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12V Dual Power Supply
Posted on Sunday, January 6, 2013   •   Category: Power Supplies


12V dual power supply has symmetrical voltage output +12V and -12V with limited current to 100mA. It has been built to power three OPA627 opamps of Audio DAC I am building with PCM1792 & PCM1794 chips. Circuit has on the primary side only fuse. I couldn't find smaller than 50mA. We can connect power cord directly to the X1 connector or via power switch on the chassis. On the secondary side of transformer are connected two fuses 100mA and after them is bridge rectifier. For filtering of rectified voltage there are C1 and C2. Next are positive and negative voltage regulators 78L12 and 79L12 with decoupling capacitors C3 to C6 close to regulators. Next are small filter capacitors and also signaling LEDs connected via resistors. Output voltages are connected to 3 pin connector. For signaling of presence of voltage is enough only one LED. We can also use 2 pin connectors for LED connecting.


Mini FM Broadcast Transmitter
Posted on Sunday, December 23, 2012   •   Category: FM Transmitters


Build your own simple mini FM transmitter. This fun project will show you how to build a mini broadcasting transmitter that can transmit an audio signal up to a quarter mile to any FM receiver. It's easy to build and a good learning experience. It serves as a hands-on learning tool for students or anybody interested in electronics. Having a range of up to a quarter mile, it's great for a house security system, baby monitoring device or simply a listening gadget that you can place anywhere!


2km Long Range FM Transmitter
Posted on Tuesday, December 4, 2012   •   Category: FM Transmitters


The power output of many transmitter circuits are very low because no power amplifier stages are incorporated. The transmitter circuit described here has an extra RF power amplifier stage using 2N3866 RF power transistor after the oscillator stage to increase output power to 250 milliwatts. With a good matching 50-ohm ground plane antenna or multi-element Yagi antenna, this transmitter can provide reasonably good signal strength up to a distance of about 2 kilometers. Transmitter's oscillator is built around BF494 transistor T1. It is a basic low-power variable-frequency VHF oscillator. A varicap diode circuit is included to tune the frequency of the transmitter and to provide frequency modulation by audio signals. The output of the oscillator is about 50 milliwatts. 2N3866 transistor T2 forms a VHF-class A power amplifier. It boosts the oscillator signal power four to five times. Thus 250mW of power is generated at the collector of transistor T2.


30V Volt Meter with PIC16F676
Posted on Friday, November 30, 2012   •   Category: Test and Measurement


This is a simple 30V volt meter using PIC16F676 micro controller with 10-bit ADC (analog to digital converter) and three 7 segment LED displays. You can use this circuit to measure up to 30V DC. The possible applications are on bench power supply or as a digital panel meter in various systems. PIC16F676 is the heart and brain of this circuit. The internal adc of the mcu with a resistor network voltage divider is used to measure the input voltage. Then 3 digits of comm anode 7 segment display is used to display final converted voltage. As you can see in the schematic the displays are multiplexed with each other. It means we switch on one display and put the corresponding digit on this while other two displays are off this cycle goes for each of the displays.


Micro Transmitter Bug
Posted on Sunday, November 18, 2012   •   Category: FM Transmitters


Presented FM transmitter bug is built using BF414 / BF324 / BF606 transistor. The 30cm antenna has a range of about 30m in the building, more in the open field. Power supply 2x AAA batteries have been used with voltage of 2.75 V. I added resistor 10K in parallel with 1.5pF capacitor so that the system works well when connected to an external source (mp3 player / computer). On the computer I had to reduce sound to about 35% of capacity, so that I do not have clipping. I managed to improve transmitter stability with simple shielding. The coil is 5 turns of enameled copper wire wound on 1 mm = 5 mm.


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