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Pragotron PJ42 Dial Backlight

The Pragotron PJ42 clock face is made of plastic and can be backlit using a strip of LEDs.

A LED controller switches the LED strip On/Off and also adjusts the brightness up and down

and has additional built in functions for various dynamic flashing and fading modes if required.

The controller is made up from a blank chrome wall switch plate and a modified

12V 12A Mini Single Colour LED Strip Tape Controller Dimmer board.


Below, modified 12V 12A Mini Single Colour LED StripTape Controller Dimmer



Item color: white
Auto memorize function
Single color dynamic modes
Work Voltage : 12V DC
Control: Brightness, flashing, flashing modes
Button:3 Keys
Used for:Single color led strip(5050,3528,5630)









Hacking the controller

Peel off the label from the heat shrink sleeve and set it aside.

Cut away the white plastic heat shrink over the circuit board to reveal the PCB with stick on buttons.

The buttons are held in place by clear tape.

Peel away the clear tape and remove the button tops.



This will leave the button contacts exposed so the wires from the PCB switches can be soldered in place.


Turn the board over and remove the soldered 12v and LED wires.








Controller Construction

Connect 3 PCB mount switches to the Vero board then fix the control board above

to the top of the Vero board with hot melt glue.

Connect 6 control wires from the old button contacts on the PCB to the new switches on the Vero board.


A terminal block is then soldered to the (LED) OUT +ve and +12v of the circuit board.

The block is hot melt glued to the Vero board.

Stick the button label removed earlier onto the Vero board.


Glue the Vero board with attached ctrl PCB to the back of the wall plate making sure the holes are aligned with the switches.

Mount the main switch in place.







Main Switch Wiring

Using the connections below the LED "Halo" light will come on when the switch is On and mimic the LEDs state.

I have connected a 100K resistor between the A terminal of the switch to the Out- of the control board to dim the Halo LED on the switch.

Main Switch Terminal Connection Block LED Ctrl Board
C -12V  
NO   -12V
LED   Out+
A   Out-

The 12 volt supply is from a 12 volt LED power converter. I have measured the current from my LEDs and the max current drawn is .6 Amp.

I have used a 2amp power supply and 2.5amp wire to connect the power and switch connections.







Blanking Plate

The controller circuit is mounted in a standard Chrome metal blanking plate made for mains wall switches.


Hole are drilled in the plate to take the three switch tops and the main On Off switch.


The cutaway show the PCB mounted switches below the blanking plate. The buttons protrude through the plate by a few millimeters.



The completed plate with main On Off switch top and three control switches below.

The "light" button puts the controller into normal light mode from auto flashing/fade mode.

While in normal light mode the "Light" and "Pwe/Brite/Spd" control the dimming up and down.

Hold the "Pwe/Brite/Spd" to turn the light On/Off.

The "mode" button will switch to auto flashing/fade mode from normal light mode.

While in auto flashing/fade mode "Light" and "Pwe/Brite/Spd" control the speed of the effects while the mode button changes effects.

When turned Off and On the last set mode is remembered.







LED Backlight

The LED backlight is just a strip of white LEDs mounted on a backboard and a strip of wood cut at a 45 angle.

The wood strip is made into an Octagon glued together then mounted on a hardboard backing.

The hardboard backing is painted white to reflect the light.

To work out the lengths of cut required to make a octagon of a set diameter I used the calculator below.

Octagon Calculator


Wood strip cut from an old piece of timber then run through my table saw to get a 45 angle is cut and glued together with mitre glue.




The octagon strip is then glued to the hardboard backing.





Hardboard backing painted white to reflect the light.




The LED strip is then stuck to the wood strip and also screwed in place using tiny wood screws and large metal washers.

The LED strip will not fit neatly around the corners as shown in the illustration below so I just hold it away from the corners with hot melt glue. 





The completed LED backlight mounted behind the clock face.