RF Signal Attenuator up to 30dB for 1MHz - 500MHz Input

Description: 

This is a PIN diode based attenuator circuit operates with 1MHz - 500MHz input. PIN diodes are one of the most common parts used in RF applications generally as switches in transmitter and receiver circuits.  They are usually placed at antenna or amplifier parts. In some applications PIN diode functions not only as switch but also only as potentiometer.

Resistance of PIN diode is high when no current passing through it. When current starts flowing, resistance decreases and diode conducts. In this circuit we use PIN diode as a potentiometer. You can use this circuit in a wide variety of applications. Some examples;

• Level control in signal generators
• Regulation of antenna signal at receivers
• Power control in RF amplifier drivers
• AGC control at receivers...

3V FM Transmitter for Short Distance - 88MHz to 108MHz

Description:

Features of the transmitter:
Supply Voltage:+3V DC
Current: 21mA
Output Frequency: 88MHz - 108MHz
Modulation: FM
Frequency Shift: ±30KHz
Input: Electret microphone or other input audio source

The important part of the circuit is formed of the Colpitts type oscillator. C3,C4,C5,C6,CD1-CD2 and L1 determines the frequency. BF982 and dual gate MOSFET are active parts in oscillator. When the input impedance of the MOSFET gate inputs are high, LC tank is not affected. However transistors force the LC tank and cause phase shift.

Two driver stages are added to isolate the antenna from oscillator. First stage (BF199) amplifies the low signal of the oscillator and works as a constant load. The second stage (BFR90) amplifies the signal going through the antenna some more. A short copper wire can be used as an antenna here. Attaching a large antenna to this circuit is unnecessary because the output power is low.

Notes: 

1. Coil 1: Winding wire must be 1 mm thick and isolated. Number of turns: 3.5 .  5mm core must be used and the distance between each turn must be 1mm.
2. You can use BF199 instead of BFR90.
3. If you can't find the varicap diode, you can use two BB405 instead.

1.2 - 12V / 1A Low Voltage LED Indicator Power Supply by LM317

Description:

This is a 1.2 - 12 V, max 1A power supply with a low voltage indicator LED. The indicator part includes three diodes and one LED. For example you are charging a battery, you can observe the charge status at that moment. Another advantage of this circuit, when the drawn current exceeds 1A (practically 0.85A), the current protector in LM317 intervenes and LED indicator warns you about the very low output voltage.

Be careful while choosing the transformer. Most of the products are specified as 10VA but their outputs are not as said.

Another good property of this circuit is the mains noise does not pass to the DC part.