The Smart Switch
The purpose of this project is to build a smart switch
A smart swith is a swith that needs a short press and the power will be turned ON.
If I press the button longer than 0.5s the power will shut down.
All contribution to this page are most welcome!

Why not using switches with two way?
Thoose swithes need a arm and are quit big. With this smart switch unit you can buy any simple low-flat, low cost one-way swith and still get a very impressive function.
In my comming bug equipment I am going to use a small flat box and I am going to use a flat button for power control.


Imagin the power is off and the capacitor C1 is not charged. Q3 will be open and no current flow.The Base of Q2 will be low and the Base of Q3 high. The circuit is in stable condition, nothing is happening. If I press SW1 power will flow through the diod and to the base of Q2. Q2 will conduct so the base of Q3 will go low and the transistor Q3 will conduct and current go through this transistor and keep feeding the transistor Q2. The circuit is locked to deliver power to the output.

At the same time you press the SW1 button capacitor C1 will start to charge through resistor R1. If the voltage is higher than 0.7V the transistor Q1 will start to conduct and pull down the current through the base of Q2. This means that when you let the SW1 button go off the Q3 will not be conducting and the circuit is shut down.

The time befor the circuit is on and off is determined by the value of R1 and C1.
You will have to experiment yourself. It depends on wich transistors you are using, and you can use almost any transistors. Make sure the Q3 transisor can handle the current you are going to draw through it.

© Max Last modified on 17th Sept 2001.