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NOISE   GENERATOR

Sometimes there is a need for a noise generator: this will work well in the audio range and, without the 680pF capacitor, will extend in the radio frequency range up to 20-30MHz. Voltage output may vary from 1Vpp to several volts depending on the type of zener used, older types have generally a higher noise; the zener voltage will influence the output level as well: the higher the voltage the higher the output level. Adjust the 1k pot for the required level.

 



 


ELECTRONIC   I CHING

This circuit is the electronic emulation of the I Ching, a form of divination originating in China. In the classical form, the response is obtained by the manipulation of 50 sticks or, more practically, by tossing 3 coins. The process must be repeated 6 times in order to complete what is called an hexagram. The various combinations given by the 6 readings are then read with the help of a book: the I Ching in fact or book of changes, used as a guide for the interpretation of the results. As the hexagrams look very much like a binary code, with additions and subtractions to go from one hexagram to the other, I thought that a hardware implementation could be feasible. The circuit reflects faithfully the chances of the I Ching: it is based on a counter which repeats a certain sequence, like a wheel. You may stop this rotation and read the counter status. In order to operate it you have to power it on and push "wait", the yellow LED will be on and all other LEDs will be off. After a short while push "read", the yellow LED will go off and the other LEDs will be enabled and the information stored in the double counter will be displayed: either red 1 or red 2 will go on representing a YIN and YANG line respectively. The same line becomes a moving line if also the green LED is lit. This is the first line of the hexagram: repeat the process 6 times by pushing again "wait" and so on. The whole circuit uses 3 ICs: one double counter and two hex inverters MC14049B. Supply voltage can be anything between 3 and 9V.




 

 


WINDSCREEN   LOOP AERIAL

If you do not like the whip antenna on your car, you may try this alternative circuit. A one-turn loop is installed in the windscreen of the car, keeping possibly away from the metal structure of the car. This loop is terminated in a 6.5mm ring core suitable for VHF use. MW and LW are gathered for by the other coil taken from an old MW radio. This is the oscillator coil of the receiver with all capacitors removed and with the slug fully in. Performance depends very much on the components used. In most cases there was no difference compared with a whip antenna although an improvement was found when traveling in tunnels.




ZERO CROSSING AC SWITCH OSCILLATOR

The circuit shown will switch on and off a resistive or inductive load up to 800VA with the possibility to adjust both the on and off period. Switching takes place during the zero crossing of the sine wave. The switch on point is around the zero crossing but pinpoint accuracy is not guaranteed due to the analogue nature of the circuit. The on period is adjustable between 0.3 to 4sec while the off period is adjustable between 0.2 and 10sec. The chosen SCR has a sensitive gate: this avoids the use of a large electrolytic capacitor. Frequency of operation is fairly stable although it is slightly affected by temperature, supply voltage and loading conditions.

 

 



 


HIGH VOLTAGE GENERATOR

Easy to build, this high voltage generator is capable of generating up to 50KV but the breakdown voltage of the coil limits the voltage to a value somewhat lower. T2 is the ignition coil of a car and also the 0.5F capacitor comes from the same place: actually I suggest using only this type of capacitor. T1 is a small transformer with a laminated iron rod, with a square section of 7x7mm, 57mm long with 75 turns on the collector side and 25 turns on the base side made with a 1mm enamelled wire. Its implementation is not critical and I expect that the circuit will work with a wide variety of transformers including ferrite ones. Try to invert one of the windings if the circuit does not oscillate. The transistor will stay quite cool and does not require a radiator if it is assembled on a metal case; otherwise a small 5C/W radiator will suffice. Frequency of operation is around 1.2KHz.





SENSITIVE GEOMAGNETIC DETECTOR

This is a rather sensitive circuit which will detect minute variations of a magnetic field, particularly the Earth magnetic field. The principle is based on an audio beat tone generated by two identical oscillators. These must be built in the same manner with the same type of components. In this way we minimize influence from temperature and voltage variations. The two oscillators, called probes, are housed in plastic boxes padded, on the inside, with copper wires terminated in one point only and running parallel to the ferrite rod. This rod, together with the coil was removed from an old Medium Wave radio and a small and powerful magnet was glued on one side. An extra magnet was placed on the outside of one of the boxes in order to set the initial or "zero" beat tone. This magnet is rotated or moved up and down until you hear the right frequency. A small hole is made in each plastic box in order to adjust the trimmer capacitor. Switch S1 will enable the sub-Hertz detector: in this way you will be able to hear the beat note even if the difference between the two probes is below the threshold level of around 20Hz and you will pick up differences well below 1Hz. The two probes are connected to the main box using standard RG58 coaxial cable tested up to a length of 15m. Operating frequency is 1.25MHz and it is sensitive enough to feel the rotation of a speaker magnet 2m away. Battery voltage is 9V.

 







 

 


BASIC INFRARED TX-RX

The transmitting section of this infrared tx-rx is unusually simple but it works rather well: the infrared LED pulses at a frequency of 160Hz and its range, with its receiver, is between 2 and 4m depending on the transformer used and the setting of the 100k pot. With other receivers it may reach a range of 15m, without any lens but with a perfect alignment between tx and rx. The receiving section uses an infrared phototransistor and an additional infrared emitter is placed next to it in order to provide a light bias thus improving sensitivity. The 100k pot. will also adjust sensitivity by setting the right operating point for the transistor. The supply should be well regulated in order to avoid self-oscillations. The audio transformer is a small output transformer recovered from an old transistor radio.



 


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