Light Sensing RF Transmitter

This transmitter is very similar to the simplest RF Transmitter. The Different is the photoresistor placed in series with R1. This could be also a thermistor or other variable resistance sensor. The frequency of the tone that is transmitted varies with the amount of light falling on tha photoresistor.
L1 is 20 to 30 turn of 24 to 32 ga. Magnet wire close wound around a 1/8 to 1/4" diameter non-conductive form and tapped 1/3 or the way from one end. The tap is connected to the emitter of Q1.
The user should be able to pick up the signal from this transmitter on any regular FM or VHF receiver. By increasing the number of turns on L1, the RF frequency can be dropped down all the way into the AM broadcast band.

via Tony Van Roon

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"Propeller Clock" Mechanically Scanned LED Clock

If you want to build some interesting clock, perhaps you can try this, Propeller Clock. In this project, scanning Led process uses spin motor. It will give Illusion of numbers in the air.

How this clock works:
A motor spins the "propeller", and a small microcontroller keeps track of time and changes the pattern on seven LEDs with exact timing to simulate a 7 by 30 array of LEDs. It is an illusion, but it works nicely.

If you want to build this clock, you will need a few things, including:
1. Skill with motors and mechanical things.
2. Prior electronic experience.
3. A dead VCR or floppy drive or other source of a suitable motor and miscellaneous parts.
4. A programmer that will program a PIC16C84 or 16F84 or AVR microcontroller.

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Reverse Polarity Protection

Protection in electronic circuits is crusial factor. especially polarity reverse protection. Can you imagine what happen if our circuit not pretected to reverse polarity supply. All digital IC will be damage, and also electrolit capacitor will blow up. There two simple way to protect the circuit from reverse polarity

1. Using diode
Connect your positif terminal in circuit to cathode and positif terminal from your power supply to anode of diode. See the picture.

The andvantage using diode protection is low in cost and simple to configurate it
The disadvantage from this configuration are the supply to circuit will decrease about V junction of diode (0,7v for silicon diode and 0,3 for germanium diode) and diode cannot handle supply with higher current.

2. Using Mosfet
Connect the drain to positif terminal of power supply, the gate to ground and source to postif terminal of circuit. See below picture

The advantage using this configuration is can handle power supply with higher current.
The disadvantage is not low cost and need heatsink (for higher current)

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