You would see in many of sites this type of wireless project for voice transmission but here I'm introducing such a project having very high gain of voice receiving. High gain causing it can sense very low strength sound from a large distance as well as it can be used to hear heart rate. The transmitted sound signal can be heared with an FM radio. Now a days most of the mobiles have the FM radio features.
Here battery operated power source is recommended from 6 Volts to 12 volts. The circuit power supply is stabled at 5V by using a regulator IC 7805.You can avoid this regulator IC but the frequency may be changed continuously with respest to power consumption.
In the first stage of mic,the sound is received and amplified using NPN transistor(BC548). In the second stage,the gain of the circuit is enhanced using an LC circuit across the collector-emitter side of NPN transistor(BC548). Here the main voice signal is modulated with a frequency modulated carrier frequency ranging from 88 to 108 MHz and transmitted via an antenna(arial). Two coils of 5 turns & 15 turns are made using 22SWG copper wire.
This project transmits voice signal around 50-100 meters. In the open space area where high distribution wires are not available,I measured it's range more than 100 meters. Hence the project is very competible for detectives.
1. Use good quality coil type mic for pure voice reception.
2. For good transmission,lenght of the arial should be proper(50 cm to 100 cm).
3. DC supply using transformer may have many of disturbance so use only battery power supply.
(Note : It is illigle to make transmitters without licence having range more than 100 meters.)