Submitted by Tom Email: XxTomxx14@aol.com

The circuit uses a 555 timer wired as an astable oscillator and powered by the emitter current of the BC109C. Under dry conditions, the transistor will have no bias current and be fully off. As the probes get wet, a small current flows between base and emitter and the transistor switches on. A larger current flows in the collector circuit enabling the 555 osillator to sound.

An On/Off switch is provided and remember to use a non-reactive metal for the probe contacts. Gold or silver plated contacts from an old relay may be used, however a cheap alternative is to wire alternate copper strips from a piece of veroboard. These will eventually oxidize over but as very little current is flowing in the base circuit, the higher impedance caused by oxidization is not important. No base resistor is necessary as the transistor is in emitter follower, current limit being the impedance at the emitter (the oscillator circuit).

By Rev. Thomas Scarborough

Cape Town

E-mail scarboro@iafrica.com

Figure 1 represents a cheap and simple Gate Alarm, that is intended to run off a small universal AC-DC power supply.

IC1a is a fast oscillator, and IC1b a slow oscillator, which are combined through IC1c to emit a high pip-pip-pip warning sound when a gate (or window, etc.) is opened. The circuit is intended not so much to sound like a siren or warning device, but rather to give the impression: "You have been noticed." R1 and D1 may be omitted, and the value of R2 perhaps reduced, to make the Gate Alarm sound more like a warning device. VR1 adjusts the frequency of the sound emitted.

IC1d is a timer which causes the Gate Alarm to emit some 20 to 30 further pips after the gate has been closed again, before it falls silent, as if to say: "I'm more clever than a simple on-off device." Piezo disk S1 may be replaced with a LED if desired, the LED being wired in series with a 1K resistor.

Figure 2 shows how an ordinary reed switch may be converted to close (a "normally closed" switch) when the gate is opened. A continuity tester makes the work easy. Note that many reed switches are delicate, and therefore wires which are soldered to the reed switch should not be flexed at all near the switch. Other types of switches, such as microswitches, may also be used.

Notes:
Please read the disclaimer on this site before making any transmitter circuit. It is illegal to operate a radio transmitter without a license in most countries. This ircuit is deliberately limited in power output but will provide amplitude modulation (AM) of voice over the medium wave band.
The circuit is in two halfs, an audio amplifier and an RF oscillator. The oscillator is built around Q1 and associated components. The tank circuit L1 and VC1 is tunable from about 500kHz to 1600KHz. These components can be used from an old MW radio, if available. Q1 needs regenerative feedback to oscillate and this is achieved by connecting the base and collector of Q1 to opposite ends of the tank circuit. The 1nF capacitor C7, couples signals from the base to the top of L1, and C2, 100pF ensures that the oscillation is passed from collector, to the emitter, and via the internal base emitter resistance of the transistor, back to the base again. Resistor R2 has an important role in this circuit. It ensures that the oscillation will not be shunted to ground via the very low internal emitter resistance, r_e of Q1, and also increases the input impedance so that the modulation signal will not be shunted. Oscillation frequency is adjusted with VC1.
Q2 is wired as a common emitter amplifier, C5 decoupling the emitter resistor and realising full gain of this stage. The microphone is an electret condenser mic and the amount of AM modulation is adjusted with the 4.7k preset resistor P1.
An antenna is not needed, but 30cm of wire may be used at the collector to increase transmitter range.

Circuit : Andy Collinson
Email: anc@mitedu.freeserve.co.uk

Description:
This Field Strength Meter is simple and also quite sensitive. It uses an ordinary digital voltmeter to measure RF signal strength up to a few hundred MHz.

Parts List:
1 OA91 Germanium Diode
1 3.3M Resistor
1 100p Capacitor
L1 7 turns on a 1/4 inch former (suitable for around 100MHz)

Notes:
The multimeter should be set to the lowest dc volts range for maximum sensitivity. This is normally 200mV DC for most meters. The circuit works well at VHF (around 100MHz) and was quite pleased with the results. L1 was 7 turns on a quarter inch former with ferrite slug. This covered the UK FM band. A digital multimeter, as opposed to an analogue signal meter offers several advantages in this circuit. First, the impedance of a digital meter is very high, around 10Meg/Volt on most meters. This does not shunt the tank circuit unduly. Second, compared to an analogue meter, very slight differences in signal strength can be more easily observed. Thirdly,a digital meter will have better linearity, responding well to both weak and stronger signals.

How it Works:
The complete schematic is shown in Fig. 1. The circuit has frequency response ranging from 100Hz to 3MHz; gain is about 30dB.
Field Effect Transistor Q1 is configured in the common-source self-biased mode; optional resistor R1 allows you to set the input impedance to any desired value. Commonly, it will be 50 ohms.
This signal is he direct-coupled to Q2, a common-base circuit that isolates the input and output stages and provides the amplifier's exceptional stability.
Last, Q3 functions as an emitter-follower, to provide low output impedance (about 50 ohms). If you need higher output impedance, include resistor R8. It will affect impedance according to this formula: R8 ~ Rout - 50. Otherwise, connect output capacitor C4 directly to the emitter of Q3.

Construction:
The circuit can be wired up on a piece of perfboard; a PC board is not necessary, although one can be used. However you build the circuit, keep lead lengths short and direct, and separate the input and output stages. You may have to install the amplifier in your receiver. Otherwise, installing it in a metal case will reduce stray-signal pickup. You'll have to provide appropriate connectors on the case. Connect the amplifier to the antenna and radio using short lengths of coax.
The circuit has only one adjustment. Connect a source of 12-volt DC power to the circuit, and adjust R3 so that there is a 1.6-volt drop across R2.
If you're not sure of the impedance of your antenna, connect a 500-ohm potentiometer for R1, and adjust it for best reception. Then substitute a fixed-value resistor for the potentiometer.
You may want to follow the same procedure with the output circuit (R8), if you're not sure of your receiver's input impedance. Common impedances are 50, 75, and 300 ohms, so the same 500-ohm potentiometer can be used.
You can connect an external antenna through the amplifier to a receiver that has only a ferrite rod antenna. Connect the amplifier's output to a coil composed of 10-15 turns of #30 hookup wire wound around the existing ferrite core, near the existing winding. To obtain best reception, experiment with the number of turns and their placement. You may need to reverse the connection to the coil if the output is weak.

 

Parts List:
R1 = see text        C1,C2,C3,C4 = 10uF, 25volt electrolytic
R2 = 270 ohms             C5 = 0.1uF, ceramic disc
R3 = 500 ohm potentiometer
R4 = 1K8 (1800 ohms)         Q1 = MPF102 J-FET, or use NTE451 
R5 = 10K (10,000 ohms)        Q2 = 2N3906, PNP-transistor (or use NTE159)
R6 = 4K7 (4700 ohms)         Q3 = 2N3904, NPN-transistor (or use NTE123AP)
R7 = 3K9 (3900 ohms)            see note about the 'AP' extension.

MORE DETAIL IN THIS LINK:http://www.uoguelph.ca/~antoon/circ/broadcast.html

60 WATTS LINEAR AMPLIFIER

The 60 Watt linear amplifier is simple all solid state circuit using power mosfet IRF840. The IRF series of power transistors are available in various voltage and power ratings. A single IRF840 can handle maximum power output of 125 watts. Since these transistors are used in inverters and smps they are easily available for around Rs: 20/-.

The IRF linear amplifier can be connected to the out put of popular VWN-QRP to get an output of 60 Watts. The circuit draws 700 ma at 60 Volt Vcc. Good heat sink is a must for the power transistor.

Alignment of the circuit is very easy. Connect a dummy load to the out put of the circuit. You can use some small bulb like 24V 6Watts as the dummy load. I have even used 230V 60Watts bulb as dummy load with my IRF840 power amplifier working at 120Volts. Adjust the 10K preset to get around 100 ma Drain current. I used gate voltage of 0.8V with my linear amplifier. A heigh gate voltage can make the power transistor get distroyed by self oscillation. So gate voltage must be below 2V and fixing at 1V will be safe.

Bifalar transformaer T1 is wound with 8 turns 26SWG on 1.4 x 1 balun core. The coil on the drain of IRF is 3 turns 20 SWG wound on 4 number of T13.9 torroids (two torroids are stacked to form a balun core). The RFC at the Vcc line is 20 Turns 20 SWG wound on T20 torroid.

500 WATT POWER AMPLIFIER
by Harry Lythall - SM0VPO

 Although I am an avid proponent of QRP (using reasonable power levels), there are times when I wish that I could run 1,000,000 watts and point it in a particular direction. If you are reading this then you know exactly what I am writing about. Unfortunately, here in the real world, it is quite expensive to buy or build BIG linear amplifiers - until now.

This is the circuit of a 500 watt linear amplifier, based upon a design by Frits Geerligs, PA0FRI, who has his own homepage at http://home.planet.nl/~fhvgeerligs. The circuit uses four PL519 TV line output valves in a very simple circuit that will deliver over 450 watts at 3.5 MHz (350 watts at 30 MHz). PL519 (40KG6A) is a more robust replacement for the earlier PL509 (40KG6) tube. Both valves will work well in this circuit. The input drive power is about 50 - 100 watts so it is compatible with most amateur radio HF transmitters. Not shown in the circuit is the cooling fan that is required to force air around the valves to cool them. In operation the 1K0 pot is adjusted to set the total valve anode current to around 50mA to 70 mA.

• T1 is a 4:1 balun wound on a 5cm ferrite rod. 9 + 9 turns. Connect the end of the first winding to the start of the second to form the center tap.
• L1 is 9 turns of 3mm Dia wire, wound on a 25mm Dia, 60mm long former.
• L2 is 18 turns on a toroidal former. Use two length of 2mm Dia wire, one with 11 turns and the other with 7 turns.

The 50 watt 100 ohm resistor recomended by PA0FRI is formed by two 50 ohm 25 watt non-inductive TO-220 resistors in series, bolted beside the fan. I use 100 x 10K carbon resistors aranged 10 x 10 between two pieces of 0.1" matrix wiring board (veroboard). My method is cheaper and avoids the need to mount input circuitry above chassis. All inputs are kept below the chassis whilst the valve anode terminals and output circuitry is kept below the chassis. The 100pf trimmer capacitor is adjusted for best VSWR from the driving transmitter at 29 MHz.

All four valve heaters (40 volts each) may be wired in series and connected to the 220 volt mains via a 6uf 250vAC capacitor for 50 Hz (5uf for 60 Hz). I personally favour the use of a 40 volt transformer winding, on a home-made transformer, to run all the valves heaters (in parallel) as well as the 40 volt fan. This places less strain on the cathode/heater insulation of old tubes that may have been kicked around in junk boxes for years.

PA0FRI sugests a power supply circuit which is switcheable and delivers 325 volts, 650 volts or 1300 volts to the amplifier. The circuit is very clever, and shown below for your interest.

I myself prefer a home wound transformer. This was constructed from an old 500 watt 120/240 volt auto-transformer. Here is the circuit of my PSU (40 volt secondary not shown).

All the old wire was stripped from the transformer as this was of a poor quality (I don't even think it was copper!!). All the laminations were varnished and the 1300 volt secondary was VERY well insulated from the other windings. The windings were:

• 120 volt primary
• 120 volt primary
• 40 volt secondary
• 1100 volt secondary

Winding transformers can be quite involved and I am writing an article for this on another page. But, here is the basic method I used. Measure the available winding area and fill 16% of it with 0.7mm enameled wire, counting the turns. Add an identical winding of the same number of turns. Add a third winding using the same guage but only 36% of the number of turns. Add a fourth winding using ten times the number of turns and using 0.2mm enamelled wire. All windings must be well insulated from each other and the fourth winding must be wound in about five sections, each insulated from the other. I use waxed paper for insulation. Do NOT use adhesive tape, masking tape or sticky backed insulating tape.

Connect the two primaries in series for 240 volt operation or in parallel for 120 volt operation. Check, with a resistance meter, that the transformer windings are isolated from each other and the case. When electrically testing the transformer, connect it to the mains without a load; the mains power in series with a mains 100 watt light-bulb. Check that the two secondaries are about 40 volts & 1100 volts. If the lightbulb lights up then you have got one of the primaries the wrong way round, or there is a fault in transformer construction.

NOTE THAT THE HIGH VOLTAGES INVOLVED WITH THIS PROJECT ARE POTENTIALLY LETHAL AND CAN KILL

As part of my laboratory project in the summer semester 2005, I have a USB programmer for Atmel AT89C2051/4051 controller. Die Schaltung, das Platinenlayout, die Firmware des Steuercontrollers (AT89C5131), eine passende Windowsanwendung zur Steuerung und weitere Informationen/Dokumentation sind auf der folgenden Seite zu finden. The circuit, the board layout, the firmware of the tax Controllers (AT89C5131), a right to control Windows application and further information / documentation is available on the following page.

Requirements and features:

• Ziel dieses Projekts war das Erstellen eines USB-Programmers für Atmels AT89C2051/4051 Mikrocontroller. The aim of this project was to create a USB AT89C2051/4051 programmer for Atmel microcontrollers. Das Gerät soll den Flash-Speicher eines eingelegten Chips beschreiben und verifizieren können, um so den fehlerfreien Upload neuer Programme zu ermöglichen. The device is designed to flash memory chips inserted a describe and verify so as to the correct upload new programmes.
• Für den USB-Programmer stehen zwei Windowsanwendungen für die Uploadsteuerung bereit. For the USB programmer are two Windows applications for the upload control. Eine grafische Version (GUI-Anwendung) und eine Konsolenanwendung für das automatische Flashen zB direkt aus einem Editor heraus A graphical version (GUI application) and a console application for the automatic flashing as directly from an editor out
• Der Atmel Controller AT89C5131 stellt das Herzstück des USB-Programmers dar. Er enthält ein Mikroprogramm, das die gesamte USB-Kommunikation und den Flash-Vorgang des eingelegten Chips steuert. The controller Atmel AT89C5131 is the heart of the USB Programmers It reflects a micro-program the entire USB communication and operation of Flash chips inserted controls.
• Da die Schaltung einen relativ geringen Strombedarf hat, ist die direkte Speisung der Versorgungsspannung aus dem USB-Bus möglich, so dass kein zusätzliches Netzteil benötigt wird. Since the circuit a relatively low power consumption, is the direct power supply voltage from the USB bus, so no additional power supply is needed.
• Es werden sowohl Intel-Hex-Dateien als auch komplette Speicherabbilder (Binärdateien) unterstützt. There are both Intel-Hex files as well as complete storage images (binary).
•  

usb_programmer_quelltexte.zip

This device was built using USBDRV firmware developed by  Objective Development.

This Low-Speed device violates following USB specification rules:

Electronic Components

You need:

• Detachable cable (must be captive for Low-Speed)
• Output drivers with 5 V (must be 3.3 V, but USB is 5 V tolerant)
• No differential data input (only one line D- is sampled)
• No data clock regenration while receiving (only at packet start)
• No error detection of input data (OUT direction), no time for CRC16
• Data transfer using BULK pipes (not specified for low-speed, but works on Windows – for Linux there is an "Alternate Setting" with INTERRUPT pipes instead)

However, new to this firmware, following rules are accepted:

• Suspend detection and lowered suspend current (below 500 µA)