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.

1.3 - 32 V / 5A Power Supply w/ Short Circuit Protection by LM338

Description:

This is a LM338 based power supply which is not complicated and easy to build. I am using the supply for a long time, have no problem yet. Only current adjust is missing but I overcome this situation by using an LCD panel ampermeter. There is no PCB for the circuit. I took a 3x16 copper plate and strip the unused areas by a knife. If you want you can use analog meters instead of LCD panels.

Notes: 

1. Use thick wires for connections.
2. When connecting the LM338 to the heatsink use thermal paste. 
3. Use external supplies for LCD panels. They can be 9V batteries. The panels I use draws 1mA current and the batteries last sufficiently long time. If you have small transformers which can supply regulated 9V, you can use them. You need separate transformers for each panel!
4. The transformer should be 100 Watt but if you don't need high current you can try transformers that you already have. (I can draw 6A from 100Watt transformer.)
5. If you have multi winding transformer you can use the diode connections shown in schematic. Diodes must be 10-15 A.
6. Be careful while connecting LM338. Don't put it inverse.
7. Output of your transformer mustn't exceed 25V AC...