LED Stimulator
(Four (+1) channel ‘ganzfeld’ unit)
Operators Manual
CH Electronics
27 Calmont Road, Bromley, Kent, BR1 4BY
UK
Tel/.Fax. +44 [0]181 460 7809
LED4 User manual
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Contents
1.0
Introduction
3
2.0
Description
6
2.1
Controller
6
2.2
Head stage
7
3.0
Controls and connections
8
Panel layout
10
4.0
Limitations
11
4.0
Calibration
12
4.1
Frequency display
12
4.2
Light intensity
12
Circuit board layouts
13
5.3
Calibration as shipped
15
5.4
Spectral output
16
6.0
Computer control
17
7.0
Modifications from standard
19
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1.0 Introduction
1.1 Light emitting diodes or LED's are near ideal light sources for many purposes. They
are small, require low voltages and currents to drive them, can be controlled by simple
electronic means to give either continuous light outputs or extremely brief flashes (or
both together) over a large range of intensities. Their light output changes by less than
10% in intensity or relative spectral emission over 50,000 hours of use (Hewlett
Packard application note 1017). These properties not only simplify calibration but also
reduce the frequency with which it is needed. Many different wavelengths of light are
available, in a variety of different physical packages with differing optical properties.
The majority of the available devices are inexpensive. The chief difficulties users are
likely to experience are due to the low intrinsic source brightness, which is considerably
less than that of arc or incandescent sources. Nevertheless, even where considerable
quantities of light are required (for example in electroretinography) LED’s can be used
to advantage.
Light emitting diodes or LED's are members of the family of epitaxial semiconductor
junction diodes. Growing a very thin crystal of a semiconductor directly onto another,
slightly different semiconductor surface forms a junction. The two layers of
semiconductor material, (frequently gallium aluminum arsenide) each contain different
impurities (dopants). As a result of these impurities, one layer contains an excess of
free electrons and the other an excess of holes (positive charge). The energy required
to move a charge across the junction against the concentration gradient of free
electrons or holes is considerable, and larger than in other types of diode, (approx.
3:1). This energy "band-gap" must be exceeded if current is to be passed through the
junction. When the device is forward biased electrons move from the negative material
to the positive and a corresponding movement of positive charge or "holes" occurs in
the reverse direction. When an electron and hole pair recombines, its energy is emitted
as a photon. The characteristic wavelength of the photon is given by the energy band
gap. Thus, it is more difficult to produce short-wavelength LED’s, since the higher
energy gap must be maintained with the controlled flow of charges. The construction of
the epitaxial layer determines the direction of light emitted, and the absence of a
resonating (reflective) cavity prevents the stimulated emission of radiation by the
photons, so the light is not coherent, and contains a number of differing wavelengths.
However, light is emitted over a relatively narrow bandwidth. For a typical red led, (for
illustrative purposes a Stanley HBR5566X) the peak is at 660nm and the half-power
bandwidth is +/- 30nm.
The construction of a 'typical' LED is shown in figure 1.
The semiconductor is mounted on a "lead frame", and encapsulated in a plastic
(epoxy) housing with an internal spherical lens. The combination of junction structure,
epoxy and lens type determine the spatial output characteristics of the device and for
many LED’s, including all the "brightest", the light is concentrated in a cone, which can
be represented in a polar diagram (figure 2), with a half-power spatial / polar
distribution of +/- 7.5 degrees.
A typical device requires fifty milliamps at around two volts to produce its maximum
output, so it is both easy to control and intrinsically safe to use in a clinical
environment. The junction of most modern devices thus exhibit a low electrical
impedance, but are nevertheless fairly robust, being able to tolerate significant
overloads for short periods.
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Figure 1
Various types of LED are available. Some are devised for special purposes, for
example alphanumeric display components or indicators. There is a considerable
range of shape, size and intensity. A number of devices are packaged with additional
circuitry that provides a constant current flow or flash the device on and off. Other
devices contain more than one junction and can produce two or even three colours, but
these are so specialised that their value for purposes other than those for which they
were designed, that is for instrument displays, is limited. However, some squarepackaged arrays may be useful for producing checkerboards or similar displays.
The colours quoted by manufacturers vary from infrared to blue, but this is may be
misleading. In general, red LEDS are by far the brightest, and have peak emission at
around 660nm. Yellow LED’s are also efficient light emitters, and peak at c. 580 nm, so
that a variety of oranges can be obtained. Green LED’s are now available with a
considerable light output. However, all have peak emission at c. 560nm, which, it
should be remembered, is the peak of the photopic relative spectral sensitivity curve.
This characteristic is unfortunate, since "green" light is often thought to selectively
stimulate rods. Various filters are also incorporated in the plastic capsules of green
LED’s to remove the longer wavelengths, but none provides a filter effective enough to
modify the output in any significant manner. If additional external filters are employed, it
will be found that the proportion of light emitted below 510nm is so low, (<5%) that it is
difficult to obtain a blue-green light of adequate intensity for most electrophysiology.
Blue LED’s are currently available from several manufacturers and are significantly
different to all others. They generally have considerably less light output, and the
voltage required at the junction is significantly higher as is the junction capacitance.
The peak of the light output is at 480 nm, but the bandwidth is wider than that of many
other devices and asymmetrical, being broader towards the longer wavelengths. The
relative spectral emission varies somewhat with the drive current. The devices are also
much more costly and 'fragile'. A single red LED may cost a few pence; a blue LED
with a much lower output costs around twenty pounds. Nevertheless, blue LED’s are
very useful in electrophysiology and psychophysics, and with suitable filtering can
provide stimuli of wavelength as low as 420nm or extending to as high as 520 nm:
thus, LED’s can be used for many purposes previously requiring incandescent sources
and monochromators or interference filters.
The relationship between applied current (or voltage) and light output of a typical
device is shown in figure 2. And for a region of about one and one half decades can be
seen to be approximately linear. Above or below this region, marked non-linearity
occurs.
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Since in general, users wish to control intensity in a simple ergonomic way over a much
larger range, a variety of drive circuits has been devised.
For a simple flash stimulator, a voltage drive circuit may be used quite effectively, and
the relationship between light output and applied voltage determined by calibration.
Figure 2
LED charcteristics
5
2.3
2.2
4
Forward voltage
2
3
1.9
1.8
2
1.7
1.6
1
1.5
1.4
0
0.1
0.2
0.5
1
2
5
10
Current (mA)
Vf
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Intensity
5
20
50
100
Relative intensity (20mA)
2.1
2.0 Description
2.1 Controller
The stimulator is designed to deliver square pulses of light rather than complex
waveforms, and as such is able to use a simple current pulse drive,
Additional intensity control is provided by means of pulse width control, this is precisely
variable over a range of microseconds to tens of seconds.
The stimulus waveform is generated by a free running oscillator, this oscillator triggers
a monostable pulse generator. The pulse generator is formed from a crystal oscillator,
the output of which is divided down by a series of counters to provide decade steps.
The output of this stage is then divided by a second series of counters giving steps of
one through fifteen. The output of this waveform generator is connected to the output
driver stages.
The four output driver stages share a common architecture. A pre-programmed power
(voltage) source, delivers power through a fixed resistor. The programmed element
being set using one of seven internal pre-set controls or one continuously variable
potentiometer to determine the attenuated levels by controlling the drive voltage. The
waveform signal drives a switching device which allows the pre-set current to pass
through the LED’s.
Figure 3
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2.2 Head Stage
The four groups of LED’s are mounted on a small planar circuit board with current
limiting resistors. This circuit board is mounted at one end of an internally partially
mirrored plastic tube (160mm long x 50mm diameter), with a diffusing screen
approximately mid way along the tube and a hemispheric translucent diffuser mounted
at the far end of the tube. On older versions of the instrument, a fixation spot is
provided via a 0.8 mm plastic optical fibre mounted through the rear surface of the
hemisphere. This fixation spot is not installed on later versions, although it can be retro
fitted if required. A white neoprene ‘eye cup’ assures patient comfort. The complete
assembly is mounted on ball joint assembly to ensure correct fitting to a wide range of
facial shapes (figure 5 shows a dual stimulator system).
Figure 4
Figure 5
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3.0 Controls
(See panel layout schematics at end of section)
3.1
Front Panel
1
Variable frequency control. Sets the frequency of the base oscillator for the
waveform (pulse) generator.
Frequency control range switch. Sets the range controlled by (1). The three
position switch selects a high and low frequency range, and in the third position,
which selects the higher frequency range, bypasses the pulse width control
circuits to provide a 50% duty cycle drive waveform.
Run. Starts and stops the waveform (pulse) generator. The set frequency is
still displayed when the output is gated using this control. In the ‘off’ position,
external signals via (24) set the frequency, or pulses initiated with push button
(4) are output.
Single shot manual pulse trigger.
Pulse width range. Determines the multiplier setting for the output pulse.
Range 1uS to 10S. Note, pulse widths below 10uS are subject to increased
error in light pulse width.
Multiplier for (5). Example; If the range is set to 1mS, and this control is set to
12, then a light output pulse of 12mS will be generated. N.B. the 0 setting is
marked, but should not be used. A number cannot be divided by zero.
Frequency display for the internal waveform generator.
Channel 1 / Red Waveform select switch. Selects one of the following; Channel
off, Positive going flash, i.e. dark to light, Negative going flash, i.e. light to dark,
Channel on, or any of these with a level set by the variable intensity control as
the ‘off’ state. Using these settings it is enables a flash to be superimposed on a
background of the same colour. N.B. The continuous intensity controls are not
linear. See calibration section for details.
Channel 1 / Red Stepped Attenuator. Attenuates the light output in steps of
1,3,10…1000, or selects the variable attenuator, (10). (See note in limitations
section for relevant notes),
Channel 1 / Red Variable Attenuator. Selected by (9). Provides a continuously
variable output across the range of the channel. (See note in limitations section
for relevant notes), N.B. This control is not linear. See the calibration graph in
section 5.1 for details.
Channel 2 / Green Waveform select switch. Selects one of the following;
Channel off, Positive going flash, i.e. dark to light, Negative going flash, i.e.
light to dark, Channel on, or any of these with a level set by the variable
intensity control as the ‘off’ state. Using these settings it is enables a flash to be
superimposed on a background of the same colour. N.B. The continuous
intensity controls are not linear. See calibration section for details.
Channel 2 / Green Stepped Attenuator. Attenuates the light output in steps of
1,3,10…1000, or selects the variable attenuator, (13). (See note in limitations
section for relevant notes),
Channel 2 / Green Variable Attenuator. Selected by (12). Provides a
continuously variable output across the range of the channel. (See note in
limitations section for relevant notes), N.B. This control is not linear. See the
calibration graph in section 5.1 for details.
Channel 3 / Blue Waveform select switch. Selects one of the following;
Channel off, Positive going flash, i.e. dark to light, Negative going flash, i.e.
light to dark, Channel on, or any of these with a level set by the variable
intensity control as the ‘off’ state. Using these settings it is enables a flash to be
superimposed on a background of the same colour. N.B. The continuous
intensity controls are not linear. See calibration section for details.
2
3
4
5
6
7
8
9
10
11
12
13
14
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15
16
17
18
19
20
21
22
Channel 3 / Blue Stepped Attenuator. Attenuates the light output in steps of
1,3,10…1000, or selects the variable attenuator, (16). (See note in limitations
section for relevant notes),
Channel 3 / Blue Variable Attenuator. Selected by (14). Provides a
continuously variable output across the range of the channel. (See note in
limitations section for relevant notes), N.B. This control is not linear. See the
calibration graph in section 5.1 for details.
Channel 4 / Amber Waveform select switch. Selects one of the following;
Channel off, Positive going flash, i.e. dark to light, Negative going flash, i.e.
light to dark, Channel on, or any of these with a level set by the variable
intensity control as the ‘off’ state. Using these settings it is enables a flash to be
superimposed on a background of the same colour. N.B. The continuous
intensity controls are not linear. See calibration section for details.
Channel 4 / Amber [ + white] Stepped Attenuator . Attenuates the light output in
steps of 1,3,10…1000, or selects the variable attenuator, (19). (See note in
limitations section for relevant notes),
Channel 4 / Amber [ + white] Variable Attenuator. Selected by (18). Provides a
continuously variable output across the range of the channel. (See note in
limitations section for relevant notes), N.B. This control is not linear. See the
calibration graph in section 5.1 for details.
Power indicator light. (Mains supply on).
Channel 4 / 5 select switch. Selects the 5th [white] channel to be controled
instead of the amber, using the controls for channel 4. Note; There is no
continuously variable control for channel 5. If ‘Var’ is selected, the channel is
effectively off.
Trigger edge select. Selects leading or trailing edge of the flash on which to
output a ttl trigger pulse, (internal waveform generator only), though the Trigger
output socket, (23).
3.2
Rear Panel
23
TTL trigger output socket. Positive going TTL pulse, 0.2mS duration, output
under the control of (22). See section on modifications for changes.
Trigger input. When (3) is in the ‘off/ext.’ position, a pulse applied to this
connector initiates an output as set by controls 5 – 19.
Not fitted.
Not fitted. Instrument ground connection. Connected to mains earth via the IEC
power lead. N.B. Not for patient connection.
….
Stimulator head connector. [25 way ‘D’ socket] Connection to Ganzfeld heads.
Note that >30 volts is present on this connector, connection to incorrect
equipment will cause damage.
Mains power switch. IEC inlet socket. Fitted with two one amp 230 volt fuses.
Normally set for 230V 50Hz input. See modifications section for alternate
power supplies.
Stimulus head connection. NB. Use only for the head supplied with the
controller. The head stages, (ganzfelds), are not interchangeable. Swapping
heads between unit may cause serious damage to either component.
Mains power ON / OFF switch.
Dual fused IEC mains power inlet connector. Use only fuses as supplied.
(1 amp 230 volt, fast blow).
24
25
26
27
28
29
30
31
32
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4.0 Limitations
Maximum intensities; All LED’s, especially those of longer wavelength emission, are
subject to a heating effect, where, when driven at high (approaching maximum) the
light output intensity falls as the junction heats. For this reason, it is recommended that
the red channel is are not used for pulses of more than 100msecs duration, or
backgrounds at attenuation settings of 1, or the variable control equivalent. These
levels will not damage the devices, but a reduction of output with time will occur.
This heating effect will especially in the shorter wavelength devices, cause a change in
the wavelength of the emitted light. These changes are small even for the blue
devices, where they are of the order of 3 to 5nm across the entire range of intensities.
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5.0 Calibration.
Adjustment. Please note that all adjustments described should only be carried out by
suitably qualified persons. If in doubt refer back to the manufacturers.
5.1.1 Frequency display. The one second base interval for the counter can be
adjusted as follows.
5.1.2 Switch the power off and disconnect the power supply.
5.1.3 Turn the controller over and remove the 4 screw-on feet. The base may now be
removed.
5.1.4 Reconnect the power supply,
5.1.5 Set the frequency to around 10Hz and using suitable instrumentation measure
the output frequency at pin X on XX. Adjust the pre-set potentiometer XX so that the
display coincides with measured frequency.
5.1.6 Repeat the comparison at the upper and lower end of the frequency output.
5.1.7 Disconnect the power and reassemble the unit.
5.2.0 Light output.
5.2.1 Switch the power off and disconnect the power supply.
5.2.2 Turn the controller over and remove the 4 screw-on feet. Turn the case over and
remove the top cover.
5.2.3 Reconnect the ganzfeld bowl and power supply.
5.2.4 Place a suitable photometer head against the ganzfeld in such a position as to
be in the same plane as the eye of the subject.
5.2.5 Switch the channel to be adjusted to ‘on’ using the waveform selector switch
5.2.6 Set the stepped attenuator to the setting to be adjusted.
5.2.7 Taking care not to exceed the maximum current levels for the channel under
test, adjust the appropriate preset, (for locations see ‘driver boards’).
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6.0 Calibration as shipped
Attenuator settings
Calibration - 20th January 2003 - IL1700
Fixed
Attenuation
1
3
10
30
100
300
1000
Red
278
92.7
27.8
9.3
2.8
0.9
0.3
Green
1916
638.7
191.6
63.9
19.2
6.4
1.9
Blue
31.000
10.333
3.100
1.033
0.310
0.103
0.031
Amber
825
275.0
82.5
27.5
8.3
2.8
0.8
White
1685.0
561.7
168.5
56.2
16.9
5.6
1.7
Spectra file; Cardiff3.wk4
Trig.op.
40uS +ve
Continuous
Dial
0
50
100
200
300
400
500
600
700
800
900
1000
Red
Green
3.78
31.3
62.8
95.6
126.9
160.0
187.4
234.0
268.0
0.178
0.815
9.24
249.0
607.0
907.0
1163.0
1386.0
1584.0
1764.0
1928.0
Blue
Amber
0.57
2.74
7.21
11.43
15.36
18.95
22.3
25.5
28.8
31.3
0.005
1.59
83.3
190.6
294
390
462
539
620
703
767
Calibration figures in Cd/m2, measured using IL1700.
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White
Not
available
Spectral distribution
(attenuation 3)
1
red/3
green/3
blue/3
amber/3
white/3
normalise output
0.8
0.6
0.4
0.2
0
350.1
424.2
387.4
496.6
460.7
567.3
532.2
636.2
602.0
wavelength (nm)
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703.4
670.0
768.8
736.3
6.0 Computer control - Not installed
Computer control connection is via the paired 37 way ‘D’ connectors on the rear panel.
The recommended interface card for computer control is the PC214E from Amplicon
Electronics, connection is via the 37 to 78 way ‘D’ cable available as an optional
component.
Control output; This socket carries lines from the front panel switches. They are
connected via the ‘Control input’ socket to the relevant points on the circuit boards. By
removing the 37 way bridging connector and connecting the external control cable from
the remote control device to the control input socket, the front panel switches are
replaced by the external control device. It should be noted that this gives direct access
to the CMOS gates of the control devices on the printed circuit boards.
All inputs and outputs concerned with external control require TTL compatible signals.
The inputs are CMOS gates tied low with 10K ohm resistors.
Care must be taken if devices other than the PC214E is used not to deviate from these
levels. Exceeding the TTL specifications could cause damage to the stimulator.
The required connections are detailed below in the following table.
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Description
SCREEN
Pulse width - period
Pulse width - period
Pulse width - period
Pulse width - multiplier
Pulse width - multiplier
Pulse width - multiplier
Pulse width - multiplier
Red - Waveform
Red - Waveform
Red - Waveform - secondary
Red - Attenuator
Red - Attenuator
Red - Attenuator
Green - Waveform
Green - Waveform
Green - Waveform secondary
Green - Attenuator
Green - Attenuator
Green - Attenuator
Blue - Waveform
Blue - Waveform
Blue - Waveform - secondary
Blue - Attenuator
Blue - Attenuator
Blue - Attenuator
Amber - Waveform
Amber - Waveform
Amber - Waveform secondary
Amber - Attenuator
Amber - Attenuator
Amber - Attenuator
Trigger - Select edge
EXT select
GROUND
GROUND
GROUND
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Pin #
1
2
4
1
2
4
8
1
2
4
1
2
4
1
2
4
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
1
2
4
1
2
4
1
2
4
1
2
4
18
19
20
21
22
23
24
25
26
27
28
29
1
2
4
1
1
30
31
32
33
34
35
36
37
18
7.0 Modifications
7.1
Trigger output has been set to 0.04ms, zero going high, TTL compatible.
[May be set internally to +5V going to 0 if required].
7.2
Power supply; Mains (line) inlet is configured for 230 volts at 50/60Hz and dual
fused with 1.0 amp ‘quick blow’ fuses.
7.3
Channel 4 now can be used with either amber of ‘white’ (see spectral output
graph). For details see the controls description section.
Please Note;Refer all servicing or modifications to suitably qualified person.
Any repairs or modifications not specifically approved in writing by CH Electronics may
void the warranty on this instrument and could also render it unsafe.
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