Wednesday, October 9, 2013

Simple Combination Lock

This simple combination lock accommodates codes from 1-9 digits long, with the only restriction being that the same digit cannot be used twice. The circuit shows strapping for a 4-digit code, in this case "2057". Any unused switches are strapped to ground. When power is applied, the 330nF capacitor connected to pin 1 of inverter IC1a is discharged, holding it at a logic low level. The high output is inverted by a second gate (IC1b), with the result being a logic low on pin 4. This pulls Q1’s emitter low via D1, causing the transistor to conduct. The falling voltage on the collector then pulls the input of IC1c low, which in turn resets counter IC2.

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).

Simple combination lock circuit schematic

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).
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Monday, October 7, 2013

A Simple Fog Lamp Sensor

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.

Circuit diagram:

Fog Lamp Sensor Circuit Daigram

Fog Lamp Sensor Circuit Diagram

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

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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.
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Thursday, October 3, 2013

PWM Modulator

If you ever thought of experimenting with pulse-width modulation, this circuit should get you started nicely. We’ve kept simplicity in mind and used a dual 555 timer, making the circuit a piece of cake. We have even designed a small PCB for this, so building it shouldn’t be a problem at all. This certainly isn’t an original circuit, and is here mainly as an addition to the ‘Dimmer with MOSFET’ article elsewhere in this website. The design has therefore been tailored to this use. A frequency of 500 Hz was chosen, splitting each half-period of the dimmer into five (a low frequency generates less interference).

PWM Modulator Circuit Diagram CompletedThe 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.

 Parts Layout PWM Modulator Circuit DiagramThe 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.
PWM Modulator Circuit DiagramThe 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.

Power supply:
Power Supply For PWM Modulator Circuit DiagramResistors:
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
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Tuesday, October 1, 2013

Voltage Inverter Using Switch Mode Regulator

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 (, 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.

Voltage Inverter Using Switch-Mode Regulator

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.

Voltage Inverter Using Switch-Mode Regulator Table 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 :

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