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Circuit Notebook

(1) Laser-Guided Parking Aid; (2) Novel White LED Torch; (3) Electronic Thermostat For Plug-In Heaters; (4) ESR & Low Resistance Test Meter

Laser guided parking

Most people have a reasonably small garage and it is most annoying when the car is left just far enough in to close the door but not far enough to squeeze past without knocking off your kneecaps!

There are lots of gizmos out there to help with parking in tight spaces but all are expensive. This unit works well and costs peanuts! All up, it shouldn’t set you back more than about $10 and will let you park the car within 1mm in both axes.

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The main working part is a low-cost laser pointer and a simple timer circuit. The laser starts operating when the remote-controlled garage door opens and then stays on for about three minutes after the door is closed. If your garage door lacks a remote controller then a limit switch can be used to control the circuit when the door is opened manually.

The circuit operation is straightforward. An existing door controller or a 9-12V DC plugpack can power the circuit. A 3-terminal regulator (REG1) reduces this to +5V to power the electronics in the laser module.

The anode of diode D1 is connected to the positive side of the motor circuit or a manual limit switch (S1), so that the gate of Mosfet Q1 is pulled high when the door begins to open. This switches on the Mosfet and powers the laser.

A 100μF capacitor in the gate circuit holds Q1 on for a short period after the motor stops or the switch opens, giving plenty of time for parking. The desired "hold on" time is adjustable with trimpot VR1.

The laser is mounted inside an adjustable light fitting scavenged from a floodlight. After removing the redundant light and socket, it is fixed to the ceiling of the garage at a location that will allow it to be aimed at the dash of the vehicle.

To set up the system, first park the car in the optimum position and then aim the laser at a fixed point on the dash. I chose a position just behind the steering wheel where the dash begins.

The car is then moved away and the point at which the laser hits the concrete floor is clearly marked with a bullseye about 100mm in diameter. A permanent marker or paint is best used for the job. The result must be visible from inside the vehicle and provides assurance that the laser has not moved since you left home (kids can do amazing things with balls and such!)

Now as you drive into the garage, the point of laser light can be seen immediately, moving progressively up the car's bonnet and (hopefully) through the windscreen and onto the exact spot on your dash!

Ron Russo,

Kirwan, Qld. ($50)

Novel white LED torch

Although this design is reproduced directly from the manufacturer’s datasheets, its use in this application is rather novel. Originally intended for high-visibility LED bargraph readouts, here the LM3914 is used as the basis of a 10-step variable brightness current-regulated white LED torch!

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The circuit has only four components in the control and regulation circuit: R1, R2, VR1 and the LM3914. The circuit can be built directly on the pins of the LM3914 to produce a package not much bigger than the LM3914 itself.

The LM3914 is set to operate in bargraph mode so that the LEDs light progressively as its input signal increases. This signal comes from the wiper of VR1, which provides a variable voltage between 0V and the supply voltage to pin 5 of the LM3914.

The internal resistor ladder network of the LM3914 has its low end (pin 4) connected to ground and the high end (pin 6) connected to the supply voltage via R2. The purpose of R2 is to give LED 10 a clear turn-on zone. Resistor R1 (620Ω) on pin 7 of IC1 sets the current through each LED to about 20mA.

As VR1 is rotated from the 0V position (all LEDs off) to the supply voltage position (all LEDs on), the LEDs will progressively light. With all LEDs off, the circuit will draw about 5mA. With all LEDs illuminated, it will draw about 205mA and dissipate 307mW with a 4.5V supply.

(Editors note: these are nominal figures only. Actual device dissipation will depend entirely on the input voltage and LED forward voltage.

In use, we recommend that a resistor (R3) be inserted in series with the positive supply, chosen so that the LM3914’s dissipation is limited to about 500mW. Typically, this would be needed for supply voltages of 6V and higher. The inclusion of the resistor necessitates a 10μF decoupling capacitor across the supply rails.)

By carefully selecting the LEDs, this torch can be as bright as 15,0000mCd while costing less than $20.

Mick Stuart,

Lambton, NSW. ($30)

Electronic thermostat for plug-in heaters

Most room heaters with a temperature control work by controlling the duty cycle of the heater, which in turn controls the energy output. However, without temperature feedback, it’s up to the user to continually adjust the heater for maximum comfort.

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A plug-in thermostat would seem to be the simplest add-on solution. However, these are not commonly available, hence the impetus for this project idea. It is based on a battery-operated thermostat from Jaycar and a plug-in electronic timer switch of the type typically available from hardware stores and supermarkets.

The need for an electronic timer switch is twofold. First, it contains a mains-rated relay that can be used to switch the maximum allowable load current (ie, 10A). And second, it also contains a timer and this can be used in addition to the thermostatic function if desired.

All that is required is to (carefully) determine the control voltage for the relay in the timer, find a supply source for this voltage in the timer circuit and use the thermostat to control the timer relay from this source. Alternatively, if the timer still works, the thermostat could just be wired in series with the timer relay coil.

As shown in the accompanying diagram, two wires connect the thermostat to the timer circuit. It can be hard-wired with the thermostat mounted permanently to the timer or connected with a longer wire and plug/socket at the timer. This would allow the thermostat to be mounted separately to better sense room temperature.

If the timer control signal to the timer relay is cut, the timer is disabled. Alternatively, the timer could be used in series with the thermostat (eg, the thermostat controls a heater under the control of the timer). A socket mounted in the timer will allow the timer to be used without the thermostat and the thermostat can be plugged in when required.

In practice, the thermostat does quite a good job of controlling room temperature.

Ian Hood,

Woden, ACT. ($30)

ESR & low resistance test meter

As electrolytic capacitors age, their internal resistance, also known as "equivalent series resistance" (ESR), gradually increases. This can eventually lead to equipment failure. Using this design, you can measure the ESR of suspect capacitors as well as other small resistances.

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Basically, the circuit generates a low-voltage 100kHz test signal, which is applied to the capacitor via a pair of probes. An op amp then amplifies the voltage dropped across the capacitor’s series resistance and this can be displayed on a standard multimeter.

In more detail, inverter IC1d is configured as a 200kHz oscillator. Its output drives a 4027 J-K flipflop, which divides the oscillator signal in half to ensure an equal mark/space ratio.

Two elements of a 4066 quad bilateral switch (IC3c & IC3d) are alternately switched on by the complementary outputs of the J-K flipflop. One switch input (pin 11) is connected to +5V, whereas the other (pin 8) is connected to -5V. The outputs (pins 9 & 10) of these two switches are connected together, with the result being a ±5V 100kHz square wave.

Series resistance is included to current-limit the signal before it is applied to the capacitor under test via a pair of test probes. Diodes D1 and D2 limit the signal swing and protect the 4066 outputs in case the capacitor is charged.

A second pair of leads sense the signal developed across the probe tips. Once again, the signal is limited by diodes (D3 & D4) before begin applied to the remaining two inputs of the 4066 switch (pins 2 & 3 of IC3a & IC3b). These switches direct alternate half cycles to two 1μF capacitors, removing most of the AC component of the signal and providing a simple "sample and hold" mechanism.

The 1μF capacitors charge to a DC level that is proportional to the test capacitor’s ESR. This is differentially amplified by op amp IC4 so that it can be displayed on a digital multimeter – 10Ω will be represented by 100mV, 1Ω by 10mV, etc.

To calibrate the circuit, first adjust VR1 to obtain 100kHz at TP3. Next, momentarily short the test probes together and adjust VR4 for 0mV at pin 6 of IC4.

That done, set your meter to read milliamps and connect it between TP4 and the negative (-) DMM output. Apply -5V to TP2 and note the current flow, which should be around 2.1mA. Transfer the -5V from TP2 to TP1 and adjust VR2 until the same current (ignore sign) is obtained. Remove the -5V from TP1.

Again, set to your meter to read volts and connect it to the DMM outputs. Apply the probes to a 10W resistor and adjust VR3 for a reading of 100mV.

Finally, ensure that all capacitors to be tested are always fully discharged before connecting the probes.

Len Cox,

Forest Hill, Vic.

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