Touch Switch_Circuit Diagram World
Technical Specifications – Characteristics
Supply voltage: 12 VDC
Max. current:30 mA
Relay rating: 250 V/2 A
How it really works
The circuit as you can see from its diagram is very simple and only uses 8 components. The guts of the circuit is the IC CD 4011 that’s connected as a FLIP-FLOP. Pins 9 and 13 of the IC are the ?SET? and ?RESET? contacts of the FLIP-FLOP. The IC is of the CMOS type and requires a really low current to in its gates to manage it. This high sensitivity of the circuit makes the touch operation possible. The two gates are held at logic state ?1? continuously via the two resistors R1 and R3 that connect them to the positive supply rail. These resistors have a very large resistance of 10 Mohm. If we now touch a set of contacts the skin resistance closes the circuit between the corresponding gate and the negative supply rail. The skin resistance for small areas of the skin is often much lower than 10 Mohm and the gate is effectively delivered to logic condition ?0? which makes the FLIP-FLOP change state. For any given state of the FLIP-FLOP touching the corresponding set of contacts will make the circuit to reverse its state of balance and in effect toggle the switch. As a switch is used a relay driven by a transistor which is driven from the out put of the FLIP-FLOP.
Initially allow us to consider just a few basics in building electronic circuits on a printed circuit board. The board is product of a thin insulating material clad with a skinny layer of conductive copper that’s shaped in such a way as to form the required conductors between the various components of the circuit. Using a properly designed printed circuit board is very desirable as it speeds construction up
considerably and reduces the chance of constructing errors. Smart Kit boards also come pre-drilled and with the outline of the components and their identification printed on the component side to make construction easier. To guard the board during storage from oxidation and assure it gets to you in perfect condition the copper is tinned during manufacturing and covered with a special varnish that protects it from getting oxidised and makes soldering easier. Soldering the components to the board is the one way to construct your circuit and from the way in which you do it depends greatly your success or failure. This work is not very difficult and when you stick to a few rules you should have no problems. The soldering iron that you employ have to be light and its power shouldn’t exceed the 25 Watts. The tip should be fine and have to be kept clean at all times. For this purpose come very handy specially made sponges which are kept wet and infrequently you may wipe the hot tip on them to remove all of the residues that are likely to accumulate on it.
Don’t file or sandpaper a dirty or worn out tip. If the tip cannot be cleaned, replace it. There are many several types of solder out there and you must choose a great quality one which contains the necessary flux in its core, to assure a perfect joint every time.
Do not use soldering flux other than that which is already included in your solder. Too much flux could cause many problems and is one of the primary causes of circuit malfunction. If nevertheless you may have to use extra flux, as it is the case when it’s a must to tin copper wires, clean it very thoroughly after you finish your work. To be able to solder a component correctly you need to do the next:
Clean the component leads with a small piece of emery paper. – Bend them at the correct distance from the component body and insert the component in its place on the board. You could find sometimes a component with heavier gauge leads than usual, that are too thick to enter within the holes of the p.c. board.
In this case use a mini drill to enlarge the holes slightly. Do not make the holes too large as this is going to make soldering difficult afterwards.
Take the hot iron and place its tip on the component lead while holding the tip of the solder wire at the point where the lead emerges from the board.
The iron tip must touch the lead slightly above the p.c. board.
When the solder starts to melt and flow wait till it covers evenly the area around the hole and the flux boils and gets out from underneath the solder. The entire operation mustn’t take greater than 5 seconds. Remove the iron and let the solder to cool naturally without blowing on it or moving the component. If everything was done properly the surface of the joint will need to have a bright metallic finish and its edges needs to be smoothly ended on the component lead and the board track. If the solder looks dull, cracked, or has the shape of a blob then you may have made a dry joint and it is best to remove the solder (with a pump, or a solder wick) and redo it.
Take care to not overheat the tracks as it is very easy to lift them from the board and break them.
When you find yourself soldering a sensitive component it is good practice to hold the lead from the component side of the board with a pair of long-nose pliers to divert any heat that would possibly damage the component.
Guantee that you do not use more solder than it is important as you’re running the chance of short-circuiting adjacent tracks on the board, especially if they are very close together. – After having finished your work cut off the excess of the component leads and clean the board thoroughly with an acceptable solvent to remove all flux residues that still remain on it.
The switch only has eight components and its construction may be very easy even for essentially the most inexperienced. As usual construction must start from the least sensitive to heat components, which in this case are the IC socket and the pins. After soldering the pins and the socket, make the two jumper connections which are marked on the component side of the board, solder the relay in its place and continue with the transistor the diode and the LED. Once everything is in its place clean the board very well from flux residues and check it for brief circuits and possible mistakes.
Then, place the IC in its socket. The IC is of the CMOS family and needs to be
handled with great care as it can be damaged very easily from static discharges. Avoid touching its pins and keep your body and the circuit board grounded during insertion. You should also take care not to bent any pins underneath the IC body during this operation.
Now connect the points marked & – on the board with 12 VDC and touch lightly the set of contacts marked ?ON?. You must hear the clicking of the relay and the LED should light up. (In case the LED turns on at power up then touch the opposite set of contacts which might be marked ?OFF?.) Touching the contacts marked ?OFF? will turn the LED off and the relay must be released. It is as much as you
to attach any device you want to control with the touch switch but please remember that you should not exceed the ability rating of the relay which is 250 V/2 A.
If it doesn’t work
Check your work for possible dry joints, bridges across adjacent tracks or soldering flux residues that usually cause problems.
-Check again all of the external connections to and from the circuit to see if there is a mistake there.
See that there are not any components missing or inserted within the wrong places.
Guantee that all the polarised components have been soldered the right way round.
Make sure that the supply has the correct voltage and is connected the right way round to your circuit.
Make certain that you’ve inserted the IC in its socket correctly and that you have not bent any pins during insertion.
Check your project for faulty or damaged components.