BASIC 
ELECTRONICS COURSE 
Page 19 INDEX

So far we have seen how the Flip Flop works. Each transistor takes it in turn to deliver current to its LOAD. A LOAD is any device placed between the collector and positive rail and may be a LED, motor, speaker, relay, globe or solenoid (a solenoid is a coil of wire wound on a former and when a current is applied, it draws a soft-iron core into its centre - this is also called a linear actuator). 
The Flip Flop (also called a Multivibrator or Square-wave oscillator) on the previous page operated at a very low frequency. This is necessary so you can see the LEDs flashing. But if you connect a speaker to the circuit, you will only get a very soft "click" every time the LED is turned on. The circuit is not designed to operate a speaker and we will show you how to change the value of the capacitors (the 100u electrolytics) to turn the Flip Flop into a MULTIVIBRATOR. But first let's look at the circuit with a CRO. 
This is a CATHODE RAY OSCILLOSCOPE and is a very handy piece of test equipment that displays the signal at any location on a circuit. 
A CRO is capable of displaying very high as well as very low frequencies signals, and when the signal has a very low frequency, the TRACE (the line going across the screen) can be slowed down to show the waveform.   The animation below shows the output of the Flip Flop on a CRO:

When the CRO is not connected to a circuit, the trace moves across the the X-axis. This indicates ZERO input voltage. When the CRO is connected, the left-hand transistor is ON and the LED is illuminated. This means the collector voltage is very low and the trace moves across the screen slightly above the X-axis. 
When the circuit changes state, (the change is very RAPID) the trace moves UP-SCREEN and continues across the screen. When the circuit "flops" back to it original state, the trace follows just above the X-axis. If the above animation is displaying too fast, the lower animation has been slowed down considerably so you can see the trace moving across the screen and rising or falling very quickly when the circuit changes state.  
 

If the electrolytics are replaced with much smaller capacitors the circuit will operate at a higher frequency and a speaker can be connected to one side to detect the output. 
The transistor will be turning on and off very quickly and this will put a pulse through the speaker to result in a tone being heard. The tone will be a SQUARE WAVE. 

When the CRO is "locked-in" on the signal, the trace (the picture) on the CRO will be stable and it does not move at all. That's the beauty of a CRO. It stabilises a "picture" of a waveform so you can see it clearly. This will only happen when the settings on the CRO are correct such as: 
1. The Sweep speed - the speed at which the trace moves across the face of the screen.
2. The Trigger is set to external so the CRO "locks-in" on each cycle of the waveform.
When the trace is stable you can read values from the screen such as time (in milliseconds) for each cycle and the amplitude of the waveform. You can also see it is perfectly square and if any distortion is occurring. 
A good quality square wave should rise and fall instantly and be square at the top. Any rounding of the wave will be due to "switching" problems and you can experiment on the circuit to see where it is coming from. 
CRO's are very easy to use. You just have to get it set up by someone "in the know" and don't fiddle with the knobs. You should be allowed to touch only two or three knobs at the beginning. 
1. The VOLTS/DIV knob attenuates (adjusts) the waveform so that its height fits onto the screen.
2. The TIME/DIV or SWEEP control determines how fast the trace moves across the screen so that the "picture" has the required number of cycles on the screen.
3. The TRIGGER (also called the SOURCE switch) stabilises the picture so that it is not jumping or flickering. 

Question 78: What is the definition of a "LOAD"

Ans: Any device being driven by a transistor. For the transistor in the multivibrator circuit above, the device is placed between the collector and positive rail. There are other ways to connect a transistor to the supply rails and the load can placed in the emitter line - more about this later. 

Question 79: What do the letters CRO stand for?

Ans: Cathode Ray Oscilloscope. 

Question 80: Name the components that control the frequency of a multivibrator:

Ans: The base resistor and capacitor. 

Question 81: When the "timing capacitor" in a multivibrator is reduced, what happens to the frequency?

Ans: The frequency increases.

Question 82: Give the two other names for a flip flop:

Ans: Multivibrator, Square Wave Oscillator.

Question 83: When a LED is illuminated in the flip flop above, is the voltage on the collector of the driving transistor HIGH or LOW? 

Ans: LOW

Question 84: In the 1kHz circuit above, why is a 300R resistor in the collector of the first transistor?

Ans: The collector of the transistor in the 1kHz circuit above must not be connected directly to the positive rail as it will create a SHORT-CIRCUIT when it is turned on and it will be damaged. The resistor limits the current and allows the voltage on the collector to vary between rail voltage and near-zero voltage.



NEXT . . .

The next page will cover another type of "flasher" circuit that flashes a LED with a completely different circuit to a Flip Flop. It's actually a very high gain amplifier and it's very interesting, once you know how it works. 

Before we go to the next page, click the button below for a memory test. The picture shows 15 components. Study the picture and start the test. You should recall about 8 items on your first attempt, 11 on your second and about 14 or 15 on your third attempt. 
This proves that you absorb about 50% of a document on the first reading, about 75% on second reading and nearly 100% on the third reading.  That's why you should come back to these pages at a later data and go through them again. You will gain a further 15% to 25% on each additional reading. So, let's start the memory test . . .


After you do the test, you will see how right I am. That's because you have not been taught HOW TO REMEMBER. 
On each additional attempt, you made a concerted attempt to remember exactly where each of the components resided. This is called "PUTTING YOUR MIND INTO GEAR". In other words, you said "I'll remember this!" 
All the test taught you is how to put your mind into FUNCTIONING MODE - it didn't help you identify any of the components with their names. But it's a start.
It did show you exactly how to put your mind into "ABSORPTION MODE."  All you have to do is tell yourself that you will remember a particular fact and then reference the fact to something you already know. It's called a "hitch" or "crutch." This way you are telling yourself that you don't have to remember anything complex. It's simply a matter of recalling something you already know and then bringing in the "new, companion fact."
It's called learning by association. If you tell yourself three things, your absorption rate will increase:
1. Tell yourself it is important to learn these facts, 
2. Keep your mind from wandering, and
3. Use the tool of ASSOCIATION.  

More on this later . . . 



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