Touch switch

A simple touch switch can be constructed using a single transistor.
Connect the collector of the transistor to the power supply with voltage about 9v or more. Connect the emitter to the load (in our case an led). Connect two wires, one to the collector and other to the base of transistor. The connections are made as below.

when we touch the two terminals shown above, the current passes from the battery positive through our skin into the base of transistor turning the transistor on. thus the led glows when we touch the terminals acting as a touch switch. 

will current through our body kill us?

usually people think that passing current through our body may kill us but to kill a human apart from current, voltage is also important. say we touch a wire carrying a current of 10Amps and 12volts then nothing will happen to us as atleast 100v is needed for us to get an electric shock. consider another wire having only 20mA but a voltage of 200v then this might kill a human on contact fro long time.

      usually our body resistance is about 100k which is enough for the current to pass from the battery to the base to switch in the transistor.

      This same design can be used to detech water level where one of the wires can be kept inside the water and the other at the level which is to be indicated. when water comes in contact with the wire it starts conducting current and the led glows.

     a series resistor of about 330ohms can be added at the collector just to limit the cuurent going to the led.

here is a video of demonstrating the above concept. 

                                

u can add multiple such transistors to increse the sensitivity of this circuit.

RESISTORS

    Resistors are  used as current limiters. Usually they are use in series or in parallel. These range from 1 ohm to 10mega ohm. They are used in battery chargers to limit current, amplifiers to bias current, regulators to set the reference values, etc.
    When we dont have required resistor values ,we can use the values we have to get the required value through series and parallel combinations. Resistor value adds in series and divides in parallel. If R1 and R2 are in series then the total value is R1+R2. Similarly if R1 and R2 are in parallel then the total resistance is given as R1R2/(R1+R2).
    When we dont have the required power ratings of the resistors we can also make parallel and series combinations to increase the power rating and in both cases the power increases. For example if we need 1watt 1kohn resistance then we can combine two half watt 2kohm resistances in parallel or two 500ohm resistance in series and we get the required resistance in both the cases.
     Apart from constant resistors there are also variable resistors called potentiometers. In these the value of resistance can be varied by turning the knob. These are also available in various power levels but the cost of higher power potentiometers is more than 20times the cost of the regular(1/10th watt) pots. Apart from normal single turn potentiometers there are also multi turn pots available to have more precision in the resistance values.

     

TRANSISTOR

    A transistor is a device which can switch on or off depending on the base current. A transistor can be called a switch because in case of a switch we use mechanical force to turn on or off the switch and a transistor can also be turned on or off but by using electrical signals. It also has an intermediate stage between on and off called the active region which has a linear  output with respect to the input at base. the off state of transistor is cutoff region and the on switch state is called as the saturation region.

    lets take an example of a water dam to understand the working of a transistor. In case of a water dam there is lots of water stored on one side and an empty area on the other side, to which water flows. the flow of water from one side to other is controlled using the dam gates.if the gates are opened slightly then water flows in less quantity and if gates are fully opened then water rushes. 

    The working of transistor is similar to this dam where the side with lots of water is the collector, the side to which water flows is the emitter and the dam gate is the base terminal.

the most used transistor is the NPN transistor which switches on with a positive base voltage while the other PNP transistor switches with a ground or negative base voltage.

SELECTING THE CORRECT TRANSISTOR

    There are thousands of transistor models present to match our needs. Depending on our need we can choose the transistor based on various parameters.

Voltage : Usually the transistors we use are rated at 24v max but if in applications such as smps, HV circuits,etc. we need to choose transistors which can handle such higher voltages.

Current : In low power circuits we can use transistors such as BC547, BC548 which handle a maximum current of 100mA but in high power circuits such as Led drivers, Amplifiers, Inverters etc we need to use transistors which can handle higher current. below are some examples of transistors and their current ratings.

2N3904-200mA

BC337-500mA

2N2222- 800mA

BD139,BD140-1.5A

TIP31,TIP32-3A

2N3055-15A

Power : power choice is also taken with respect to both the voltage and the cuurent ratings of a transistor.

Frequency : In circuits such as FM transmitters, High frequency oscillators, High speed switchs, RF circuits etc we cannot use normal transistors, we should choose transistors which can operate at such high frequency to satisfy the speed and also the output.

here are some examples of Frequency values of transistors

2N3904-300Mhz

2N3866-500Mhz

BFY90-1Ghz

2SC3358-7Ghz

Gain : For applications such as inverters and switching circuits amplification is not needed so gain may not be considered in these cases but for amplifiers we need to consider the gain to know the number of stages of amplification needed to get the required amplitude from smaller signals. usually it is given as hfe. here are gains of few transistors.

BC547-110-800

2N3055-20

TIP142- >1000

BD139-40-250

Apart from these parameters we can also  consider other parameters such as base current, package type, heat dissipation etc.
 Package :

here are two tables with various parameters of few transistors. we can use them to select the appropriate tansistor.

Testing components with a multimeter

  While using a electronic component like a resistor, capacitor, diode, etc. it is important to test them to avoid any damaged component in the circuit. a multi-meter would be really useful in testing the components.
 The following are the ways we can test different components using a multi-meter

RESISTORS : resistors are devices which are used to limit the current flow. the value of a particular resistor can be read using the color coding over it. to test it we put the multi-meter in resistance mode of desired range and measure the resistance if the value shown in the multi-meter is close to the value from color coding then the resistor is in good condition. if the multi-meter shows 00 or 1 then the resistor is damaged (i.e shorted or open circuited).

DIODE: A diode should conduct only in one direction. turn the multi-meter knob to diode or continuity mode. connect the black probe to the anode of diode (the side with silver band) and the red probe to the cathode. now the multi-meter should make a small beeping sound and meter should show some value. if the meter shows open circuit then the diode is damaged. reverse the terminals and the meter should show open circuit, if it shows some value then the diode is damaged.

CAPACITOR: if the multi-meter has the mode to measure the capacitance then directly measure it and compare to the value on the capacitor. if the meter doesn't have that mode then put the multi-meter in continuity mode and connect the capacitor to the two probes (the capacitance should be more than 10 uF to test in this method) the capacitor will charge due to the current from the multi-meter,after about 5 seconds turn the multi-meter knob to DC voltage mode. now the multi-meter should show some voltage reading which will be dropping. This indicates the capacitor is storing charge and working.