With the PWM system, that is to say, pulse width modulation, it is possible to gradually turn a light bulb 12 V DC in a time adjustable from 2 to 25 seconds. Very useful in the car (if you have a little mind "tuning") or at home if you use light bulbs low voltage DC (solar house) or if you want to further increase the retro aesthetic of your tube amp .
If we connect a generator of sawtooth wave and a ramp generator to the inputs of a comparator, we obtain circuit PWM (Pulse Width Modulation) for gradually turning on a light bulb in 12 VDC. This assembly will also adjust at will the time it takes for the bulb to go off at the full brightness (adjustable between 2 and 25 seconds).
This circuit can be installed, for example, inside an amplifier (the one that we suggested, why not?) So that the bulb illuminating the VU meter lights gradually (in a tube amplifier, it needed almost). After all audiophiles also have two eyes between ears.
But maybe in your car that you want to mount it then you can turn on the ceiling or even the parking lights and lanterns in a progressive manner (you'll even have the satisfaction of avoiding the light bulbs of vehicle shock heat can shorten their life). Again, if you have a car collection, it almost goes without saying ... Sick of "tuning", you've understood.
Figure 1: This drawing represents the lighting of a lamp at three different times. Its brightness is inversely proportional to the length of time the duty cycle of the signal remains at logic high. In other words, the lamp brightness increases gradually and progressively as the high logic level signal shrinks. The wiring diagram
soon as power is supplied to the circuit, the current generator IC1 LM334 (see wiring diagram in Figure 2) C3 load up at its terminals and produces a voltage ramp from 0-12 V variable length from 2 to 25 seconds depending on the cursor position trimmer R5. When we cut this supply, the transistor TR1 becomes conductive and discharges C3 so that the next power cycle begins again with the same period, according to R5. The operational
IC2 / A, contained in the LM358 is used as oscillator for generating sawtooth wave at a frequency of about 1600 Hz is the value measured on our prototype, but yours will be be a few hundred Hz above or below (this will not affect the proper functioning of the assembly). The sawtooth wave present across C4 is fed to the non-inverting 3 of IC2 / B, a voltage comparator consisting of second operational amplifier contained in the LM358. The voltage ramp, channeled by the inverting input 2 of the same comparator, determines the variable duty cycle of PWM square wave signal at the output of IC2 1 / B.
The lamp brightness is inversely proportional to the time during which the duty cycle of the PWM signal remains at logic high, as shown in Figure 1: the wider the pulse duty cycle, the longer the signal remains at logic upper and lower the brightness of the bulb; gradually as the pulse width shrinks logically high, the brightness of the bulb increases until it reaches the maximum value. We used a P MOSFET IRF9540 "Beefy": it is indeed capable of withstanding a load up to 10 A, so we can use a bulb very greedy (over 100 watts! Beware the car battery anyway).
In addition, through the PWM, we can connect to our circuit bulbs pretty powerful (and greedy ...) without the MOSFET does not heat excessively because, working as a switch and not in its linear region, it dissipates minimal power and only during the phase of power.
What this does not preclude you however, if you plan to use light bulbs that use multiple amps to mount a heatsink on the MOSFET (type ML26) or well, as shown in Figure 8, against the wall of the metal casing. But this last consideration out of the wiring diagram to enter the ground floor in the practical realization.
Figure 2: Diagram of the PWM circuit for variable light bulb 12 V gradually in a time freely adjustable from 2 to 25 seconds (with trimmer R5). For a PWM system, we connected to the inputs of the comparator IC2 / B generator sawtooth wave (the operational IC2 / A) and a ramp generator (the integrated circuit IC1 with capacitor C3). 
Figure 3: Pinouts of the integrated circuit LM358 (dual op-amp) seen from above, the MOSFET IRF9540 front view and the BC557 transistor and integrated circuit LM334 (both boxes TO 92) viewed from below . 
Figure 4: Diagram of component layout on the deck of the progressive lighting lamp for 12 V The P-channel MOSFET that we chose to support a load 10 A. To assemble, its sole metal must "watch" R1-R2. 
Figure 4b-1: Drawing scale 1, the PCB double sided plated through holes of the plate for the progressive lighting lamp 12 V EN1648 side seams. 
Figure 4b-2: Drawing to scale 1, double-sided PCB with plated through holes of the plate for the progressive lighting lamp 12 V EN1648 side components. 
Figure 5: Photograph of a prototype of the plate for the progressive lighting lamp 12 V EN1648. The trimmer is visible at the bottom to adjust the duration of the ignition between 2 and 25 seconds. Iist
R1 ........ 47
R2 ........ 10 k R3
........ 10 k R4
........ 1 k
R5 ........ 10 k
trimmer R6 ........ R7 100
........ 10 k R8
........ 10 k R9
........ R10 100 k
....... R11 47 k
....... R12 10 k
....... R13 100 k
....... 47
C1 ........ 100 uF electrolytic
C2 ........ 10 uF electrolytic
C3 ........ 10 uF electrolytic
C4 ........ 3.3 nF polyester
C5 ........ 100 nF polyester
DS1 ....... 1N4150
DS2 ....... 1N4150 TR1
....... BC557 PNP
MFT1 ...... IRF9540 MOSFETs P
IC1 ....... LM334 IC2
....... LM358
Note: the resistors are 1 / 4 W.
The practical
This arrangement is within the scope of a beginner.
To build this small (but powerful) device, you need the double sided PCB with plated through holes EN1648, where all components are mounted, except perhaps the power MOSFET (see Figure 8) you can fix cons a wall of the metal case, inside: 4b-1 and 2 shows drawings of two faces on the scale 1. Make it or get it.
When you have before you mount the first bracket IC2 and make sure that first job (or short-circuit between tracks or pads or cold solder joints) and then climb all the other components from the lowest (resistors, diodes) the highest (trimmer, capacitor, transistor and LM334 TO packages in 92, IC-see Figures 7 and 8 - and terminals).
Check before welding orientation of polarized components (electrolytic diode-rings to and from C5-R7, transistor and LM334-flats to the left-and the integrated circuit, only insert it into the socket after installation in the case and made the last connection).
No problem if you look closely 4a to 8 and the component list. Check
well, several times, identification and orientation of components and quality of all welds, then go to installation in the housing.
installation in the housing
course, this arrangement in a metal box is optional: if you use it, as we have suggested, for the progressive lighting of the backlit VU meters your amplifier, you climb the small stage directly in the latter.
If this is not the case, take the small aluminum enclosure (see Figures 6 and 8) and drill four holes at the bottom by using the platinum (already drilled) as a template, in the aftermath, drill two holes about 10 mm in the two major opposing sides .
Within four holes, mount four metal spacers of 5 mm in both holes insert two hobbies son rubber; attach the plate on the spacers with four screws and take in / out both son and supply 12 VDC both son going to fly to the bulb. Use of wire whose diameter is commensurate with the current consumed by the load (do not skimp on the diameter).
If you chose the option of fixing the inner wall MOSFET cons "or if you should equip it with a mosfet heatsink, do not forget to interpose between the metal and the metal flange of a sink support kit insulation consists of a bushing for the bolt and a mica surface of the sole.
All connections are made and verified, you can insert the chip into its socket with great care and in the right direction-cue-keyed U to C5. If you are a beginner, take everything away this excellent habit.
Figure 6: Photograph of a prototype of the plate for the progressive lighting lamp 12 V EN1648 MOX30 installed in its case (you'll need to drill to attach the plate to the bottom with four spacers, to let the son of two 12 V power and leave out the two son up to the load). 
Figure 7: To ensure the MOSFET MFT1 high heat dissipation, mount it directly on the metal case (Figure 8), without forgetting to use the insulation kit (grommet and mica) . 
Figure 8: Before inserting the platinum in the metal casing, drill four holes at the bottom of it (using the deck as a template) and tighten four metal spacers 5 mm (this slight elevation of the plate from the bottom of the metal casing will prevent any short circuit). Drill two holes for passage of the son of input and output voltage in the two long sides and place of hobbies son rubber. Attach the legs of the SDG MOSFET points of the plate with the son of different colors so as not to invert (see Figure 7).
Figure 9: Tension 12 V to power this circuit is applied to the terminal and left the lamp to the right. 
Figure 10: If you sample the voltage 12 V vehicle battery, the son of the negative input as the output can be connected to the chassis (ground) of it. setting
With a small screwdriver adjust the slider trimmer mid race: thus, upon powering up the circuit you can check the progressive lighting of the bulb.
This course is a 12 V and you feed him in DC (eg vehicle battery or power supply amplifier: in the latter case, if necessary, install a controller 7812). See Figure 9.
Turn: The brightness of the bulb gradually increases at a rate depending on the setting of the trimmer. To set the duration (time it takes for the bulb to reach its maximum brightness), you will proceed by trial and error each time acting on the trimmer, but always proceed with this setting switched off.
Turn the trimmer screw and then powering the device and see if this setting is right for you, otherwise turn off the power and edit the trimmer in one direction or the other and then turn the power on, etc..
If you are considering a used car for a gradual switch, ceiling lights or as position and why not lights, see Figure 10: Mass for the son (both input and output), be sure to link them to the body, but made good ground connection (the metal must be free of paint and rust). If you apply the gradual ignition circuit of the headlights of the car, get two circuits (with one you could melt the MOSFET) and set the duration to a minimum.
If you are not keen, get help from a friendly mechanic (yes it exists!) Or by an auto electrician: this will avoid perhaps disappointments (in the vehicle, what is more dangerous, just after the driver is the fuel tank and electrical system).
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