FM Stereo Transmitter

You'll find that this is a very easy project to build. It will transmit good quality sound in the FM band ( 88 - 108 mhz ). One inportant item is that the IC chip operates on 3 volts DC. The chip will get destroyed if it is operated on any voltage higher than 3.5 volts. The antenna can be a standard telescopic antenna or a 2 foot length of wire. The input is in the millivolt range and you may need to add additional pots for the inputs. I was able to use this circuit for a walkman and a portable CD player in my car. I used the headphone jack on both and varied the signal with the volume control.
To adjust the circuit tune your FM radio to a quite spot then adjust the trimmer capacitor C8 until you hear the signal that you are transmiting. When you have a strong signal adjust the resistor R4 until the stereo signal indicator lights. If the input is to high of a signal you may over drive the IC chip. Use two 15 turn pots on the input signals to bring the level down. You can balance the signal by using headphones. The inductor L1 is 3 turns of .5 mm wire on a 5 mm ferrite core.

Intercom

This circuit was requested by an school teacher. It is a simple intercom that anyone can put together and get to work. It is based on the LM380 IC chip. This chip is able to put out 2 watts of power if it is heat sink properly. The following pins should be grounded and attached to a foil to dissipate the heat. Pins 3,4,5,10,11,12 should all be grounded. The circuit works as follows. Switch 1 is a double pole double throw switch. In one position is the talk position and in the other is the listen position. In the diagram shown the switch is in the talk position for the speaker on the left. Talking into the speaker inputs a signal to the IC chip through the matching transformer T1. The output from the IC chip goes to the speaker on the right. If you put the switch in the other position the speaker on the right is the talking unit and the speaker on the left listens. Volume is controlled by the 1meg ohm pot R1. This circuit is very basic but is a good start for a child or anyone starting new in electronics

Tri-Waveform Generator

The Tri-Waveform Generator can be used for a number of different uses. The one that I use it for is a signal generator to test circuits. The frequency range is 20 to 20khz. and can be adjusted by R1. The duty cycle or the time that the waveform is high and the time that the waveform is low can be adjusted by R4. The purpose of R2 and R3 are to clean up any distortion on the sine wave output. To do this you must hook up the sine wave output to and oscilloscope and adjust R2 & R3 to make the sine wave as accurate as possible.

12 Volt to 120 Volt Inverter

Ever needed a low power 120volt AC power source for your car, van or truck? Well this circuit should do the trick for you. It will supply 15 watts of AC power to a device. It should power lamps, shavers, small stereos and small appliances. If you draw to much power the circuit will shut down all by itself. The output of this circuit is a square wave so there may be some noticeable hum on audio units plugged into it. To reduce some of the hum increase the value of the output capacitor which is at .47uf now. That transistor in the circuit are high power PNP transistors. Radio Shack part number 276-2025 are good ones to use or TIP32. The transformer is a 24 volt 2 amp center tapped secondary Radio Shack part number 273-1512 or equivalent.

Voice Record / Playback Circuit

The ISD1000A is a Direct Analog Storage device which allows you to store 20 seconds worth of voice data on an IC chip which can be play backed anytime. The data stored will stay in memory even if the power is removed. To use the circuit below simple apply power to the circuit, press the record button and hold. Speak clearly into the microphone. You have up to 20 seconds of voice message that you can store. If you talk beyond that time the chip will only store the first 20 seconds. After recording, release the record button. To playback the message, press the playback message and the message you recorded will play back. The microphone is an electret mic and the speaker is a 8 ohm speaker. If you use a 16 ohm speaker then the 10 ohm resistor marked optional, can be eliminated. This circuit can be the basis of many other larger projects. For example it could be part of an alarm circuit which plays back a voice warning when the alarm circuit is triggered.