Most of the time, students in the back rows of a large classroom can't hear the teacher. So, the teacher has to literally yell for everyone to hear her. Here is a Wireless PA circuit that can be used by teachers as a wireless speech aid so that their voice can reach every student in a big classroom. The circuit is made up of a wireless microphone and an amplifier for the receiver. The teacher speaks into a wireless microphone, and what he says is sent over the FM band. You can cover the whole room by putting several receivers with speakers in different places. Fig. 1: Circuit of wireless microphone Part of a wireless microphone The transmitter part of a wireless PA for classrooms is made up of an electret microphone (MIC), two BC547 npn transistors (T1 and T2), and a few other parts. The electret microphone MIC picks up the sound signal, and the transistor T1 boosts it. The signal that has been amplified is sent to the base of transistor T2. Here, the signal at the base of transistor T2 changes its junction capacitance, which affects the L-C tank circuit made up of L1 and VC1. A Colpitts oscillator is similar to the tank circuit. The 27pF

capacitor attached to the antenna keeps the L-C circuit from being affected by the capacitance of the antenna. The inductor L1 is made by wrapping seven turns of 24 SWG copper wire around an air core that is 5mm in diameter. Use any wire that can bend as an antenna. Section of the receiver amplifier The receiver part of a wireless PA system for classrooms is made up of an audio amplifier called TDA2030 (IC1), an FM receiver kit, a loudspeaker called LS1, and a few other parts. There are many Sony ICs like CXA1019 and CAA1619 in FM receiver kits. Some of them also have Philips ICs. Both 6 V and 12 V can be used to power an FM kit. Choose 12 V for this project and power it with a 12V, 1A adapter. Fig.2.Circuit of receiver amplifier section Choose a kit that doesn't have a gang capacitor but does have trimmers instead. Even a kit with ganged capacitors will work. But once the circuit is aligned with the FM transmitter/wireless microphone, it is hard to keep the gang in one place.

The FM kit's output is hooked up to the sound amplifier made with IC1. A pair of diodes, D1 and D2, and a 100-kilo-ohm resistor, R8, are used to create a "virtual earth" that lets IC1 work with just one power source. Use a heat sink to keep IC1 from shutting down because it's too hot. How the circuit works is easy to understand. Once the transmitter and receiver are tuned (see the section on calibration), the microphone picks up the teacher's voice and sends it over the FM band. The sound is picked up by the FM kit, and the amplifier makes it louder. This louder sound can be heard at loudspeaker LS1.

Keep the transmitter and receiver about three meters away from each other. First, turn on the receiver and tune it to a fixed frequency that is not on any of the regular channels. Put the volume in the middle. Now, use switch S1 to turn on the wireless mic. Using a small plastic screwdriver, turn trimmer VC1 a little bit at a time (no more than 5 degrees) until the radio stops "hissing" and the null point is found. This means that the frequency of the wireless microphone is now the same.