Wiring Schematic Diagram

This FM wireless microphone transmitter can transmit speech over a short range. It can be used as a simple cordless microphone. This circuit has good frequency stability and has range over 1 Km (under good conditions). This project features RF amplifier buffer (10dB gain), an AF preamplifier to boost the modulation and good microphone sensitivity. You can use it for guitars and remote control system.

This circuit quite simple to build. The two BC547 transistors can be replaced with any small-signal NPN transistor, such as the 2N2222. L1 is 3.25 turns in spiral form and is an integral part of the PCB foil pattern. The final stage is a BC557 PNP general purpose device. If you use different devices then you should select the 1M0 resistor for 5-volts DC at the collector of the first transistor. Select the 47K resistor for 3 – 4 volts on the collector of the third transistor.

 The inductance L is the critical part and should be handmade. Use an AWG24 (0.5mm) enameled copper wire and make 2-4 loose turns with about 4-5mm diameter. Try to loosen or tighten the coil, or trim the variable capacitor, and try to receive the signal on around 90MHz. Don’t forget to tighten the coil using a glue to fix the coil, avoiding mechanical deformation that change its inductance value.

This circuit uses three bipolar transistors to accomplish the Same result (previous post) with the touch contact referenced to the negative or ground end of the supply. The 12volt 20W lamp will be turn on when the contacts are touched with skin resistance about 2M or less. This circuits are available also for other applications, for additional current the lamp could be replace with a 12V relay and diode across the coil.

Since the base of a bipolar transistor draws current and the current gain is usually Less than 200, three transistors are needed to raise the microamp current levels through the touch contacts to a couple amps needed by the light.

Here is a circuits light a 12V/20 watt lamp when the contacts are touched and the skin resistance is about 2 Megs or less. This circuit uses a power MOSFET (IRF510) which turns on when the voltage between the source and gate is around 6 volts. The gate of the MOSFET draws no current so the voltage on the gate will be half the supply voltage or 6 volts when the resistance across the touch contacts is equal to the fixed resistance (2 Megs) between the source and gate.

Drain to Source Voltage        : 100 V
Drain to Gate Voltage          : 100 V
Continuous Drain Current (ID)  : 5.6 A
Pulsed Drain Current (IDM)     : 20 A
Gate to Source Voltage (VGS)   : ±20 V
Maximum Power Dissipation (PD) : 43 W 

When the wiper of the potentiometer is at the uppermost position, the capacitor will charge quickly through both 1K resistors and the diode, producing a short positive interval and long negative interval which dims the lamp to near darkness. When the potentiometer wiper is at the lowermost position, the capacitor will charge through both 1K resistors and the 50K potentiometer and discharge through the lower 1K resistor, producing a long positive interval and short negative interval which brightens the lamp to near full intensity. The duty cycle of the 200 Hz square wave can be varied from approximately 5% to 95%. The two circuits below illustrate connecting the lamp to either the positive or negative side of the supply.