Keyfob Torch

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Keyfob Torch

 


 

The keyfob torch is not strictly an electronics project because it does not contain any active devices (transistors).  However, it is a great introduction to electronics because it is low cost, quick to make and easy to get working.  PCB price and parts list is given here.

 

Keyfob Torch instr

 

 

 

W A R N I N G

 

This project uses button cells which can cause serious injury if swallowed.

 

Always take care to ensure that button cells are kept out of reach of small children.  In the event that it is suspected that a button cell has been swallowed, seek immediate medical assistance.

 

Please ensure that all students making this project are aware of the hazards posed by button cells.

 

 

 

The Torch can be constructed by conventional soldering or by using solder paste.

 

 

Conventional soldering

 

¡Use sandpaper or a fine file to remove the breakout tabs at the edge of the PCB.

 

¡Start with the push button.  Apply solder directly to one of the push button pads.  Place the push button into position.  Use the soldering iron to heat the lead which rests on the pad you have applied solder to.  This will fuse the lead into place.

 

¡Check that the push button sits squarely in place.  If it does not, gentle pressure can be applied to line it up.  Solder the remaining leads by pressing them down with the soldering iron tip.  Apply solder to the joint as you heat each lead up.

 

¡Rest a cell clip in place on the PCB.  NB the finger spring must point to the centre of the PCB (see photograph).  If the PCB is resting on a level surface, the cell clip should stand up on its own.  (Take care not to bend the cell clip as it is already formed to the correct shape.)

 

¡Carefully heat one corner of the cell clip pads (without touching the cell clip - or it will fall over!).  When the pad is hot enough, apply solder to the pad (not the iron).  If the pad is hot enough solder will flow across the pad and under the clip.  Remove the soldering iron.  The clip should now be held firmly in place by the solder joint.  Repeat for the other end of the cell clip.  If you are unable to solder the cell clip using ask someone else to hold the clip in place by resting a screwdriver on top of it until it is secure.

 

       WARNING The cell clip remains very hot some time after it is fitted!

 

¡Fit the second cell clip (remember to check its orientation).

 

¡Clip the LEDs leads to fit.  Check they are the correct length by holding against the PCB.  Solder LED to the PCB.  This is easiest to do one when someone else holds it in place while you solder.

 

¡Slide the button cells into place and check the torch works.

 

¡Fold the outer cell holder tabs down to stop the cell falling out. NB this is best done by a teacher because care is needed to ensure the finger spring tension is not reduced (causing the cell to rattle and loose connection with the PCB).

 

 

Using solder paste

 

 

 

W A R N I N G

 

Leaded solder paste is harmful if ingested.

 

Avoid touching solder paste with your fingers.  Wash hands thoroughly after working with solder paste. 

 

 

 

 

¡Use sandpaper or a fine file to remove the breakout tabs at the edge of the PCB.

 

¡Apply solder paste to all the PCB pads EXCEPT the circular pads for the button cells.  You do not need to load the whole pad.  Just use enough paste to hold each component in place.  (It should just about be possible to hold the PCB upside down without placed components falling off.)

 

¡Place the push button carefully on its pads.

 

¡Using tweezers place the cell holders on their pads.  Make sure you copy the photograph to ensure they are the correct way round.

 

¡Clip the LEDs leads to fit.  Check they are the correct length by holding against the PCB.  Place the LED onto the pad.  The paste should hold it ion place.

 

¡Carefully place the PCB onto a gauze and then into the oven.  Make sure it is horizontal (so the cell clips do not fall over as the paste is heated).

 

¡Heat the PCB until it reaches 210 (for lead free solder) 225 (for lead free solder)

 

¡Switch off the oven and carefully open the door to allow rapid cooling.

 

       WARNING The PCB and cell clips remains very hot for some time!

 

¡Slide the button cells into place and check the torch works.

 

¡Fold the outer cell holder tabs down to stop the cell falling out. NB this is best done by a teacher because care is needed to ensure the finger spring tension is not reduced (causing the cell to rattle and loose connection with the PCB).

 

¡Wash your hands.

 

 

Circuit diagram

 

Torch circuit diagram

Torch circuit diagram

 

Hyperlink jumpto 3D printed key fob torch case