The main problem faced by the electronics engineers are they want a variable power supply for testing their circuits.the cost of the variable power supply is nearer to $5.here I am going to make the variable power supply in the range of 0-12V .this instructable costs less than $1.
This instructable may help the electronic engineers for their experiments.
Step 1: Gathering Tools & Materials
Resistor -5K(potentiometer type ) 
Capacitor -1 uf /25v
LED -red 
IC -LM 317
Heat sink with screws – required size
Positive negative knobs –one pair
DC input port-
Wires - required
Solder legs - required
Tooth paste cap-
Plastic box-small size
-hot glue gun
Step 2: Assembling the Circuit
Take the common pcb board and mark required size with the help of marker and cut it out.
Assemble the circuit with proper planning&make the solder.
place the soldering leg where the external parts are connected.
Take the potentiometer and solder the wire in pin 1 and pin 2.
Connect to the potentiometer with the pcb board.
Take the DC input port and switch, solder the wire with them.
Step 3: Make Cover for the Power Supply
Take the plastic box and marker, mark out the places where the knobs,switch, input port are placed.
Using the sharp knife cut the marked places.
Insert the positive and negative output knobs on top of the box and tight the screw strongly.
Place to potentiometer on centre of two knobs with help of bolt.
Take the plastic pushes,and place it on the PCB board with the help of hot glue gun.
place the DC input port ,switch and PCB board into the plastic box with the help of super glue.
Make the soldering between the circuit board and output knobs.
Close the plastic box,stick the tooth paste cap on top of the potentio meter with the help of super glue.
now the WORLD'S SMALLEST VARIABLE POWER SUPPLY is ready
Step 4: Working
After complete the finishing work, the variable power supply is ready to work.
Connect the 9 volt DC adapter to the power supply and turn on the switch.
This variable power supply don't have any display device for showing the output voltage.
Connect a simple multimeter with the power supply output knobs in DC mode( in the range of 20 v) as a display device.
By vary the potenetio meter to vary the voltage.
The voltage is increased from 0-9v.
The circuit works very well.
Any suggestions for this instructable please mentioned it with your comment.
the working of this power supply. https://www.youtube.com/watch?v=jMUQsKTttc8