Fun With Transistors

Playing with Transistors lets you in on what makes just about everything in our world work. Transistors are easy with 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 them 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.
MosFet Basic performance is like NPN transistor, but they are so 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 paper work. 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 so much more.

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 use tri-state outputs. Hi-z is one state of a tri-state output where the pin is floating. Biasing keeps a transistor from being affect 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 transistor with very little current and drive the load with maximum current. These very basic circuits will drive a lot of useful things. Our world is driven by transistors. Transistor do so much that there is a ton of information out there for you to do anything that you want to with them.

MosFet drive much higher current with much finer control. Metal-Oxide-Semiconductor Field-Effect Transistors are fun. Like a touch switch is easy. They remind me of vacuum tubes because the Gate is not directly connected to anything. Other transistors you can test with volt-ohm meter between the Emitter-Base you should read a diode and between Collector-Base another diode. The MosFet uses 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 ready for use.

The touch switch is so simple, but powerful. MosFets can drive up to 500 mA and these can handle about 60 volts. Do not use a touch switch for people with 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 need to insulate your contacts. Mylar tape or electrical tape works well for this. Even hot glue.

Wire one or more of the contacts to the Gate lead.
Wire other contacts to the V+ lead.
Wire other contacts to the Ground V- lead.
Touch a V+ contact and a Gate contact at the same time to turn ON the MosFet.
Touch a Ground contact and a Gate contact at the same time to turn OFF the MosFet.
Anything is possible.

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. You can test your LEDs with my ‘Testing an Led’ picture and document. LEDs need about 2 volts and about 2-6 mA each to work. 12 volts lights about 6 LEDs.
 
A simple PNP 2907 transistor can drive 6 LEDs. My circuit is a mini LED lighting a mini LDR driving the base if the PNP to light 6 LEDs on 12 volts. The LDR is a Light Dependant Resistor. LDRs run about 160k dark to about 25k 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 circuit to find your balance point to keep the transistor off until the LED lights. Generic transistors like the 2907 and 2222 have a wide operating window and they can leak. I used a 10k-25k potentiometer to find my balance point. It is fun. You can light other strings of LED with this method.