATU For 27 MHz CB Radios
The internal diameter of the coil is 15 mm, and it is stretched to a length of about 4 cm. The tap for the antenna cable to the CB radio is made at about 2 turns from the cold (ground) side. Two trimmer capacitors are available for tuning the ATU. The smaller one, C1, for fine tuning, and the larger one, C2, for coarse tuning. The trimmers are adjusted with the aid of an in-line SWR (standing-wave ratio) meter which most CB enthusiasts will have, or should be able to obtain on loan. Select channel 20 on the CB rig and set C1 and C3 to mid-travel. Press the PTT button and adjust C2 for the best (that is, lowest) SWR reading. Next, alternately adjust C3 and C2 until you get as close as possible to a 1:1 SWR reading.
C1 may then be tweaked for an even better value. No need to re-adjust the ATU until another antenna is used. In case the length of the wire antenna is exactly 5.5 metres, then C3 is set to maximum capacitance. Although the ATU is designed for half-wavelength or longer antennas, it may also be used for physically shorter antennas. For example, if antenna has a physical length of only 3 metres, then the remaining 2.5 metres has to be wound on a length of PVC tubing. This creates a so-called BLC (base-loaded coil) electrically shortened antenna. In practice, the added coil can be made somewhat shorter than the theoretical value, so the actual length is best determined by trial and error. Finally, the ATU has to be built in an all-metal case to prevent unwanted radiation. The trimmers are than accessed through small holes. The connection to the CB radio is best made using an SO239 (‘UHF’) or BNC style socket on the ATU box and a short 50-W coax cable with matching plugs.
Sunday, October 6, 2013
Speach Amplifier Circuit Diagram
This circuit is intended to be placed in the same box containing the loudspeaker, forming a compact microphone amplifier primarily intended for speech reinforcement. A device of this kind is particularly suited to teachers, lecturers, tourists guides, hostesses and anyone speaking in crowded, noisy environment.
The circuits heart is formed by the TDA7052 Audio power amplifier IC, delivering a maximum output of 1.2W @ 6V supply. An external microphone must be plugged into J1, its signal being amplified by Q1 and fed to IC1. R1 acts as a volume control and C3 tailors the upper audio frequency band, mainly to reduce the microphone possibility of picking-up the loudspeaker output, causing a very undesirable and loud "howl", i.e. the well known Larsen effect. Therefore, C3 value can be varied in the 4n7 - 22nF range to ensure the best compromise from speech tone quality and minimum Larsen effect occurrence. Dynamic or electrets microphone is warmly recommended. It has a useful feature that can be used to momentarily mute the microphone by connecting SW1 shown in diagram.
Circuit Diagram:
Speach Amplifier Circuit Diagram
Parts | Description |
R1 | 22K |
R2 | 1M |
R3 | 15K |
R4 | 470R |
R5 | 47K |
R6 | 4.7K |
C1 | 100nF-63V |
C2 | 100nF-63V |
C3 | 100nF-63V |
C4 | 10nF-63V |
C5 | 220uF-25V |
C6 | 10uF-25V |
Q1 | BC547 |
IC1 | TDA7052 B1 |
J1 | Mono Jack Socket |
B1 | 6V Battery |
SW1 | SPST Slider Switch |
SW2 | SPST Toggle Switch |
Notes:
- Please note that hands-free, uni-directional headset or ear clip microphone types are very well suited for this device, as also are Clip-on Lavaliere or Lapel microphones.
- If a small electrets capsule is used for the microphone, R5, R6 and C6 must be added to the circuit to provide power supply.
- Choose a loudspeaker as large as possible, in order to increase circuit performance.
- You can use also two 4 Ohm loudspeakers wired in series or two 8 Ohm types wired in parallel in order to obtain better results.
- The box containing the amplifier and loudspeaker(s) can be fitted out with a belt and carried like a shoulder-bag or, if you build a smaller unit, it can be used as a Pick & Go Belt Clip Speaker.
Source : www.redcircuits.com
Saturday, October 5, 2013
Fuse Box BMW 318i 1995 Diagram
Friday, October 4, 2013
4 20mA Current Loop Tester
The digital PWM signal is converted to an analog voltage using a low-pass filter formed by the 1kω series resistor and a 4.7μF tantalum capacitor. By varying the PWM duty cycle and therefore the DC signal level out of the filter, the program can indirectly vary the current flow through the transistor. A 100 resistor in series with the emitter of Q1 converts the loop current to a small voltage, which is fed into the micro on input1. The program uses this feedback signal to zero in on the desired current level with the aid of the micros analog-to-digital converter. Details of this can be seen in the accompanying program listing.
If the PICAXE senses an open circuit, it shuts down the output and goes into an alarm state, to alert the operator and protect the circuit under test. The switch can be pressed to reset operations to the start once the open circuit has been rectified. The LED flashes a code for various milestones, as follows: one flash at 4m and one flash to confirm a switch press two flashes at 12m when ramping up (for the first 5 cycles); three flashes at 20m and continued fast flash sequence for open-circuit alarm. For portable use, the circuit can be powered from two 9V batteries, whereas for bench testing, a 12V DC plugpack will suffice.
Thursday, October 3, 2013
LED Volt Meter Circuit
Here is a Simple LED Volt meter to Monitor the charge level in Lead Acid Battery or Tubular battery. The terminal voltage of the battery is indicated through a four level LED indicators. The nominal terminal voltage of a Lead Acid battery is 13.8 volts and that of a Tubular battery is 14.8 volts when fully charged. The LED voltmeter uses four Zener diodes to light the LEDs at the precise breakdown voltage of the Zener diodes. Usually the Zener diode requires 1.6 volts in excess than its prescribed value to reach the breakdown threshold level. When the battery holds 13.6 volts or more, all the Zener breakdown and all LEDs light up. When the battery is discharged below 10.6 volts, all the LEDs remain dark. So depending on the terminal voltage of the battery, LEDs light up one by one or turns off.
Circuit diagram:
LED Volt Meter Circuit Diagram
Author: D. Mohan Kumar Copyright: electroschematics.com
Wednesday, October 2, 2013
Adjustable Zener Diode
Now assume that 4.2V is present at the input. The result is that the maximum positive voltage is present at the opamp output, but the diode prevents this from having any effect on the signal. However, if the voltage rises above 6.5V, the output of the opamp goes negative and pulls the voltage back down to 6.5 V. The current is limited by R3. Another example is a situation in which exactly the opposite is required. In this case, the voltage must not drop below a certain value. This can be easily achieved by reversing the polarity of the diode. Another option is a voltage that is only allowed to vary within a certain voltage window. It must not rise above a certain value, but it also must not drop below another specific value. In the circuit shown in Figure 3, the left-hand opamp provides the upper limit and the right-hand opamp provides the lower limit. Each opamp is wired as a voltage follower.
Tuesday, October 1, 2013
Model Railway Short Circuit Beeper
An adequate coil is provided by several turns of 0.8–1 mm enamelled copper wire wound around a drill bit or yarn spool and then slipped over the glass tube of the reed switch. This technique generates only a negligible voltage drop. The actuation sensitivity of the switch (expressed in ampère-turns or A-t)) determines the number of turns required for the coil. For instance, if you select a type rated at 20–40 A-t and assume a maximum allowable operating current of 6 A, seven turns (40 ÷ 6 = 6.67) will be sufficient. As a rule, the optimum number of windings must be determined empirically, due to a lack of specification data. As you can see from the circuit diagram, the short-circuit detector is equally suitable for AC and DC railways. With Märklin transformers (HO and I), the track and lighting circuits can be sensed together, since both circuits are powered from a single secondary winding.
Coil L1 is located in the common ground lead (‘O’ terminal), so the piezoelectric buzzer will sound if a short circuit is present in either of the two circuits. The (positive) trigger voltage is taken from the lighting circuit (L) via D1 and series resistor R1. Even though the current flowing through winding L1 is an AC or pulsating DC current, which causes the contact reeds to vibrate in synchronisation with the mains frequency, the buzzer will be activated because a brief positive pulse is all that is required to trigger thyristor Th1. The thyristor takes its anode voltage from the GoldCap storage capacitor (C2), which is charged via C2 and R2.
The alarm can be manually switched off using switch S1, since although the thyristor will return to the blocking state after C2 has been discharged if a short circuit is present the lighting circuit, this will not happen if there is a short circuit in the track circuit. C1 eliminates any noise pulses that may be generated. As a continuous tone does not attract as much attention as an intermittent beep, an intermittent piezoelectric generator is preferable. As almost no current flows during the intervals between beeps and the hold current through the thyristor must be kept above 3 mA, a resistor with a value of 1.5–1.8 kΩ is connected in parallel with the buzzer. This may also be necessary with certain types of continuous-tone buzzers if the operating current is less than 3 mA. The Zener diode must limit the operating voltage to 5.1 V, since the rated voltage of the GoldCap capacitor is 5.5V.