# Simple circuit, uses 2 transistors & LED - does not work

Discussion in 'General Electronics Chat' started by svk, Aug 11, 2006.

1. ### svk Thread Starter New Member

Aug 11, 2006
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Hello all,

This is my first post on this forum. I recently started out with electronics, and after having bought a breadboard and some components, I started experimenting.

I built a simple circuit that I hoped would have worked. Basically, I'm trying to light a LED through a couple transistors. Using MSPaint as a schematic editor eek: ), here's what it looks like:

My logic here is this: the two resistors that you see act as a voltage drop. The bases of the transistors, after all, don't require 9V. At the same time, these resistors should lower the amount of current that flows through the LED so that I don't burn it. Since the bases of the transistors are always connected to the power supply, they should allow current to flow through them and light the LED.

However, nothing happens. The LED does not light.

Frankly, I'm not very experienced with a multimeter, so I don't know what to test and where.

Can anyone explain why nothing is happening?

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
Hi,

The bases of your transistors have no voltage or current available, so they will not turn on. The total resistance in series with the battery is much higher than necessary, so the led would be very dim.

A led is a current operated device. Most require an electrical energy level of about 1.5 volts to force the electrons across the internal structure that makes the light. They also tend to work with a current of 10 milliamps.

You subtract the 1.5 volts needed to run the led from the source voltage. That leaves you 7.5 volts. Then you use R = E/I to get the series resistance. That would be 750 ohms. That is a standard value, so Radio Shack will have one. With that current a 1/4 watt resistor will do.

The circuit is the battery, the led, and the 750 ohm resistor - no transistors needed.

Aug 7, 2006
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4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
Hi,

If you have made mistakes in setting up your SPICE simulation, I am not the person to ask. I have never used it.

You are also hijacking another person's thread. This is not acceptable. Please do not do so again.

Refer to the semiconductors section in the electronics texts that are on this site. See if you can learn why NPN nad PNP transistors are used in different situations. Ask specific questions, and someone can probably help.

5. ### Chris Wright Well-Known Member

Jul 26, 2006
62
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Check out this circuit simulator java applet, it has lots of examples and if you choose the "blank" circuit you can draw your own and watch it in animation and it "solves" the circuit. Download the zip file and you can run it off line:

6. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
12
Your circuit is drawn upside-down and reversed from the usual convention (rotate 180 degrees).

The base-emitter junction of your "top" xstr (to neg) is 0.7V and is quite effectively shorting out the rest of your circuit (the "lower" xstr and LED).

The circuit is drawing a little over 6mA thru the 1.8k and the "top" xstr and that's it!

Please add reference designators such as Q1, Q2, etc. to better aid in describing the circuitry and function.

7. ### Chris Wright Well-Known Member

Jul 26, 2006
62
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Remove R1 from the circuit.
Add a resistor between the base of each transistor and the right leg of your circuit to drop to the small voltage needed to forward bias the base.
Calculate R2 to keep the voltage and current in the limits of the LED/transistors

Now it will work with the two transistors, but the bottom transistor is not needed, Remove it also.