In a previous article I wanted to know if I could use the recording medium from old floppy disk as an infrared (IR) filter to shoot infrared photography on an iPod. I built a simple IR detector using Snap Circuits to test how well the floppy disk would absorb visible light yet let infrared wavelengths pass through. In that circuit I used the photodiode Snap Circuits block (Infrared Receiver U24) as the infrared detector. Even though a photodiode was useful for building a simple IR detector, another electronic component that is often used to detect infrared light is the phototransistor. We’ll use a phototransistor to build an audible IR detector circuit.

You may recall that a photodiode, like the one in your TV that receives the pulses of infrared light from your TV remote, converts light into energy like the solar cells in your solar garden lights. Since photodiodes are so small they don’t produce much energy from light, so they wouldn’t be useful to charge the rechargeable battery in your solar garden light, but they do produce enough energy to do useful work in electronic circuits such as detect the infrared pulses from your TV remote.

In some cases, it might be necessary to amplify the energy produced by the photodiode like a lever amplifies the force, or effort to lift a load. A transistor is an electronic version of a lever. (Source) It can take the small amount of current or voltage and amplify it. A transistor is a little more complicated than a diode so it might be simpler to think of a transistor using the “two diode” model, or imagine “the transistor is composed of two diodes wired back to back.” (Source)


Using Snap Circuits, we can build the two diode model. When the circuit is switched on, you’ll notice that no current flows through the circuit since the LED on the left is reversed in the circuit blocking the flow of current.

As a comparison to the above circuit I've switched the LED on the left around so that both LEDs are forward biased. Current can flow through the circuit and both LEDs light up. I installed the 1K resistor to prevent damage to my LEDs.

In the next picture, I returned the circuit to its initial conditions with both LEDs wired back to back.

In the following picture, I’ve created a base wire for our two-diode model of the transistor (connected to the 1K resistor--again to protect the LED). So, the left LED would be the transistor’s collector (C), the right LED would be the emitter (E), and the two snap conductor connected to the 1K resistor would be the base (B). When I switch the circuit on I apply voltage across the base (the electrical path through the 1K resistor) and the emitter (the right LED). Current can now flow from the base (B) of the two-diode model through the emitter (E), or right LED and the LED lights up.

To demonstrate how a transistor can be used as an amplifier like a lever amplifies the force, or effort to lift a load, I’ll remove the two-diode model and install an actual transistor. The 1K resistor is, of course, installed to protect the LED.

With nothing connected to the base (B) of the transistor, no current will flow through the LED and the LED stays dark. To use the transistor as an amplifier, I can use a small amount of voltage or current at the base to control a much larger voltage between the collector (C) and emitter (E) like a lever where you can use a small amount of effort to lift a heavy load. Human skin has a resistance from 1K to 100k (skin isn't a very good conductor), but I can use my fingers to close the circuit between the two snap conductor and the base of the transistor to light up the LED. I can lick my fingers to increase their conductance and make the LED burn brighter. You can also think of my fingers as an on/off switch—when my fingers touch the two snap conductor and the base of the transistor, this closes the circuit (switches it on) and when I remove my fingers the circuit is open (switches it off).

Now that you understand how an ordinary transistor works, you can take advantage of a phototransistor as an infrared detector. A phototransistor works like a transistor except that instead of a wire connected to the base it has photosensitive material (at the base and collector junction) and a lens to focus light onto the photosensitive area (looks a lot like an LED). When light falls on the photosensitive area, it converts the light into energy—a small amount of current or voltage. Since it is like a transistor it can use this small current or voltage to control a much large voltage between the collector and the emitter. As above, you can also think of a phototransistor as an on/off switch. When light falls on the photosensitive area, the transistor switches on. When no light falls on the photosensitive area, the transistor switches off. In the next photo, I've removed the transistor and resistor and replaced them with the photo transistor.

When I switch the circuit on, I can point a TV remote at it and push buttons on the remote. The red LED flashes to indicate the pulses being transmitted by the infrared LED on the TV remote.

We have a visible representation of the invisible flashes of the TV remote using the red LED, but it might be interesting to have an audible representation of the pulses transmitted by the remote. To build an audible IR detector, we can use the “the 555 Test Circuit” and install the phototransistor to see if it can generate a tone that corresponds to each pulse transmitted by the TV remote.

Parts needed:

555 Timer IC (I used a KIA555p, but the NE555 will do just fine)

Snap Circuits Parts:

1 Base Grid (11” x 7.7”) # 6SC BG
1 Eight-Pin IC Socket # 6SC ?U8
1 0.2uF Capacitor # 6SC C1
1 Variable Resistor #6SC RV
1 Whistle Chip # 6SC WC
1 4.5 Volt Battery Holder # 6SC B3
1 Slide Switch # 6SC S1
1 Phototransistor # 6SC Q4
1 Single Snap Conductor # 6SC 01
5 Conductor with 2-snaps # 6SC 02
3 Conductor with 3-snaps # 6SC 03
4 Conductor with 4-snaps # 6SC 04
1 Conductor with 5-snaps # 6SC 05

Build the circuit shown:

In the following video I demonstrate how the audible IR detector circuit works. It turns out that the phototransistor is sensitive even to indoor lighting so I had to cover the phototransistor with the TV remote to reduce the noise from the overhead light.

You can now compare the two circuits: the visual IR detector and the audible IR detector. Which is the better IR detector circuit? The visual IR detector is simpler to build, but all it does is flash an LED that corresponds to the invisible light pulses transmitted by the TV remote. The audible IR detector is more complicated to build, but when you listen to the tones, there seems to be something there that wouldn't be apparent with just a flashing LED. One would expect to hear just beeps that correspond to the flashes of the IR LED from the TV remote, but the beeps seem to contain some sort of static or interference instead of a clean tone. Could be nothing. Could be something. Either way, it might be interesting to try to figure out what’s causing the apparent interference in the tone.