Planck constant apparatus using LEDs
Water clear LEDs of different wavelengths are mounted in a line on PCB
board and jumper links are used to connect one at a time. Using external
ammeters and voltmeters readings of the threshold voltage are taken by
adjusting the finger potentiometer for each LED. The circuit incorporates a
transistor for current amplification and diodes, resistors and zener diodes to
protect damage to circuit from incorrect connection to the power supply.
Components:
LED
Red
Orange
Yellow
Green
Blue
Violet
625nm
605nm
590nm
565nm
470nm
405nm
PCB header plug
Jumper link pull-tab
10K pot finger adjust
BC238B NPN transistor
1N4001 rectifier
BZX85C zener diode
1K resistor
3K3 resistor
4mm PCB sockets
Feet
Photoetch PCB
4mm sockets
Rectifier
1K
10K pot
Zener diode
3K3
Rapid
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55-2476
55-2486
55-2478
55-0135
55-2482
551818
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x
x
x
x
x
1
1
1
1
1
1
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19-0076
22-0695
68-0242
81-0048
47-3130
47-3065
62-0370
62-0382
17-0595
31-0516
34-0110
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x
x
x
x
x
x
x
x
x
x
1
1
1
1
1
1
1
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6
6
1
Pull-tab
LED
NPN transistor
PCB Header
PCB used (actual size)
The design
We decided to build new equipment for this experiment as the previous set up
had loose LEDs which got mixed up, lost and broken legs due to bending. The
new design has made the labelling wavelength of the LEDs clear. The
equipment is easy to store and put out as everything is attached to one board.
When physics students have worked with LEDs in the past we have come
across the misconception that the colour of the light emitted was due to the
filtering action of the coloured plastic. When choosing LEDs for this apparatus
we chose LEDs with clear plastic casings so that it is clear to the students that
it is coloured light that is being produced by the electrons moving across the
junction. We also thought that labelling the LEDs just with their wavelength
made the students draw a stronger link between colour and wavelength and
made them practice converting wavelength in nm to frequency in Hz.
We have built in some protection against damage by misuse:
 A diode to stop problems with reverse polarity connection to the power.
 A zener diode, resistor and transistor to limit the voltage and prevent
damage by too high a supply voltage
 A resistor to limit the current so that the intensity of light from the LEDs
does not damage the eyes
We spent some time choosing the LEDs to get a fairly good straight line. The
problem with the experiment is that not all of the LEDs are made from the
same substance so there are different work functions for different colours. By
being pragmatic in our choice of LEDs we have avoided some difficult
explanations of anomalies which would obscure the main teaching point for
most students. If you decide to select your own LEDs it is worth buying a
range of devices and experimenting to get a good spread of points which
produce a fairly good straight line – you may even decide to use uncertainty of
the wavelength in the specification to aid the production of a straight line.