A Transistor Controlled Light Emitting Diode Circuit

Using A Transistor To Control A Light Emitting Diode

simple transistor controlled light emitting diode circuit
If you were able to build the last circuit, "Light Emitting Diode Circuit" without any problems, then you should have no trouble with this one. It's basically the same circuit with the addition of a silicon bi-polar transistor to turn the LED on and off electronically.

Transistor Controlled Light Emitting Diode Circuit Construction

Start by laying out all the parts in front of you. You will need a 9 volt battery, 9 volt battery connector, a 1K ohm (1000 ohms) 1/2 watt or less resistor, a 10K ohm resistor 1/2 watt or less, a NPN general purpose transistor, and one led light.

Start with the transistor. There are three leads you need to idenify, the emitter, the base and the collector. These are often shown by a small letter "e" "b" and "c" on the case near the lead or on the package the transistor came in.

Connect the emitter of the transistor to the black wire on the battery connector. Next connect one lead of the 10K ohm resistor to the base lead of the transistor, the other lead of the resistor goes to a terminal called "B" or whatever you want. These terminals can just be bare wire if you want. Now connect a wire from terminal "A" to the red or positive wire on the battery connector. You will want to be able to connect these terminals together later.

Now we will add in the led. All diodes have a anode and a cathode, you need to determine which wire is the cathode. There's some kind of indicator, one lead can be longer or a small flat on the side of the led can mark the which lead is the cathode, it will also be marked on the package it came it. Connect the cathode of the light emitting diode to the collector lead of the transistor. The anode connects to the 1K ohm resistor and the other end of the resistor connects to the red or positive wire of the battery connector.

Circuit Operation

Double check all the wiring and particulary the connections to the transistor. You can easily blow out the transistor if it's miswired. If everything checks out, connect the battery. This time nothing should happen. The transistor is in its "off" mode so no current can flow to the led and light it. Now connect a wire between points "A" and "B". The led should come on. If it does not, make sure the light emitting diode is not in the circuit backwards. Again like before this is 99.9% of the problem.

When you connect points "A" and "B" you are applying a forward bias voltage to the base emitter junction of the transistor. This puts the transistor in the "conduct" mode between the collector and the emitter. Since the cathode of the led is connected to the collector, it's now at "ground" and the led is now forward biased and lights. Exactly the same as in circuit 1. Disconnecting points "A" and "B" removes the forward bias from the transistor, that causes the collector emitter junction to stop conducting and the led goes out. If you are wondering what good this is, we are leading up to building the led flasher circuit and these steps help make the operation of that circuit much clearer.

What can you use this circuit for as it is? One thing that comes to mind is a water level detector. If you place points "A" and "B" where water can reach them, the led will light. This could be a warning of a flooded basement. You can think of many more uses I'm sure.

This type of circuit is covered in more detail in book two of my beginning basic electronics program.

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