Page 18: SIMPLE SIREN

Build your own Simple Siren and learn how it works
This is Project #5 on the PC board in “5-PROJECTS”

This project is built on the third section of the PC board, identified by “SIREN” and “Project 5.” You will notice the similarity between this circuit and the LED FLASHER circuit from project 2. The only differences are the LED has been removed and the 22R resistor has been replaced by a mini speaker.

The 10u has also been replaced with a smaller value of capacitance (10n) and this makes the circuit operate at a much higher frequency. The result is a tone from the speaker. (If you connected the speaker to the LED Flasher circuit you would get a “click - click - click” at 2 clicks per second.

This SIMPLE SIREN circuit uses components we have covered in the front of this book and up to now you have seen how a varying current on the base of the first transistor affects the flash rate of the circuit. Now you will HEAR how the varying current alters the frequency at which the circuit operates.

PARTS LIST

• 1 - 1k resistor (brown-black-red-gold)
• 1 - 100k resistor (brown-black-yellow-gold)
• 1 - 10n greencap or ceramic
• 1 - 47u 16v electrolytic
• 1 - BC 547 NPN transistor
• 1 - BC 557 PNP transistor
• 1 - 8R speaker
• 1 - touch plate - (from project 2)
• 1 - 20cm hook-up wire for speaker

1 - “5-PROJECTS” PC Board

HOW THE CIRCUIT WORKS

There are many ways to explain how a circuit works and we have presented three different approaches in this course - the voltage approach, the current approach and the resistance approach. Sometimes we have combined all three.

In this final project we will explain how the circuit works using the resistance approach. We start with the touch plate. When the touch plate is touched, the 47u electrolytic gradually charges via the resistance of the plate. In the notes we have explained how the touch plate works, with its resistance varying according to the pressure of your finger.
The base of Q1 sees the voltage on the electrolytic and when it is about .6v, the transistor starts to turn on.

Since Q1 is directly coupled to the second transistor, Q2 turns on too. When Q2 turns on, the resistance between its collector and emitter reduces and allows current to flow. This causes current to flow in the voice coil of the speaker and pulls the cone towards the magnet.

This is the first half of the cycle for the speaker.
Also connected to the collector of the PNP transistor is one end of a 10n capacitor and when one lead of the capacitor rises, the other side rises too. (This is because it is uncharged at the moment). This has the effect of turning on both Q1 and Q2 even harder. This action runs around the circuit until both transistors are turned on fully.

At this point the 10n capacitor begins to charge via the base-emitter junction of Q1 and the collector-emitter junction of Q2. When the capacitor becomes nearly charged, the charging current reduces and it cannot keep Q1 turned on as much and it begins to turn off slightly.

This begins to turn off Q2 and the voltage on the collector of Q2 falls. The 10n capacitor is connected to this and both ends begin to fall and turn off Q1. This action turns both transistors off and the voltage on the base of Q1 is below the negative rail (as explained in Project 2).

Current through the voice coil of the speaker ceases and the cone is released. This completes the cycle for the speaker and it’s the action of pulling the cone towards the magnet and releasing it that produces the tone.

The charge on the capacitor is now cancelled by the current from the 100k resistor and it begins to charge in the opposite direction so that the voltage on the base of Q1 rises to .6v. At this point the NPN transistor turns on again and the cycle repeats.

If the touch plate is kept touched, the tone from the circuit gradually rises as the time taken to charge the capacitor at the end of the cycle will be shortened. This is due to a higher voltage being present on the electrolytic and thus a higher current will flow through the 100k resistor to charge the capacitor faster.

Most of the explanations of how the circuits work have opened up more questions than they answered. This is only a commencement book and future books will elaborate on the operation of the circuits in more detail. Even if you have only learnt the resistor colour code and got the projects to work, you will have achieved all this book has intended to get across.

Furthermore, if you like what you have learnt, electronics will be buzz and a very rewarding hobby. Look out for the next books in the series.

ASSEMBLY

All the components fit on the section of the PC board marked “SIREN.” The two resistors lay flat on the board and the other components are pushed up until they are about 3mm (3/16”) from the board. Use the layout diagram on this page to see where they go and how they fit. Don’t forget to hold each part as you solder it to make sure it doesn’t get too hot.

Now for the assembly.

Collect the parts and lay them on the work bench.
You are now ready to start. Mark off each step as you do it.

( ) Bend the leads of the 1k resistor to 90° and push them through the holes identified by the 1k symbol on the board and hold the resistor while soldering it. Cut the ends of the wires with a sharp pair of side cutters making sure you do not cut any of the solder joint, as this may damage it.

( ) Repeat with the 100k resistor.

( ) Fit the 10n capacitor by pushing the leads through the holes until the body of the capacitor is almost touching the board. Solder the leads quickly so that the component does not get too hot. Cut off the leads neatly.

( ) Fit the 47u electrolytic with the negative lead close to the edge of the board and the positive lead down the hole marked with a ”+.”

( ) Fit the PNP transistor at the position marked on the board with a “D” symbol, making sure the leads are correct for the transistor you are fitting.

( ) Fit the NPN transistor in the same way.

( ) Connect the touch plate to the holes marked on the board via the two wires attached to it, (you may have to remove it from the other section of the board.)

( ) Fit the speaker wires to the speaker and solder the other ends to the board.

The project is now complete.
Slide the power switch on and the touch the touch plate. After a short while the siren will start up. Keep your finger on the touch plate and the tone will increase. You can regulate the tone by pressing lightly or with more force.

The Siren components fitted to the board.
Touch the Touch-Plate to increase the Siren tone release to decrease tone.

IF THE CIRCUIT DOESN’T WORK

If the circuit doesn’t work, you should go over the construction notes again, making sure you have not left anything off the board. Look at the solder side of the board and make sure no joints have been left unsoldered.

If the other projects are flashing when the power is switched on, the battery will be ok. This circuit will work down to a voltage of 3-4v, so it is not voltage critical.

Look at the speaker to make sure the leads are soldered correctly. The other major cause of a mistake is the transistors. Make sure they are the correct types for both the NPN and PNP positions and make sure they have been fitted correctly.
Get someone else to check this for you as it is difficult to check your own work.
If you have held the transistors while soldering, they will not be damaged, but if you had to let go, they could be damaged.

Buy two more as “spare parts” and fit them to the board.

In later pages we will show how to test the circuit using a multimeter and other test equipment but we have not covered these yet and the only thing you can do is visually inspect the board for correct parts placement and make sure the soldering is neat.

If you have made a mess of your PC board, it would be a cheap price to buy another board (or book), get another kit of components and start again.
This time you will make a much neater job and learn a lot in the process.
You really have to feel you have been successful with this e-book before you should go on to the next in the series.

INCREASING THE VOLUME

The output volume of the siren can be increased by adding 4 components. The 10R and BC338 create a buffer stage to increase the current.
The BC557 is not capable of delivering more than about 50 to 100mA and the sound is not very loud.
By adding a BC338, the current to the speaker will be increased and the output volume will increase.
These 4 components are available for FREE by emailing Colin Mitchell at Talking Electronics and they are soldered to a small matrix board and added to the project.

WHAT HAVE YOU LEARNT?

These are the areas we have covered in this e-book: Tick those you have understood:
( ) Recognising components such as transistors, resistors, capacitors.
( ) Placing components correctly on a Printed circuit board.
( ) Soldering components neatly to a PC board.
( ) Holding components while soldering so that they do not get too hot.
( ) Understanding the concept of resistance - high resistance and low resistance.
( ) Understanding the concept that a capacitor stores energy - it “charges up.”
( ) Understanding a speaker produces a tone by current flowing through
the voice coil then ceasing to flow and repeating the process to produce a tone.
( ) Understanding a LED produces coloured light when current flows through the special type of crystal it is made of. The colours are: red, green, yellow, orange and blue.
( ) Understanding a transistor is an amplifying device with the
base as the input and the collector as the output.
( ) Understanding current flows through a circuit when it is switched on. In the siren circuit for example, there are a number of different current paths and a different value of current flows through each path.

You don’t have to FULLY UNDERSTAND any of the concepts, just be aware that they exist and be prepared for further study in future pages.

THE END

This completes the 5 projects.
There is a lot to be learned from the circuits in this project as they are often used in our other projects.
You can experiment with the circuits to create different effects. By placing a different value resistor across those on the board, you will be able to see the effect of lowering the resistance.
Some of the capacitors can also be changed and the effect will be quite noticeable. This is a very good way to find out the effect of various components on the operation of the circuit.

The only page remaining is the TEST PAGE. See how much you have learned by taking the test …

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