Fun-with-Transistors.pdf

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Fun with Transistors
Fun with Transistors
Written By: Steven Robert Cypherd
SUMMARY
Playing with transistors lets you in on what makes just about everything in our world work.
Transistors are easy to work with, requiring just a few basic steps. In most cases you need
to use biasing resistors to control when they turn on and off. Biasing resistors also lower the
current needed to switch a transistor on and off. Biasing keeps a transistor in one of its
states so it only changes states when you want it to. Biasing helps with signal flow. Generic
transistors need biasing. Transistors are current devices and voltage only plays a small part
in their operation.
Resistor 1 Main biasing resistor keeps the transistor in the Off or On state. Changing this
resistor's value adjust the turn On voltage and current.
Resistor 2 Input voltage and current adjustment to lower the load on what is driving the
transistor. Transistors can change state with as little as 2 mA of current.
Resistor 3 Protects the load the transistor is driving. LEDs need a current-limiting resistor to
protect them.
You can get packs of transistors at Radio Shack.
NPN (Negative-Positive-Negative): Switches negative voltages with positive voltage control.
It has the effect of an inverter.
PNP (Positive-Negative-Positive): Switches positive voltages with negative voltage control.
Also an inverter.
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Fun with Transistors
MOSFET: Basic performance is like an NPN transistor, but they are much more.
Parts of a transistor -------------------------- MOSFET
Emitter Input Voltage ----------------------- Source
Collector Output Voltage ------------------- Drain
Base Controls the states of the transistor. -- Gate
Transistors come in many formats so check the data sheets. Even generics like the 2N2222
or the 2N7906 can handle about 30 mA or maybe up to ten LEDs. They can drive small
motors on small vehicles and small 5-volt relays. You can build oscillators, timed switches,
animated things and much more.
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Fun with Transistors
Step 1 — Fun with Transistors
Most microprocessors cannot drive more than 20 mA on one output pin at a time. This is
where transistors come in to drive the outside world. Through biasing you can connect
transistors to tri-state outputs. Hi-Z is one state of a tri-state output where the pin is
floating. Biasing keeps a transistor from being affected by this type of open circuit. You
must use a resistor between the microprocessor pin and the transistor to protect the
microprocessor. Use 220 to 1000 ohms.
Look at the pictures and see that transistors are very easy to use. For each circuit you
adjust the biasing resistors so that you drive the transistor with very little current and the
load with maximum current. These very basic circuits will drive a lot of useful things. Our
world is driven by transistors. Transistors do so much that there is a ton of information out
there for you to do anything that you want to with them.
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Fun with Transistors
Step 2
Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) are fun. Creating a touch
switch with them is easy. They remind me of vacuum tubes because the Gate is not
directly connected to anything.
You can test bipolar transistors with a volt-ohm meter; between the Emitter and Base you
should read a diode and between Collector and Base another diode. The MOSFET uses the
principle of field effect, like a Hall Effect device/transistor that switches when you pass a
magnet over it. MOSFETs seem rugged, but they are susceptible to static discharge so
keep them in their grounded holder until they are ready for use.
The touch switch is simple, but powerful. MOSFETs can drive up to 500 mA and can
handle about 60 volts. Do not use a touch switch when controlling more than 12 volts. Use
another transistor or relay to drive a high-voltage circuit from the touch switch. The touch
switch wires can be greatly extended. You can make a secret lock by placing metal tacks,
thumbtacks, nails or wires on anything. If the surface is conductive, like metal, you will
need to insulate your contacts.
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Fun with Transistors
Step 3
Lighting multiple LEDs is easy; just add up the voltage and the current of each LED to
match your power supply and/or your transistor. You connect them Cathode (-) to Anode
(+) down the line. LEDs need about 2 volts and about 2-6 mA each to work. 12 volts lights
about 6 LEDs.
My circuit is a mini LED lighting a mini LDR driving the base of the PNP transistor to light
6 LEDs on 12 volts. The LDR is a Light Dependent Resistor. LDRs have a resistance of
about 160kΩ in the dark to about 25kΩ in the light.
The biasing resistor worked out to be 2.2k connected to V+ and the Base of the transistor.
You have to play with this to find your balance point to keep the transistor off until the
LEDs lights. Generic transistors like the 2907 and 2222 have a wide operating window. I
used a 10k-25k potentiometer to find my balance point. It is fun. You can light other strings
of LEDs with this method.
Transistors are fun.
This document was last generated on 2012-11-01 03:28:02 PM.
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