Headlight reminder

With the storm season recently upon us, it’s not uncommon to switch car headlights on during the daytime. Unfortunately, it’s easy to forget to turn them off again when parking, with the result being a flat battery. This circuit will sound an alarm if the ignition switch is moved to the "off" position while the car lights are on, reminding you to turn the lights off before leaving the vehicle.

The circuit is simple but effective. A 555 timer (IC1) is configured as a free-running oscillator to drive a small piezo transducer. The pitch of the transducer is set by the resistor and capacitor connected to pins 2 & 6.

Power for the 555 is derived from the dashboard lighting circuit. However, the piezo does not sound during normal operation, because the 555’s reset input (pin 4) is held low by transistor Q1. This transistor is switched on whenever accessory power is present, pulling its collector towards ground (0V).

If the ignition is switched off but the lighting circuit remains powered, the loss of accessory power results in Q1 switching off and releasing the reset signal to IC1, sounding the alarm.

A 220Ω resistor in series with the piezo protects the 555’s output (pin 3). Although most piezo elements have relatively high impedance, this drops as the frequency increases due to their capacitive nature. The square-wave output on pin 3 includes many harmonics, some extending well into the ultrasonic range.

The unit fits easily into a small plastic box. I spliced mine into the wiring running to the cigarette lighter, which includes both accessories and panel lamp circuits as well as a chassis ground wire. The result fits neatly behind the ashtray, with no chassis bashing required!

Bruce Colledge,

Ferny Hills, Qld. ($30)

El Cheapo cable tester Many cable testers have been published before, some quite complex, but here’s a cheap and simple alternative. It uses only a 9V battery, two mating connecators and a few resistors, as well as a multimeter for voltage measurements. Begin by soldering the leads of a 9V battery clip between the two pins furthest apart on one of the test connectors and then add a ladder of resistors between them for each of the other required pins. Any junk-box resistors will do but values between 1kΩ and 50kΩ are best. In the example shown, a five-core data cable terminated with D-9 connectors is to be tested. Connect 0V to pin 5, +9V to pin 1, and four resistors in between. Plug in a 9V battery and then probe the connector at the other end of the cable with a multimeter as indicated. Broken, shorted or incorrect connections are all quickly evident using this method. Grant Walker, Lutana, Tas. ($25)

Aviation intercom

Before its move offshore, I was lucky enough to be involved in developing the avionics system for the Flightship Ground Effect FS8 craft (see www.pacificseaflight.com/craft.shtml). Although officially classed as a boat, it has wings and can travel at 180km/h some three metres above the water.

The communications system was adapted from an aircraft unit and was a particular problem. It was expected to allow speech between the two pilots and radio, as well as receive audible warnings from the onboard computers and feed sound to the onboard data logger.

Initially, the system was very noisy due to ground loops and incompatibility problems. A circuit similar to that shown here was the solution. Although optimised to suit Softcom brand headphones with active noise reduction, it should be suitable for most aviation sets.

The plugs indicated are standard aviation types but are insulated from the instrument panel to eliminate earth loops.

The inputs from the two pilots' microphones are summed and amplified by transistors Q1 & Q2. When one pilot presses his or her transmit key (mounted on the yoke), the transmit relay (RLY1) closes, muting the other pilot’s microphone via the optocoupler (OPTO1).

The outputs from the microphone preamp, computer audio transformer (T1) and radio speaker transformer (T2) are summed via 10kΩ resistors and applied to the input of IC1, an LM386 audio amplifier. Note that transformers are used here to avoid creating additional earth loops.

The output of the LM386 drives the pilots’ headphones via transformers T3 & T4, which are needed for impedance matching.

Each audio source has its own level control (VR1, VR3 & VR4). The main volume control (VR5) is included to allow for ambient noise level. VR2 is used to set the signal level for the data logger.

Gary Smith,

Montrose, Tas.

Touch lamp remote A bedside table touch lamp can be awkward to reach in the dark. However, its very easy to position one or more additional "touch pads" wherever you desire, using just a length of insulated wire and any suitable metal object. How? Well, simply strip both ends of a length of insulated wire, attaching one end to the touch surface of the lamp and the other end to a conductive surface. That’s it! The touch pad can be almost any conductive surface that is not earthed and is safe to touch. You could even use a metal bed head but this is not recommended – I found the lamp kept being switched on by accidental touches to the bed head. I’ve used a small metal costume jewellery broach but you could even use a thumbtack. Caution: don’t push the wire inside the lamp, as contact with the internal 240VAC wiring may prove fatal! Andrew Hicks, Harris Park, NSW. ($20)

Simple under-voltage cut-out

Sensitive low-current relay coils often operate at much lower voltages than their typical ratings. This can be undesirable in some applications, where low supply voltages can result in erratic system behaviour.

In some instances, this problem could be overcome simply by inserting an appropriate value zener diode in series with the relay coil. This reduces the voltage seen by the relay coil, so causing it to drop out earlier.

If desired, a LED could be included when a spare set of "normally closed" contacts are available to indicate relay dropout.

Jay Williams,

Baldivis, WA. ($20)

Reverse polarity protector A series diode is often used as a means of protecting equipment from accidental power supply reversal, particularly in battery-powered equipment. Due to forward voltage losses, this is sometimes impractical. One solution is to use an enhancement mode P-channel power Mosfet (Q1) in series with the positive supply rail. A device with low drain-source "on" resistance can be selected to minimise voltage losses, which in turn extends battery life and reduces heat dissipation. Zener diode ZD1 must be included to protect against excessive gate-source voltage, while a 100kΩ resistor limits zener fault current. A second 100kΩ resistor across the output ensures that the gate doesn’t float when the input is disconnected. A series fuse and bidirectional transient voltage suppressor (TVS1) could be included to provide over-voltage protection, if desired. If common input & output grounds are unimportant, then a version of this circuit employing an N-channel power Mosfet in series with the negative (0V) rail could also be employed. Bruce Griffiths, Hamilton, NZ. ($25)

PICAXE freezer thermostat

We have a portable 12V 18-litre freezer that’s had a chequered career as the boat’s freezer. Its innards were replaced about 18 months ago but then the thermostat failed in the "on" position.

Being part Scottish, I was loath to throw it away. I was sure I could get a PICAXE-08 to make it work so I bought a thermistor from DSE and proceeded to experiment.
The circuit shown was constructed and the trimpot (VR1) initially set to about 100kW. The freezer was then switched on and the voltage at pin 6 of the PICAXE was recorded at 5-minute intervals, along with the values from the readadc command.

After the freezer reached about -14°C, power was switched off and more readings were taken to determine the warm-up rate. From this data and after some experimentation, I decided to run the compressor for five minutes if the temperature was above a certain threshold or three minutes if below.

Once the temperature reached a lower limit, it would stay off for five minutes. This method adds hysteresis into the system, preventing rapid on-off cycling. It also helps to minimise current drain on the batteries.

As a result, the unit runs to about -11°C and with the 5-minute off period, it comes back to about -5°C or -6°C; enough to make ice cubes for the Scotch. More importantly, it should also keep any frozen meat in good condition for our trips!

The freezer will eventually get down to -21°C if allowed to run continuously but with the law of diminishing returns, it takes far more energy to get down that far and of course, it warms up more quickly.

Paul Rodenhuis,

Port Bundaberg, Qld. ($40)

' Thermostat for Waeco 18-litre freezer
symbol LED = 0
symbol relay = 4
symbol voltage = b1
symbol on_time = b2
symbol off_time = b3
symbol minute = b4

start:
  high LED		   'show program is running 
  wait 2
  low LED 
  wait 2

main:
  readadc 1, voltage    'read from thermistor
  debug voltage
  if voltage <= 43 then relay_off
  if voltage > 43 and voltage <=64 then relay_on2
  if voltage > 64 then relay_on1
  goto main

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relay_on1:
  high relay
  on_time = 5
  for minute = 1 to on_time
  high LED
  wait 60
  next minute
  goto main

relay_on2:
  high relay
  on_time = 3
  for minute = 1 to on_time
  high LED
  wait 60
  next minute
  goto main

relay_off:
  low relay
  low LED
  off_time = 5
  for minute = 1 to off_time
  wait 60
  next minute
  goto main

NOTES & ERRATA PICAXE Freezer Thermostat (Circuit Notebook, March 2005): The serial programming input (pin 2) and output (pin 7) for IC1 (page 73) are shown connected in reverse.