# Basic Question: Transistors as Switches

Discussion in 'General Electronics Discussion' started by Noura, Jun 19, 2014.

1. ### Noura

19
1
May 13, 2014
Hello everyone!

I am struggling to understand how you use a transistor to switch a circuit on or off. The reason that this came up is that I have this infrared detector and emitter pair and when the detector senses IR it will drop the voltage. I was just playing around and wanted to use this info. to swtich a circuit on and off with an NPN transistor. I couldn't get it to work Could someone here, in very simple terms (still a noob here!) explain how you get a transistor to switch something on/off?

I do know that you have to have a voltage difference of so much between emitter and base for it to allow current to go through to the collector in an NPN.

Any help is appreciated. Thanks guys!!

Noura

2. ### Harald KappModeratorModerator

10,771
2,427
Nov 17, 2011
Here's a tutorial on the use of transistors as switches.

Noura and KrisBlueNZ like this.
3. ### Noura

19
1
May 13, 2014
Thanks! So since the IR sensor made the voltage drop in the output would I have to use a pnp? What I gathered from the section on NPN transistors is that you close the circuit by applying a more positive voltage to the base...if I wanted to do the opposite and apply the relatively low voltage I'm still a little confused.

4. ### BobK

7,682
1,686
Jan 5, 2010
A PNP the transistor is on when the voltage on the base is low compared to the emitter.

If the voltage you are switching is the same as the voltage supply for the IR sensor, you simply connect the output of the sensor, through a suitable resistor to the base of the PNP.

The emitter of the PNP is connected to the positive supply voltage.
The collector is connected to the load.
The other side of the load is connected to ground.

If you are switching a voltage different than the supply to the sensor, it gets a little more complicated.

Bob

5. ### Harald KappModeratorModerator

10,771
2,427
Nov 17, 2011
For a more detailed answer we would love to look at a schematic diagram of your circuit.