Wednesday, January 6, 2016

Push To On and Push To Off Switch

Good morning everyone, hope you guys are doing great! So in this brief post I will try to explain how a single push button can be connected with couple transistors to create a push to On and then again push to off switch. Yes a simple mechanical switch will just do the job fine by itself, all you will be doing is flipping it to On or Off position but I want something a bit fancy and this circuit is also a great one to understand more about transistors.

So let's jump in and see the diagram first.

Circuit diagram:


So this circuit needs to turn on the Light emitting diode D1 whenever the Press button PB is pressed and it will turn of D1 whenever the PB is pressed again.

How it works:

So now that we know what will happen let's see if we can understand how it will happen. Let's assume the initial state, where the source is supplying power to the circuit. If we look at the A point we will see that it is basically the ground pin via resistor R5, which will keep transistor Q1 and Q3 off. On top portion the Q2 is also off because of the resistor R7 and as Q1 is not conducting no current can pass through it hence the LED D1 will not glow. At the same time the capacitor C1 will be charged via the current through R1 and R2 and will be fully charged shortly.

Now let's press the button. Upon pressing it we are allowing the current stored in the capacitor to reach the base of Q1 via the resistor R6 which will turn on the transistor Q1. As soon as Q1 turns on it will start conducting and it will pull down the base of the transistor Q2 low which will turn it on. And as soon as Q2 turns on it will start conducting and via the resistor R4 it will supply enough current to the base of Q1 via R6 that the transistor Q1 will stay On. So the point A is high right now. Which will eventually turn on Q3 as it will get sufficient current to it's base via the R3. This transistor will dissipate any energy stored in the capacitor C1 via R2. And as the Q1 is conducting the the LED D1 will glow via the current through resistor R9. So we have pressed the button once and the LED is on.

Time for pressing it again. This time, remember the capacitor is fully discharged, so the current on point A will charge the capacitor and for that brief moment of time Q1 won't have enough base current to stay On, so it will turn off. As soon as Q1 turns off, the resistor R7 will pull the base of Q2 high so it will turn off too and no more current will be passed via Q2 to point A. So eventually Q3 will turn off too. And as the Q1 is no longer conducting there will be no current through it hence the LED D1 will not glow anymore.  So second press of the button and the LED is off. If we press that button again it will light up again.


Apparently it might seem like this circuit is useless but I think it is an awesome circuit to understand transistors and their work as switches, also think about the possibilities, you can replace the LED with any other kind of load and control it.

Anyway so that was all for now. Check out my other posts here and happy experimenting!