Wednesday, October 9, 2013
On reset, output O0 (pin 3) of IC2 goes high, charging the 330nF capacitor via D2 and the 33kΩ resistor. If switch S2 is now pressed, Q2s emitter will be pulled high and so Q2 conducts, applying a rising positive voltage to one end of the 1MΩ resistor. This resistor and the 33nF capacitor act as a switch "debounce" circuit, delaying the pulse through IC1e by about 33ms. After the delay, the output of IC1e goes low. However, counter IC2 does not increment at this stage, since it needs a positive-going edge at the clock input (pin 14). When the switch is released, Q2 turns off, IC1e’s output goes high after the debounce period and the counter advances to the next state (ie. O0 goes low and O1 goes high).
When output O0 (pin 3) goes low, the 330nF capacitor starts discharging through the 33kΩ and 10MΩ resistors. This allows about 3s for the operator to press the next button. If no button is pressed within this period, IC1b’s output goes low, which pulls Q1’s emitter low and resets the counter via IC1c. Hence the code entry must be restarted. When the second digit of the code is entered (0 in this example), Q2’s emitter is again pulled high. Q2 thus turns on and after the debounce delay, IC1e’s output goes low. When the switch is released, Q2 turns off, IC1e’s output goes high and the counter advances to state 2.
Note that while the switch is pressed, IC1d’s output is high, recharging the 330nF capacitor and therefore resetting the 3s delay. Thus, the operator is allowed another 3s to press the next digit. This process is repeated for each digit in the sequence. If the wrong switch is pressed at any point, IC2 is reset as described above. Conversely, if the correct code is entered, IC1 advances to state 4 (for our 4-digit example) on release of the fourth switch. Output O4 then goes high and turns on Q3 and relay 1. Q3 can handle up to about 300mA of load current. If more current is required, then either a Darlington or power Mosfet can be substituted. D4 is required if the load is inductive (eg, a relay, solenoid, etc).
Monday, October 7, 2013
For several years now, a rear fog lamp has been mandatory for trailers and caravans in order to improve visibility under foggy conditions. When this fog lamp is switched on, the fog lamp of the pulling vehicle must be switched of to avoid irritating reflections. For this purpose, a mechanical switch is now built into the 13-way female connector in order to switch of the fog lamp of the pulling vehicle and switch on the fog lamp of the trailer or caravan. For anyone who uses a 7-way connector, this switching can also be implemented electronically with the aid of the circuit illustrated here.
Here a type P521 optocoupler detects whether the fog lamp of the caravan or trailer is connected. If the fog lamp is switched on in the car, a current flows through the caravan fog lamp via diodes D1 and D2. This causes the LED in the optocoupler to light up, with the result that the photo-transistor conducts and energies the relay via transistor T1. The relay switches of the fog lamp of the car. For anyone who’s not all thumbs, this small circuit can easily be built on a small piece of perforated circuit board and then fitted somewhere close to the rear lamp fitting of the pulling vehicle.
Author :Harrie Dogge Copyright :Elektor Electronics 2008
Saturday, October 5, 2013
To startup the Evaluation board, set the EN1 jumper and EN2 jumper to the “OFF” position, apply power to the board, and then move the EN jumper(s) to the “ON” position. This is the expected startup operation in the typical application where VIN is tied to a voltage rail and the EN pins are controlled via logic signal.
Thursday, October 3, 2013
The first timer is configured as a standard astable frequency generator. There is no need to explain its operation here, since this can easily be found on the Internet in the datasheet and application notes. All we need to mention is that the frequency equals 1.49 / ((R1+2R2) × C1) [Hz] R2 has been kept small so that the frequency can be varied easily by adjusting the values of R1 and/or C1. The second timer works as a monostable multivibrator and is triggered by the differentiator constructed using R3 and C3.
The trigger input reacts to a rising edge. A low level at the trigger input forces the output of the timer low. R3 and C3 have therefore been added, to make the control range as large as possible. The pulse-width of the monostable timer is given by 1.1xR4xC4 and in this case equals just over a millisecond. This is roughly half the period of IC1a. The pulse-width is varied using P1 to change the voltage on the CNTR input. This changes the voltage to the internal comparators of the timer and hence varies the time required to charge up C4.
The control range is also affected by the supply voltage; hence we’ve chosen 15V for this. The voltage range of P1 is limited by R6, R7 and R5. In this design the control voltage varies between 3.32V and 12.55V (the supply voltage of the prototype was 14.8V). Only when the voltage reaches 3.51 V does the output become active, with a duty-cycle of 13.5 %. The advantage of this initial ‘quiet’ range is that the lamp will be off. R8 protects the output against short circuits. With the opto-coupler of the dimmer as load, the maximum current consumption of the circuit is about 30mA.
R1 = 270k
R2,R3 = 10k
R4 = 100k
R5,R8 = 1k
R6,R7 = 220R
P1 = 2k2, linear, mono
C1,C4 = 10nF
C2,C5,C6 = 100nF
C3 = 1nF
C7 = 2µF2 63V radial
C8 = 100µF 25V radial
D1 = 1N4002
IC1 = NE556
IC2 = 78L15
P1 = 3-way pinheader
K1 = 2-way pinheader
Tuesday, October 1, 2013
This circuit uses a step-up switch-mode regulator, which is usually used to produce a positive supply, to generate a regulated negative output voltage. The device used here is the MIC4680 from Micrel (www.micrel.com), but the idea would of course work with similar regulators from other manufacturers. Because of coil L1, which performs the voltage conversion by the intermediate storage of energy in the form of a magnetic ﬁeld, the output is effectively isolated from the input. We can therefore connect the right-hand side of L1 to ground rather than to the positive output without causing a large current to ﬂow. Then we connect the ground pin of the regulator IC and all the components connected to it as the negative voltage output, isolated from ground.
The components on the output side of the regulator are connected as usual: ﬂywheel diode D1, coil L1 and the voltage divider formed by R1 and R2. These last two components set the output voltage, according to a formula given in the data sheet. Example component values for the MIC4680 used here are given in the table. The input voltage should lie within the permitted range for the regulator used, and must in any case be at least as great in magnitude as the desired output voltage (here +5 V or +12 V), so that the step-down regulation technique can wor.
It is important to take care when building this circuit to mount the regulator using an insulator, since generally the GND pin of the device is connected to the heatsink tab. Also, the ON/OFF control input cannot be driven using a normal logic signal, since the regulator’s ground reference is the output voltage rather than ground itself. If the ON/OFF function is required, a level shifter or optocoupler must be used.
Copyright : www.elektor.com
Sunday, September 29, 2013
Almost all 24V power systems in trucks, 4WDs, RVs, boats, etc, employ two series-connected 12V lead-acid batteries. The charging system can only maintain the sum of the individual battery voltages. If one battery is failing, this circuit will light a LED. Hence impending battery problems can be forecast. The circuit works by detecting a voltage difference between the two series connected 12V batteries. Idle current is low enough to allow the unit to be permanently left across the batteries.
R1 = 2.K
R2 = 4.7K
R3 = 39K
R4 = 39K
R5 = 1.5K
R6 = 1.5K
Q1 = BC547
Q2 = BC547
Q3 = BC557
D1 = 3mm Red LED
D2 = 3mm GreenLED
B1 = DC 12 Volt
B2 = DC 12 Volt
Source : www.extremecircuits.net
Friday, September 27, 2013
The State Jal Boards supply water for limited duration in a day. Time of water supply is decided by the management and the public does not know the same. In such a situation, this water alarm circuit will save the people from long wait as it will inform them as soon as the water supply starts.
At the heart of this circuit is a small water sensor. For fabricating this water sensor, you need two foils—an aluminium foil and a plastic foil. You can assemble the sensor by rolling aluminium and plastic foils in the shape of a concentric cylinder. Connect one end of the insulated flexible wire on the aluminium foil and the other end to resistor R2. Now mount this sensor inside the water tap such that water can flow through it uninterrupted. To complete the circuit, connect another wire from the junction of pins 2 and 6 of IC1 to the water pipeline or the water tap itself.
The working of the circuit is simple.
Timer 555 is wired as an astable multivibrator. The multivibrator will work only when water flows through the water tap and completes the circuit connection. It oscillates at about 1 kHz. The output of the timer at pin 3 is connected to loudspeaker LS1 via capacitor C3. As soon as water starts flowing through the tap, the speaker starts sounding, which indicates resumption of water supply. It remains ‘on’ until you switch off the circuit with switch S1 or remove the sensor from the tap. The circuit works off a 9V battery supply. Assemble the circuit on any general-purpose PCB and house in a suitable cabinet. The water sensor is inserted into the water tap. Connect the lead coming out from the junction of 555 pins 2 and 6 to the body of the water tap. Use on/off switch S1 to power the circuit with the 9V PP3 battery.
Source:w w w. e f y m a g . c o m