L32, Simple Siren


The 10uf 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-

  • Resistor = 1k resistor

  • Resistor = 100k resistor

  • Capacitor = 10nf

  • Capacitor = 47uf 16v

  • Transistor = BC 547 NPN

  • Transistor = BC 557 PNP

  • Speaker = 8-ohm mini

  • Touch plate

Circuit’s working-


There are many ways to explain how a circuit works, but we will discuss about three different approaches - the voltage, the current and the resistance approach. Sometimes we have combined all three.

The circuit works using the resistance approach.When the touch plate is touched, the 47uf capacitor gradually charges via the resistance of the plate. The touch plate resistance varying according to the pressure of your finger.


The base of Q1 sees the voltage on the capacitor 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 10nf 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 10nf capacitor is connected to this and both ends begin to fall and turn off Q1.


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.

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