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