New Tech Embd Labs

Jhotwara, Jaipur, Rajasthan

To begin time keeping and initiate the bell mechanism using the Automatic School Bell circuit, the peon needs to momentarily press switch S1 to mark the beginning of first period. Thereafter, the bell sounds every 45 minutes to indicate the end of consecutive periods, except immediately after the fourth period when it sounds after 30 minutes to indicate that the lunch break is over. When the last period is over, LED2 glows to indicate that the bell circuit should now be switched off manually. In case the peon has been late to start the school bell, the delay in minutes can be adjusted by advancing the time using switch S3. Each push of switch S3 advances the time by 4.5 minutes.

The Quality FM Transmitter provides good signal strength up to a distance of 500meters with a power output of about 200 mW. It works off a 9V battery. The audio-frequency modulation stage is built around transistor BF494 (T1), which is wired as a VHF oscillator and modulates the audio signal present at the base. Using preset VR1, one can adjust the audio signal level. The VHF frequency is decided by coil L1 and variable capacitor VC1. By reducing the value of VR2, a greater power output can be achieved.