Magic Lamp


You have probably seen "magic lamp" circuits in which an ordinary
incandescent bulb is lit by a match. These circuits rely on a hidden temperature
or light sensor and are not particularly interesting. I decided to make a magic
lamp, too! But, to make it more interesting, I decided to just use plain old
magic for this circuit instead of resorting to any hidden components.

This circuit should be constructed only by persons with the qualifications to
work on and design high voltage circuitry. Necessary safety considerations are
not indicated. All parts of the circuit should be considered "hot".
The use of magic is frowned upon in higher places.


The components are not critical and substitutions are possible. The SCR is an
ordinary sensitive-gate type like the 2N5064 or other type designed for 200
volts or more, the PNP transistor is a high-voltage type like a MPSA-92 or a
2N6520, the 1N4003 diode is an ordinary rectifier with a breakdown voltage of
200 volts or more. The 1uF capacitor is any low voltage type and the resistors
are ordinary 1/4 watt types. The 200k potentiometer could be a trimmer type but
a panel-mount type may be more desirable since this circuit does require a lot
of tweaking. Use an insulated shaft, however. All points in the circuit can
shock. The lamp is a clear 7-watt bulb typically used in night lights. White or
frosted bulbs will not work as well.

Note: If the lamp ever lights during the following adjustment process, remove
the power for several minutes before starting over and wait a couple of minutes
after applying power before making adjustments. This is an
"expert tweaker" level circuit and might drive the beginner crazy. A
beginner shouldn’t build line-powered devices anyway!

The DC voltmeter is an ordinary digital VOM set to the 2 volt scale and is used for
adjusting the potentiometer. Set the pot to the highest resistance and then
apply power to the circuit. If the bulb lights immediately, you will need to
increase the 100k resistor in parallel with the pot. If it doesn’t light
immediately, slowly reduce the resistance on the pot until the voltmeter starts
to show a voltage. The SCR will trigger and the lamp will light when the voltage
reaches about 250mV. Remember the exact voltage. Turn off the circuit, wait 5
minutes, reapply power with the pot set to the high end and begin approaching
this trigger voltage again. Stop when the voltage is about 50mV below the
triggering point. Hold a flame near the bulb and
the voltage should begin to climb toward the trigger point. (You can use a
flashlight instead of a flame for testing purposes.) After a few seconds
the lamp will light. Magic!

Unfortunately, this "magic" isn’t particularly stable and frequent
adjustment may be required. Some bulbs may require a larger or smaller
resistance across the SCR than can be obtained with the values shown. If the
bulb will not light at any pot setting, reduce the 100k  in parallel with
the pot to 47k. If it won’t stay out, increase the 100k and possibly the 47k.
After a little experience you will be able to make the adjustments without the
meter. The bulb in the prototype measures 150 ohms at room temperature. Another
identical looking bulb measured 300 ohms which is more typical of the little
night-light bulbs that vary from 4 to 7 watts.

By now you probably get how it works! The tungsten in the bulb
has a pretty steep temperature coefficient and since it is in the good thermal
insulation of a vacuum, it is easy to heat with moderate light levels. In
fact, a small flashlight can heat the filament to 50 C and a nearby 60 watt bulb can
raise the temperature to over 200 C! (I recently changed
these numbers after additional testing. The original value of 400 C rise for a
60 watt bulb seemed too high to be possible and it apparently was. Maybe I
switched the two bulbs or I was
working in Fahrenheit! I’ve also noticed that some filaments don’t absorb the
radiant energy as well as others, perhaps due to surface color or texture. I
wonder if a used bulb is more sensitive due to a darkened surface.) A tiny penlight will raise the temperature
of the prototype’s bulb over 1/2 degree from across the room! These temperature
increases result in significant resistance changes as the chart below indicates:


Tungsten Filament 7-watt Lamp

The resistors and potentiometer across the SCR cause a couple of
mA to flow in the dark lamp and when the value is set just right, the transistor is on the
verge of conducting at the peaks of the line voltage. A bright light heats the
filament and, as the bulb’s resistance increases, the voltage across the bulb
goes up. The transistor starts passing pulses at the peaks of the line voltage
that charge the 1uF triggering the SCR. Once the SCR triggers the lamp lights
and shoots up in resistance, and the circuit latches on. Power must be removed for
several minutes to reset the circuit because it takes quite a while for the
filament to cool to room temperature. The 1uF capacitor prevents the SCR from
triggering due to the turn-on transient.

This fun demonstration of some properties of the ordinary
incandescent bulb suggest other projects. The filament is a low resistance and
will exhibit little noise so it should be possible to place a couple of bulbs in
a bridge to achieve an extremely sensitive radiometer (one bulb shielded from
light).  Some other ideas include a light meter, optical isolator,
beam-break detector, solar radiometer (Just how hot does the sun get a
filament?), flame monitor, and solar panel positioner. Or an unusual gain
control; many years ago Hewlett Packard used a similar bulb to stabilize their
audio oscillator but the energy to heat the filament came from current flow, not
light. One bulb illuminating another for gain control would be unique, if not
practical.  Many photoelectric projects will seem amazing with a light bulb
as a sensor in place of a photocell! Other type bulbs may be worth
investigating, too. Choose a high voltage, low wattage bulb to get a high
resistance filament. For example, a tiny #387 type (28 volts at 40 mA) has about
1/2 the resistance of the larger night light bulb and it makes an excellent
detector. For most light sensing applications, apply only a volt or two across
the bulb to keep self-heating down.

More experimental results:

Using two #387s in the bridge configuration described above and
feeding a differential amplifier with a gain of 500, a radiometer with
surprising stability and sensitivity was realized. It drifts around a little but
this detector can easily detect the infrared from an IR LED and a soldering iron
and the sensitivity to light is also excellent. (It is fascinating to see.)

A 100uA meter was connected across the bridge in place of the
differential amplifier and a 60 watt lamp held near one bulb gave a near
full-scale reading. (Exposure meter!) The bridge voltage was increased to 9
volts for this experiment. The bridge circuit is nothing more than the two lamps
in series across 9 volts for one leg and a 1k ohm potentiometer connected
across the 9 volts for the other leg. The diff amp or meter connect between the
wiper of the pot and the point where the two bulbs connect together. The pot is
adjusted for a zero reading.

Here is a somewhat impractical but interesting idea:

Car taillights have two filaments in one bulb. How about a
circuit that watches the turn signal filament to determine if the taillight is
functioning properly? It would only work when the brakes and turn signals were
not in use but it would still be handy and could be done remotely without
running additional wires or fibers as done in some cars. A bad brake/turn signal
filament is already easy to spot since the rate of flashing changes when one is

Update – I just checked a taillight bulb and determined the
higher current filament increases from about 0.5 ohm to near 1.5 ohm when the
taillight filament is illuminated.  A 3 to 1 change should be easy to
detect!  Reversing the roles of the filaments results in a 2 ohm to 9 ohm

Reader Tom Bruhns suggests using a light
bulb to measure and possibly control your oven’s temperature! In this
application the bulb is acting as a simple temperature sensor. Ordinary light
bulbs can take the extreme temperatures in an oven – there is already one in
there, after all. Come to think of it, other lamps might serve as thermometers
when not "lit", the refrigerator being one example. The use of the
lamp for light might degrade the calibration due to filament aging but
temperature measurement/control applications could benefit from the use of a
dedicated light bulb sensor.  Bulbs, after all, are made from corrosion and
temperature resistant materials and the filament is sealed inside a protective
vacuum.  I can’t think of a superior temperature sensor available at the
local convenience store!

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