LED Flip Flop Project

You’re a newbie in electronics and you need to practice your electronics knowledge and then I suggest you to build this LED Flasher (Flip-Flop). This circuit is very simple and very easy to understand…

INTRODUCTION:

Flashing circuits are very interesting. They grab your attention and can be used in many applications. They consume very little energy and a single cell can last as long as 12 months. This is the simplest circuit capable of alternately flashing two LEDs.

SCHEMATIC DIAGRAM:


COMPONENT LIST:

The component list is organized by part type, description and colour codes where applicable. You may wish to sort the parts into a compartmented container as you inventory them. This will aid you in building the kit.

[ ] 470 Ohm Resistor ( ¼ Watt)(yellow, violet, brown and gold)
[ ] 470 Ohm Resistor ( ¼ Watt)(yellow, violet, brown and gold)
[ ] 10 K Resistor ( ¼ Watt) (brown, black, orange and gold)
[ ] 10 K Resistor ( ¼ Watt) (brown, black, orange and gold)
[ ] Red LED
[ ] Red LED
[ ] 100 uF 25V Electrolytic Capacitor
[ ] 100 uF 25V Electrolytic Capacitor
[ ] 2SC1815 NPN Transistor
[ ] 2SC1815 NPN Transistor
[ ] 9V Battery Clip


CIRCUIT INFORMATION:

The Flip Flop is a symmetrical arrangement using two transistors with cross-coupling. Each transistor has a base bias resistor (10k in our case) and a LED with 470R resistor in the collector lead to form the collector load.

The circuit consists of two identical halves and is called a Flip Flop because one half is ON while the other half is OFF. The ON half is keeping the OFF half OFF but it cannot keep it off indefinitely and gradually the OFF half turns ON via the 10k base-bias resistor.
This drives the ON side OFF and the circuit changes state. In other words it flips over. The same events occur in the other half of the cycle and the circuit eventually flops back again.
In reality the circuit is quite simple as one half is exactly the same as the other and there’s only 5 components in each half.

The circuit is self-starting and only one LED is on at a time. It is a free-running multivibrator (this means it does not stop) and we will describe its operation in a non-technical way. A free-running multivibrator is also called an astable multivibrator (meaning is has no stable states) and that is why it flips from one state to the other continuously.

The standard way to draw this type of multivibrator is to show the two capacitors crossing at the centre of the circuit, this also gives the circuit symmetry and makes it easy to recognize.
The other way to identify an astable multivibrator is knowing that it has two capacitors. (The monostable multivibrator has one capacitor and the bistable multivibrator has no capacitors.)

In simple terms, the astable [pronounced (h)ay-stable] multivibrator has two states. When one transistor is turned on it operates (supplies current to) a LED (or other device) in its output line and at the same time keeps the other transistor off. But it cannot keep the other off forever and eventually the other transistor begins to turn on. When it does, the action turns the first transistor off slightly and a change-over begins to occur. This produces the flip action. After a short period of time the other half of the circuit cannot be kept off and the whole arrangement flops back to the first state.

The components that determine the frequency are the electrolytics and two base-bias resistors. If these values are changed, the frequency will alter. For instance, if the electrolytics are reduced in value, the frequency will increase and if the resistors are decreased, the frequency will increase. If you increase the frequency of this circuit to more than 20 cycles per second, it will appear as if both LEDs are on at the same time. But the fact is the circuit will be operating faster than your eye can see and that’s why we have chosen large values of capacitance to slow it down.

When the electrolytics and resistors are made equal value (as in our case), each LED flashes for the same length of time. This is called an equal mark-space ratio: (50%:50%). This means the flip time is the same as the flop time.
Theses components can be changed to any ratio, to give different effects.

CONSTRUCTION:

VERY IMPORTANT: Pay careful attention to the following:

1. Read this entire document thoroughly prior to starting construction of your kit.
2. Identifying each component as per component list.
3. Observe general construction precautions when handling and assembling components.
4. Lightly scrape the legs of all the components clean with a sharp cutting knife. This is to remove corrosion.
5. Use steel wool to clean the copper track on the PCB before soldering any components to it.

Do not reverse polarity of battery/power or wires!!! THIS WILL DAMAGE THE LED FLASHER!!!

You are now ready to begin construction of the LED FLASHER. Follow standard construction practices when building the unit.
Use a temperature-controlled, fine-tipped soldering iron of relatively low wattage (25 watts maximum, 15 watts is ideal) and a good quality solder for construction. Keep the tip of your soldering iron bright and clean, wiping it frequently on a wet rag or sponge. Make solder joints carefully, but swiftly. Prolonged heat on a PC board pad can be as disastrous as it can ruin the PC board. Two to three seconds should be enough time to apply heat to any joint. Due to the proximity of some of the tracks on the PC board, solder bridges are a very distinct possibility. However the layout of this PC board was done to assist builders with little or no experience. Enough space between components exists for the ease of soldering and fitting of the components, hence the larger PC board. Following these points could eliminate several hours of troubleshooting (or worse). This is good practice when working on any kit.

You will need small flush or semi-flush cutting pliers (I use my wife’s nail clipper) and small-tipped long nose pliers. A magnifying glass may prove helpful to identify the values of the small components


Refer to the above diagram for clarification of parts placement and install one component at a time.

THE PCB OUTLAY: (Not to scale)


Resistors

Resistors have a lead spacing of 10.16 mm and should lie flat on the PC board. You may wish to use a lead former to pre-form the resistor leads for neatest appearance. Installation of the resistors on the board is not critical. WARNING: Be careful when clipping leads, as they have a tendency to fly towards your eyes! Take appropriate precautions (grasp leads and wear eye protection).

Capacitors

All capacitors should be mounted as nearly flush to the board surface as practical without stressing the leads. Electrolytic capacitors are polarized. The positive lead goes in the hole on the board marked with a “+”. Be careful! Typically the negative lead is marked and sometimes the positive lead is marked.

Now check your work. All leads should be soldered. There should be no solder bridges or cold solder connections.

Light Emitting Diodes (LEDs)

LEDs are polarized components. The flat side on the body and the shorter lead of the two identifies the cathode lead. NOTE: Cathode = positive; Anode = negative.

Transistors

Be careful when installing the transistors, be sure that the E, B, and C terminals are connected properly). Now find the 2SC1815 NPN transistors; it looks like a black bead
with three legs and bears the mark, “C1815”. The component outlay diagram will guide you as to how to orient this component. Please note that the middle legs must be bent slightly to accommodate the whole patterns for transistors. Avoid excessive bending as this can damage the component.

Power Connector

Solder the power connector to the PCB. The red lead to + (positive) and the black lead to – (negative).

FINALLY:

Finally you have finished installing components. Before connecting a battery it is essential that you check your work. Check that all components were installed correctly.
Check for solder bridges. Check for dry-joints. Once you satisfied that every thing is correctly installed it is time to turn the power on. SWITCH ON!!! No smoke and LED’s working, your project is working. Congratulations! Have fun with the project!