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Circuits designed by David Johnson, P.E.
Last Updated on:
Monday, December 25, 2017 02:05 PM
List of Dave's Circuit Designs
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Timers |
3 Minute Closet Light #2 |
3 Minute Closet Light #2
I have a coat closet
by my front door. The thing is very dark at night and really needs some kind of
illumination inside. I could hire an electrician to install a light in the
closet ceiling and then wire it to a toggle switch outside the door but that would
cost me quite a bit of money. That doesn’t sound like a good investment for
something which does not get much use. I looked around for some battery powered
LED lights which would emit enough light for the small space and I found one made by
Sylvania. |
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I think it will do the job. I figured I could mount one or two of
the things in the closet and connect them to a pushbutton activated timer circuit. A
tap to the timer’s switch would turn on the light for some period of time, and then turn
the light off. This timing function would insure that the light would not be
accidently left on, which would kill the batteries after a few days. Since I would
not need to use the light very often, the set of batteries in the light should last for
many years. |
As shown below, the Sylvania “Light Flute” has four bright white LEDs,
housed in a nice cylinder about half an inch in diameter and about 9 inches long. It
is powered by 3 AAA 1.5v alkaline cells, which snap into holders inside the housing.
It has an on/off switch on one end. The light assembly can be snapped into a plastic
holder but I was planning on mounting the light right above the inside top of the door
molding, so the lights would shine up to the ceiling. The diffuse light would then
provide illumination to both the shelves in the closet and the coat rack below. I bought
one of these things and tested it in my closet. It did indeed produce enough light.
If my wife says that she wants even more light, I could wire the timer circuit to two
such devices, which would double the illumination level. |
I sketched up several ways to go with the timer design. One classic
timer circuit requires three wires soldered to the light assembly. The timer circuit would
draw power directly from the light fixture. In two other designs only two wires
would need to be connected to the light. One circuit used a quality super capacitor
with a low leakage current feature while a third circuit used a coin style 3v lithium
battery to power the timer circuit. If I carefully designed the circuit for an ultra
low current draw, the lithium battery might outlast the batteries in the light fixture,
perhaps up to 10 years. After some head scratching, I decided to use the design with
a 3v lithium battery. |
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Sylvania Light Flute |
Sylvania Light Flute |
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Sylvania Light
Flute Front with Metal Case Removed |
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Sylvania Light
Flute Back with Metal Case Removed |
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The timer circuit I came up with is shown below. It uses a tiny LMC7215
ultra low power voltage comparator from National Semiconductor. The non-inverting
input is connected to a half supply reference while the inverting input is connected to a
RC timing circuit. When the trigger switch is pressed, the timing capacitor C1 is
discharged. In this state, the output of the voltage comparator swings high, which
turns on the transistor Q2 through R4. The transistor Q1 and the resistors R5 and R6 form
a constant current sink circuit, set at about 150ma. The LED light circuit is
modified by unsoldering its 6.8 ohm resistor and shorting out the on off switch. The
capacitor C1 should be a low leakage tantalum or a surface mounted ceramic. |
I measured the standby current for the circuit at about 2 microamps.
This is such a low current that the 3v lithium battery should last for at least 5 years.
I measured the total current going into the 4 white LEDs at about 150ma. If I use
800ma-hours as the capacity of the 3 AAA cells, the batteries should have enough energy
for at least 1000 light operations. That means the batteries should last about 5
years. |
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Click
on Drawing Below to view PDF version of Schematic |
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