We’re now going to build an invisible alarm system using an infrared LED and phototransistor.
The LEDs we have been using so far emit light that is visible to the human eye. However, infrared LEDs emit light at a wavelength which is not visible to the eye. The beam can be seen by using cameras that don’t have an IR filter – for example, the front-facing cameras of most phones. IR LEDs are commonly used in all sorts of TV remote control units.
The theory is this: if you have an invisible IR beam, then you can place a detector directly in its path which will monitor this beam. If the beam is broken, for whatever reason, you can identify this and sound the alarm.
The detector that we will be using is called a phototransistor and it does not have a base terminal but it is switched ON when light shines on it. It also has a coating applied to it which makes it less sensitive to visible light.
The resulting circuit is shown above. We have an IR LED and just like a regular LED, it has an anode and cathode. We have an LED controlled by a PNP transistor – which means that the base has to be at lower potential compared to its emitter for it to switch ON.
Under normal circumstances, light from the IR LED will shine onto the photo-transistor and this will cause a current to flow through it. This current will form a voltage drop across R3 which is given by Ohms Law and positive voltage will switch OFF Q2. When the beam is interrupted, no current will flow through the phototransistor and R3 will pull the base of Q2 to the ground potential – switching it ON. This will turn ON the LED.
Let’s build and test the circuit. Make sure the IR LED and photo-transistor are facing each other as shown here.
We’re only using an LED here to keep things simple and we will learn about relays, buzzers later which can be used to control more devices or to create an audible alarm. You can also add multiple sets of emitter/detector pairs to create a complex alarm system. Let’s move on to the next project.