Electronic Siren with 2 transistors

Posted on Dec 19, 2012

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 The only differences are the LED has been removed and the 22R resistor has been replaced by a mini speaker. 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.

Electronic Siren with 2 transistors
Click here to download the full size of the above Circuit.

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....

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