Here is an idea for a low cost simple electronic project. It's a combination of two oscillators and an audio amplifier.
One of the oscillators is a low frequency two LED light blinker the other is an audio frequency oscillator.
When connected together you will have a device that might drive your neighbors crazy, your wife or husband and perhaps even your parents, so
please be careful!
This project also introduces
the bi-polar transistor.
Transistor theory is a complex subject and too big for this website. I can however
show you some useful things about how to use a transistor in various circuits.
A transistor is a semiconductor whose conductance can be controlled from very high conductance to very
low conductance using a very tiny control current. The connections to the transistor are the "base", "emitter", and "collector".
The base controls the current flow between the emitter and collector. Like the diode, the transistor must be
biased correctly to operate. There are two types a PNP and a NPN version. The letters indicate the
three layers of semiconductor material sandwiched together to make the transistor. This also determines voltage polarity to bias
them correctly. A PNP is a positive-negative-positive device and the NPN is a negative-positive-negative device. Don't
worry, you will understand after awhile. The schematic symbol looks like this 
for
an NPN and like this
for PNP. Notice the direction of the arrows. They indicate the
emitter and the polarity of bias required. The horizontal wire above the emitter is the 'base' and the last connection is the
collector.
In this project we will be using two NPN transistors. The center letter of the three tells the polarity required
to bias the transistor into the "on" mode. We will be using the transistors in the common emitter mode which means that
both emitters will be connected to the negative side of the power supply or ground as it is called. We will apply a small
positive voltage to the base to cause the transistor to begin conducting. We will apply a positive voltage to the collector
through the LED diode so the diode will 'light' when the transistor conducts.
Here is the schematic: It is what is known as a "classic" multivibrator.
(Strange because nothing actually vibrates!)
OK, let's start at the top and work down. The first thing is the battery connector.
It has two wires one red and one black. Of course the red is positive and the black is negative. The red connects to two wires,
one goes to the resistor R1 and the other goes to R2. Now these wires can be the wires that come as part of the resistor or
they can be separate wires, the choice is yours.
Here is the surprise, the black wire connects to a new symbol.
This is the symbol for ground or "common".
It is
usually the chassis or metal box the circuit is built
in. But in any case it is where the negative terminal of the battery reaches all the needed points. Notice that
the both emitters of transistors Q1 and Q2 connect there, as well as both base resistors R3 and R4. They are all
connected to the same common point or ground.
At this point, we will look at only one half of the schematic since both halfs are identical. On the left side, the other
end of R1 connects to LED diode D1's anode and D1's cathode connects to the collector of transistor Q1.
Then Q1's emitter goes to ground. Now look at Q1's base. It connects to three parts, R3,D3 and the other is C1.
R3 and D3's anode connect to ground. The other end of C1 connects over to transistor Q2's collector. There are a
couple of very important things to watch here. One is to be very sure you have identified the wires correctly
for the transistors emitter, base and collector. They are shown on the package they come in. Next capacitor
C1 and C2 are polarized. They will have a black mark indicating the negative lead. (usually) Or a big red + marking
the positive lead. Be sure to observe the polarity which is shown on the schematic.
How it works:
When the battery is connected both capacitors C1 and C2 begin to charge through the diodes and the base resistors R3 and R4
This will begin building a positive voltage on the transistor bases but because not all the parts will be exactly identical,
one transistor will turn on before the other.
As soon as one turns on the collector of that transistor will pull down toward ground completing the circuit through
one of the LED's. That LED will light because it's cathode is pulled to ground through the transistor but look closely
and you will see that capacitor connected to that diode will begin to disharge back to ground. This will place a
positive voltage on the opposite transistors base and turn on that transistor. Now that action will cause a positive voltage
to be applied to the other transistor's base turning it back on again which will cause the second transistor to turn
on again and so on over and over again. Diode D3 and D4 discharge the capacitors C1 and C2 between cycles so they are ready to charge
again on the next cycle.The flash rate is determined by the value of R3, R4 and C1, C2.
Now here is the Audio Oscillator
This is a schematic of a type of audio oscillator known as a twin "T" phase
shift oscillator. It gets its name from the phase shift network made up of R3, R4 and C3 plus C1, C2 and R2 which shifts the phase of
the signal fed back from the collector to the base by 180 degrees to make the oscillator oscillate.
You will notice that the emitter of Q1 is not connected. Bringing this connection to ground will turn on the
oscillator or 'key' it as it's called. You could connect this to a telegraph key and make a code practice oscillator.
We will be connecting this to one of the collectors on the LED flasher. Then when that LED comes on the oscillator
will start.
Near the bottom of the drawing you will see the output which will connect to the audio amplifier.
The Audio Amplifier
This is the last part of this project and it uses an intergrated circuit or 'IC' as the active component. IC's
contain many parts such as transistors, diodes and resistors all inside a very small package. This IC is designed
to be an small low power audio amplifier and is not too easily damaged.
But be careful not to try driving a large load
like big speakers with this small IC.
Build these circuits one at a time or put them all on the same board. If you build them separately be sure to keep the
connecting cables as short as possible.
On the right is a diagram of the LM386 IC. Use this to find the correct pins to solder to. Actually, it's best
to use an IC socket and not to solder directly to the pins of the IC. Also make sure that all the grounds are tied
together on all the boards if you do it in pieces.
If you build a second audio oscillator, you can change the value of the phase shift network to change the frequency
of the oscillator. If you connect the second oscillator to the other
collector on the LED flasher you will have two tones that alternate with each LED. Some values to try are .05uF for
C1 and C2 as well as 18K for R3 and R4. That will make a lower pitched note. Just have fun experimenting!
SIMPLE TEST EQUIPMENT
This is an easy to do one evening project.
It's a Zener Diode Tester you can build with just a few parts.
Often you can find a great bargin at a surplus store or at a flea market
on un-marked zener diodes. This device here will not only test for
good diodes but will tell you the zener voltage of that diode.
Zener diodes are used as voltage regulators and will begin to conduct in the
reverse or "breakdown" mode when enough voltage is applied. This voltage will
remain constant or "regulated".
To use this tester, supply enough voltage from a battery or power supply that will
cause the highest voltage zener you might have to operate. If you might have some
24 or 48 volt types then you need that voltage as a minimum. I use 12 volts on mine
since most of the diodes I use operate below that voltage. Next turn the pot all the
way down (zero volts). Attatch the diode and begin to increase the voltage. Watch the meter
and when it stops going any higher you have reached the "zener point" for that diode.
Now, just read the voltage on the meter and mark the diode. If the meter doesnt
read as you increase voltage, try reversing the diode. If that dosent work you have
a shorted diode.
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