Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

Soldering iron tip preserver

Although 60/40 solder melts at about 200°C, the tip temperature of a soldering iron should be at about 370°C. This is necessary to make a good quick joint, without the risk of overheating delicate components because the iron has to be kept on the joint for too long.

Unfortunately, at this temperature, the tip oxidises rapidly and needs constant cleaning. That's where this circuit can help - it keeps the soldering tip to just below 200°C while the iron is at rest. Oxidisation is then negligible and the iron can be brought back up to soldering temperature in just a few seconds when needed.

In addition, normal soldering operation, where the iron is returned to rest only momentarily, is unaffected because of the thermal inertia of the iron.

Two 555 timers (IC1 & IC2) form the heart of the circuit. IC1 is wired as a monostable and provides an initial warm-up time of about 45 seconds to bring the iron up to temperature. At the end of this period, its pin 3 output switches high and IC2 (which is wired in astable configuration) switches the iron on - via relay RLY1 - for about one second in six to maintain the standby temperature.

The presence of the iron in its stand is sensed by electrical contact between the two and some slight modification of the stand may be necessary to achieve this. When the iron is at rest, Q1's base is pulled low and so Q1 is off. Conversely, when the iron is out of its stand, Q1 turns on and pulls pins 2 & 6 of IC2 high, to inhibit its operation.

During this time, pin 3 of IC2 is low and so the iron is continuously powered via RLY1's normally closed (NC) contacts.

Note that the particular soldering iron that the circuit was designed for has its own 24V supply transformer. Other irons may need different power supply arrangements. The warm-up time and standby temperature can be varied by altering R2 and R5, as necessary.

Alan March,
North Turramurra, NSW. ($40)

TV relative signal strength meter This circuit was designed to assist the installation of TV antennas. The signal is monitored using a small portable TV set and this circuit monitors the output of the TV's FM detector IC via a shielded lead. To initially calibrate the meter, adjust trimpot VR2 to zero the meter. Trimpot VR1 is a sensitivity control and can be set for a preset reading (ie, 0dB) or can be calibrated in millivolts. Rotating the antenna for a minimum reading on the meter (indicating FM quieting) gives the optimum orientation for the antenna. Ted Sherman, Kawhia, NZ ($30)

Simple card access control system

This card access control system for medium-to-low security situations can be built at a relatively low cost and is more fun than a keypad. The circuit can be driven by the smallest of microcontrollers, say a PIC12C508A, and only requires simple assembly code to run.

The concept is quite simple - a cardboard or plastic card (with holes punched) is slipped between two PC boards separated by a plastic spacer. This spacer also helps to guide the access card into position.

When the access card is inserted all of the way to the back of the spacer, it hits a small rubber mat attached to a flexible metal clip which makes up one half of the "card present" switch. This flexible clip is forced against a small metal contact plate, resulting in the trigger line going low. This tells the microcontroller that a card is present.

The microcontroller can then pulse the clock line while reading the sense line. A low signal on the sense line indicates the presence of a hole.

If you are using the unit in bright sunlight, it is advisable to read the sense line with an A/D converter input, to allow for variations in ambient light conditions. If no A/D converter input is available, a simple op amp input circuit would work just as well.

The circuit uses infrared LEDs and phototransistors, while the access card is covered with IR lens material on each side. This not only minimises problems with ambient light but also means that the holes in the access card are hidden (infrared lens material is readily available in sheets from Farnell Electronics).

IC1, a 4017B decade counter, counts up on every clock cycle (only one pin high at any time) from the microcontroller. Its outputs in turn drive LEDs
1-7 via IC2, a ULN2003 Darlington array. If a LED shines directly onto its corresponding phototransistor (Q2-Q8) via a hole in the access card, the phototransistor will turn on, pulling the sense line to ground.

The sense line is fed back to the microcontroller. If the correct sequence of phototransistors turns on, the microcontroller turns on transistor Q1 to activate the relay and the door strike mechanism.

David Kadow,
Norwood, SA.

Petrol gas switch for a Pajero My current vehicle, a Pajero, was modified for dual fuel - ie, petrol and gas. However, it's necessary to run the vehicle on petrol at regular intervals to stop the injectors from clogging up. This simple circuit allows the vehicle to be started using petrol and then automatically switches it to gas when the speed exceeds 45km/h and the brake pedal is pressed. Alternatively, the vehicle may be run on petrol simply by switching the existing petrol/gas switch to petrol. You can also start the vehicle on gas by pressing the brake pedal while starting the vehicle. The circuit is based on an LM324 dual op amp, with both op amps wired as comparators. It works like this: IC1a buffers the signal from the vehicle's speed sensor and drives an output filter network (D1, a 560kΩ resistor and a 10μF capacitor) to produce a DC voltage that's proportional to the vehicle's speed. This voltage is then applied to pin 5 of IC1b and compared with the voltage set by trimpot VR1. When pin 7 of IC1b goes high, transistor Q1 turns on. This also turns on transistor Q2 when the brake pedal is pressed (pressing the brake pedal applies +12V from the brake light circuit to Q2's emitter). And when Q2 turns on, relay 1 turns on and its contacts switch to the gas position. Trimpot VR1 must be adjusted so that IC1b's pin 7 output switches high when the desired trigger speed is reached (ie, 45km/h). In effect, the speed signal is AND'ed with the brake light signal to turn on the relay. The vehicle has been running this circuit for several years now and is still running well, with no further injector cleans required. J. Malnar, Gordon, ACT. ($40)