Automatic Telephone Answering Machine

This is not very critical and any other device (such as tape player with message recorded on an endless tape) which does not overload the circuit or the telephone line, may be used. The incoming line is protected by metal-oxide varistor (MOV) RDN 130/14 followed by polarity guard circuit comprising diodes D1 through D4 in bridge configuration. Transistor T1 (MPSA92), having a high breakdown voltage (Vce max.) is used as electronic switch/telephone line interface. It is controlled by transistor T2. Ring detector section comprises capacitor C1, resistor R10 and opto-coupler NEC 2505. (In case of its non-availability, one may substitute it with MC2TE or a similar opto-coupler with additional diode 1N4148 placed with its cathode connected to pin 1 and anode to pin 2 of the opto-coupler.)

Color Sensor

Colour sensor is an interesting project for hobbyists. The cir- cuit can sense eight colours, i.e. blue, green and red (primary colours); magenta, yellow and cyan (secondary colours); and black and white. The circuit is based on the fundamentals of optics and digital electronics. The object whose colour is required to be detected should be placed in front of the system. The light rays reflected from the object will fall on the three convex lenses which are fixed in front of the three LDRs. The convex lenses are used to converge light rays. This helps to increase the sensitivity of LDRs. Blue, green and red glass plates (filters) are fixed in front of LDR1, LDR2 and LDR3 respectively. When reflected light rays from the object fall on the gadget, the coloured filter glass plates determine which of the LDRs would get triggered. The circuit makes use of only ‘AND’ gates and ‘NOT’ gates.

Interfacing the DS1620 with a DS5000/8051 Microcontroller

This application note introduces the user to software for interfacing a DS5000 (8051 compatible) microcontroller to the DS1620 digital temperature sensor. The DS1620 communicates via a 3-wire serial digital interface. Software code is provided that reads the DS1620 temperature register and calculates high-resolution results based on data from the counter and slope accumulator registers.

Moving Message Display

This project uses the Phillips 87C750 microcontroller with an Agilent Technologies HDSP-2112, 8-Character, 5X7 Dot Matrix, Alphanumeric Programmable Display to create a marquee style moving message display. Your custom message scrolls by from right to left facing the display.

Thermistor Thermometer

<span style="background-color: #ffffff">The hardware block diagram is shown in Fig. 2. The sensor is epoxy molded thermistor. The circuit for signal conditioning is simple voltage divider. The ADC is 12-bit SPI interface LTC1298 analog-to-digital converter. The microcontroller is ATMEL 89S52. The display has four digits 0.5 inches 7-segment LED. The segment driver provides 32-bit CMOS output.</span> <p>This is the simple project of Thermistor Thermometer developed by Wichit Sirichote. Schematic and associated source codes are availabe for download.</p>

Dallas DS1307 64 X 8 Serial Real Time Clock IC to build a Battery Backed Alarm Clock.

The DS1307 is a Serial Timekeeping Chip that will count seconds, minutes, hours, date of the month, month, day of the week, and year with leap year compensation. It has 56 bytes of nonvolatile RAM for data storage, 2-wire serial interface, a programmable square wave output, and automatic power fail detect and switch circuitry. The DS1307 requires the use of a 32.768 KHz quartz crystal with a load capacitance of 12.5pF for the internal clock oscillator circuitry. An external 3 volt battery is used to maintain the internal RAM data.

TDA7000 for NBFM

For some months I have been nursing the thought of making a "universal receiver": a receiver that can be used from DC to light, or something like that. The idea was to make a receiver using the TDA7000 receiver chip, and changing the components for either WBFM or NBFM. If the TDA7000 receiver is used at, say, 5MHz, then by adding a harmonic converter the complete receiver could be used to tune from 1MHz to perhaps 200MHz.

REMOTE ANTENNA TUNER

After doing a bit of messing around with frame and loop antennas I found that they all worked much better outside, but their very narrow bandwidth required them to be manually tuned. The obvious answer was some form of remote antenna tuner so that the antenna could be remotely tuned from inside my shack. This I did using a cheap (as always) RC-servo, of the type that is used for control of radio controlled models. Suitabale servos are made by Futaba and cost typically £7.50p (UK) - $11.00 (USA) - SEK 120 + Moms (SWE). 'Second user' (seen better days) servos are quite adequate in this application.

80 METER FRAME ANTENNA

The construction of the antenna is shown above and is a five turn loop of one cable from 5 ampere mains cable. The cable must have a multi-stranded conductor. The antenna uses over 20 meters of the cable, so I stripped down 7 meters of 3-core cable and soldered the ends together. Construction is otherwise quite straight forward if you follow the above drawing. Note that all cable lengths shown are approximate.

The two boom poles - I have used both cane and a plastic clad tin (metalic) pipes, of the sort that are sold in garden shops. Both worked very well in spite of the difference in materials. If you do use metal booms then insert some form of insulation in the holes before you pass wire through them. I used plastic drinking straws from MacDonalds. This will prevent the metal from digging into the cable insulation, as well as improving the insulation.

7dB COLINEAR ANTENNA

A little while ago (about 6 years to be exact) the best antenna I had for 145MHz was the 1/4-wave mag-mount antenna on the roof of my car. So I decided I wanted a bit more gain. I was able to get more by using a pair of end-fed 1/2-wave antennas. This gave about +3.5dBd, which is about 6dB more than the 1/4-wave antenna. The antenna is still nailled to the side of the house and I can access four repeaters, between Uppsalla and Stockholm.

Traffic Light System

This project operates red, amber and green LEDs in the correct sequence for a single UK traffic light. The time taken for the complete red - red & amber - green - amber sequence can be varied from about 7s to about 2½ minutes by adjusting the 1M preset. Some amber LEDs emit light that is almost red so you may prefer to use a yellow LED.