I am just learning how to understand a schematic. I tried to copy a basic schematic from a book onto a breadboard. The circuit seems to work, but I was wondering if someone could tell me whether or not I hooked it up correctly.
Note: The black wire coming from the LED is going to ground, it's just not in the shot. Also, I used different resistors, because I didn't have the exact Ohms handy. I just want to know if the connections are in the correct places.
Thanks!
Looks good to me?
Looks fine, but as you do not say what transistor you have it is impossible to say if the collector and emitter have been swapped over. If you do swap them over the transistor still works only the gain is drastically reduced.
If you are simply wanting to turn turn the LED on using a switch (or arduino), the normal way of connecting it is to have the LED in the collector circuit with the 470 ohm R and remove the 2k R as it is not needed.
Weedpharma
Grumpy_Mike:
Looks fine, but as you do not say what transistor you have it is impossible to say if the collector and emitter have been swapped over. If you do swap them over the transistor still works only the gain is drastically reduced.
I just have the packaging from the transistor. It says: Mfg P/N: 2N3904 BJT NPN.
I didn't look up the data sheet on the transistor, I just used one I had laying around. I know the transistor is positioned correctly, because the EBC pins are marked on the case. The schematic didn't call for a specific transistor, so I just randomly grabbed one.
My other goal with this was to try and figure out what a transistor really did. I have an encyclopedia set for electronic components. I was reading the section on transistors today. Some of it is pretty technical so I didn't understand it.
I just got done learning about machine language in class. I learned about how computers are made up of millions of transistors, which are 2-state switches. So I wanted to use a transistor to see it in action.
The schematic confuses me, because I don't see anything useful with it. When I press the switch, the LED comes on, but I can do that exact same thing with a switch, resistor, and LED, throwing away all of the other components, including the transistor.
So basically, what I am getting from this, is that the transistor is nothing more than a simple switch (in this schematic). You can use a transistor when you want the computer to open/close a circuit. It's the exact same thing as the push button switch I am using, except the push button switch has a external button that can be opened/closed by a person, instead of a computer.
In this case, the transistor was used with a push button switch (for demonstration) only because there was no other way to activate the transistor. Hooking the positive wire directly up to the collector would cause the circuit to be permanently closed.
Hopefully, I am understanding it correctly.
The second thing that confuses me, is the schematic listed specific resistors that needed to be used. Again, I just used some random resistors I had laying around, and the circuit still works. I know one resistor will protect the LED. I have no idea what the other two resistors are for, or why the circuit still works when I used the wrong resistors. What difference would I see if I found the correct resistors?
The last thing, is that 2 resistors are used between the switch, base and ground. What is the point of that? Why not just use a single resistor of a higher Ohm value? 12k Ohm instead of 10k and 2k.
Thank You.
weedpharma:
If you are simply wanting to turn turn the LED on using a switch (or arduino), the normal way of connecting it is to have the LED in the collector circuit with the 470 ohm R and remove the 2k R as it is not needed.Weedpharma
I was just trying to see how a transistor worked, and also seeing if I could read a schematic and put it onto a breadboard.
Thanks.
The schematic confuses me, because I don't see anything useful with it
You could only make a statement like that about that schematic if you didn't know Ohm's Law.
So , do you ? (what confuses you about the schematic ?)
raschemmel:
You could only make a statement like that about that schematic if you didn't know Ohm's Law.
So , do you ? (what confuses you about the schematic ?)
Hi,
I explained in the next several paragraphs what I found confusing about the schematic. You only read the first sentence.
Thanks.
The transistor in this circuit is a current amplifier. A small current through the base allows a larger current to flow through the collector emitter circuit.
The 10k limits the base current. The 2k limits the voltage as it forms a divider. It is not needed normally.
The purpose of the circuit is to allow a low current circuit (Arduino) to control a larger current or voltage circuit such as a motor or high power LED.
Weedpharma
Also that is just one configuration of using a transistor. That one is called an emitter follower when the voltage on the emitter follows the voltage on the base only about 0.7V less.
That circuit has a voltage gain of 1 but a high current gain determined by the transistor.
There are two other configurations, common emitter and common base as well as analogue applications, that means none switch applications.
Keep on reading.
Thanks!
gentle people am i the only one:
first yes your translation from diagram to breadboard is correct.
however the diagram is not the best
9 Volt supply
gives me 1.5 volts on the Base.
so the transistor is taking 0.7 Volts
the LED is another 0.7 Volts
so it will not light very much and the transistor is on the edge so will be hotter as needed.
as another poster said no need for the 2 kiloohm resistor, unless it is a FET.
Give us more of the book you are using, i want to know who wrote it.
Shooter is correct, only more so:
1.5V on the base, but in addition to 0.6 to 0.7V across the base, even a red LED requires about 1.8 to 2V to light up. An IR LED typically lights up around 1.5V.
So it probably won't light up at all.
As mentioned, to drive an LED, lose the 2k resistor, move the LED so it goes 9V, 470 ohm, LED, collector. Then connect the emitter to common, aka the negative terminal of the battery.
The only reason this lights up is because you are using different resistor values. Is that two 220 ohm resistors and a 1k?
This example running from 5V uses a lower series resistor:
5V - 0.2V (saturation voltage across the transistor) - 2V = 2.8V. 2.8V/150 ohms = 18.7mA.
Hi, if you can all go back to picture and read the caption on the page please?
use to breadboard test a transistor empirically
So it is not intended as a common way of lighting an LED, its a transistor tester.
polymorph's circuit is the more common way.
Tom..... 
You can use a transistor when you want the computer to open/close a circuit. It's the exact same thing as the push button switch I am using, except the push button switch has a external button that can be opened/closed by a person, instead of a computer.
Maybe in that circuit, but small signal transistors are often used as amplifiers to amplify a varying signal
(like an audio signal) , in which case they are not like a switch at all but because the current varies by
small amounts and the signal is a varying analog signal, as opposed to an ON/OFF digital signal you would
get with a transistor switch being used as an inverter or on/off led driver.
You could , for example , have a photo resistor that drives a transistor and lights a led based on how much
light (or absence of light) the photo resistor is exposed to, in which case it would be a light controlled
dimmer. (ie, the less light, the brighter the led). If you replaced the small transistor with a larger one, and the led with an incandescent (or Halogen) light, you could have a circuit that functions like a security light
that gets brighter as the daylight decreases until it is on full after dusk. It would not be operating in an on/off mode light the circuit you were referring to in the above quote.
