Monday, August 1, 2011


Mono state switches are again divided in two parts as,
1. Normally Closed(NC)
2. Normally Open(NO)


Two way switch


Switch designs(Step by Step)


When we are choosing a resistor for this should not be too big or small in size. Why?

When resistor value is too high – As you all know when switches get old there is a possibility of reduction in the resistance (Rs) when it is not pressed (This happens due to corrosion, dust, etc.). When it is reduced it become close to the Rs value (NOTE: resistor value is too high). This makes V­1 higher even it is not pressed.
Ex: Think you put a 1000 ohm resistor, which is too big, and with time you switch resistance when not pressed get to low value like 4000 ohm. There V1 will be Vcc/5.
When resistor value is too low – It is so simple, this lead to a greater sinks current.
NOTE: Before you connect this switch to a circuit, you may want to connect it through a resistor, which allows us to control the current when the switch is pressed.

What if you want circuit to be active when the button is not pressed?

Only a small change is needed here as shown in the below illustration. When we want to control the current, we will do it by changing the current controlling resistor. We do not change the pull-up or pull-down resistors to do this as that can lead to a change in V1.

Use of pull-up or pull-down resistors


So do we need a pull-up or pull-down resistor?

Yes, this is a mechanical switch, when it is changing from ON to OFF or vice versa, there is a floating voltage. This can be a problem if we need fast operation. That is why we need a resistor to have a steady voltage.
This should be corrected to below.