Starts transmitting when it hears a sound! Each section is clearly laid out on the circuit board. Peaking circuit included to peak the FM Tx for maximum efficiency. Trim pot adjusts the level of sound which will trigger the switch. Full instructions included in the kit.
This kit combines two kits in one - a powerful two stage FM transmitter and a sound activated switch.ﾠ These two parts of the circuit can clearly be seen from the schematic diagram.ﾠ The sound activated switch is at the top of the circuit.ﾠ The FM transmitter is underneath.ﾠ A trimpot adjusts the level of sound which will turn on the kit.ﾠ When there is no sound, the transmitter turns off after a short delay.
The kit is constructed on a single-sided printed circuit board (PCB).ﾠ It has a silk screen overlay on top to aid construction.ﾠ On the bottom there is a solder mask to help in soldering.ﾠ
The microphone and first audio amplifier stage are common to both the transmitter (Tx) and the sound activated switch (VOX) stages.ﾠ Let us look at the electret microphone in more detail.ﾠ An electret is a permanently charged dielectric.ﾠ It is made by heating a ceramic material, placing it in a magnetic field and then allowing it to cool while still in the magnetic field.ﾠ It is the electrostatic equivalent of a permanent magnet.ﾠ In the electret microphone a slice of this material is used as part of the dielectric of a capacitor in which the diaphragm of the microphone forms one plate.ﾠ Sound pressure moves one of its plates.ﾠ The movement of the plate changes the capacitance.ﾠ The electret capacitor is connected to an FET amplifier. ﾠ These microphones are small, have excellent sensitivity, a wide frequency response and a very low cost.
The amplifier is a standard self-biasing common emitter amplifier.ﾠ The amplified signal is taken directly to the Tx section.ﾠ The signal goes to the VOX stage via a 100K potentiometer sensitivity control.ﾠ Two 100n capacitors isolates the microphone from each of the following stages which allows only alternating current signals to pass.
The input to the VOX has a second amplifier stage to amplify the signal so it is suitable for the use by the Schmidt triggers.ﾠ The input to this digital section is biased mid-way between the supply voltage by the two 1M resistors.ﾠ Before we look at the digital section let us review the operation of the 74C14, the Inverting Schmidt Trigger IC, from National Semiconductor.ﾠ
At the operating voltage (6V) there is about 2.4V hysterisis gap between 1.6V and 4V.ﾠ This means that as the input voltage increases from zero to six volts the Schmidt circuit will not rigger until it get to about 4V.ﾠ But as the voltage falls from six volts to zero the circuit must fall to 1.6V before it changes state.
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