Lab 6

We want to add a transistor to our circuit in order to make the infrared LED shine brighter. That way, we will be able to tag people from further away. As we said in Lesson 6, we need to use a transistor, or electrical switch, in order to have more current flow through the infrared LED. The more current that goes through the LED, the brighter it will be.

We're going to need to adjust our circuit in order to put the transistor in the circuit. What we want to happen is to have the electricity flow from positive voltage, through a resistor, through our infrared LED, and then through our transistor to ground. Our transistor will act like a switch or button, that our arduino will "press" or turn on by applying a high voltage to it. Based on that description, you should have an idea about how to wire the transistor (remember, we are building a pipe for the electrons to flow through).

But what resistance to use? Well, we know that we want to put more current through the LED. But how much is too much? The infrared LED's data sheet says that it can take up to 1 amp of current if we pulse it on and off (which we are doing 38,000 times a second, remember?). However, a 9 Volt battery (which we may be using to power the project) can't really output 1 Amp of current. That's too much for it. So we will stick to a lower current for this project. If we were using bigger batteries, we could use a different resistor and have up to 1 Amp go through the infrared LED. So, with our voltage equal to 5 volts, and current equal to 0.25 Amps, what is the resistance?

The answer is 20 Ohms. Be careful to only use resistors that have this low of a resistance with transistors. Otherwise you may burn up your arduino.

We're going to wire the transistor in place like the picture below. After that, try playing laser tag again. How much farther can your signal travel?


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