Narrow-bandwidth Television Association
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Chapter 4
Camera / Monitor assembly
Les Robotham & R. Pedro
This project is a natural progression
from the monitor features in the previous chapter. Les Robotham and Pedro have
developed the project over a number of years. The unit itself forms a
self-contained combined camera and monitor, which, at a later date can be
modified to produce video signals for external equipment.
No synchronising is required as the
monitor and camera are on the same disc. A special disc is used having the holes
distributed over one and a half turns to ensure the image on the viewing section
is the right way round. Construction is not critical but it is important that
external light entering the disc area is kept to a minimum. Areas within the
camera and detection head are normally sprayed matt black to reduce reflections.
Many variations have been made of this unit including the 'hidden disc' 'biscuit
tin' method where the disc is completely enclosed, the only apertures being for
the viewing area and the lens and detection head.
What you will need for this
project is:
1. LED Cluster and drive electronics
as in the monitor (sync pulse circuit not needed) The video output from the
detector head is fed directly into this circuitry which may be separate from the
head assembly.
2. Motor speed control electronics (regulator circuit to keep
motor speed fairly constant.... at about 750 rpm)
3. Cassette motor (club
sales)
4. Special one and a half turn disc (club sales)
5.
Enclosure for disc and mounting base.
6. Power supply unit for
electronics
7. Lens head assembly ( see next chapter)
8. Detection head
assembly
Fig. 1. Plan view of the Camera / Monitor
Fig.2. A possible design for an enclosure.
One suggestion for a suitable
enclosure is shown in fig. 2. Whilst the design is not critical, stray light
must be kept out of the area between head amplifier section and the disc this is
achieved by spraying all the inside of the box matt black. The advantages of the
square box are that different scanning formats can be achieved. Either by
changing the direction of motor, reversing the disc or positioning the box in
another plane.
This box contains all the sub-assemblies plus the power
supply. You may however decide to enclose the disc totally in a light-proof unit
and mount all the peripherals on the outside of the unit.
The disc
The club disc has a
spiral of 48 holes over 1.5 revolutions. The camera masks the inner 16, the
monitor the outer 16 holes. In this way the camera and monitor share the same
disc. There is an extra circle of 32 sync holes at the rim.
The motor
Whilst the speed
of a monitor motor is controlled by the incoming video signal, the speed at
which a dc camera motor rotates is directly proportional to the current flowing
through it. An AC motor is governed by the supply frequency and the number of
poles on the rotor on the device. Whilst the dynamo is normally a generator,
applying an ac supply to it will act as a motor. At 50 Hz an eight- pole dynamo
will rotate at 750 rpm which is the correct speed for NBTV. Whilst small motors
of this type are rare the common bicycle dynamo can be used although it will
need to be turned by hand to start. By using dynamos on both camera and monitor
the common ac supply will keep both units in synchronism. The most popular motor
used however is the small dc motor 'liberated' from an old cassette recorder or
purchased at a small price from the NBTV club shop. It is important to ensure
the motor has not got an internal speed regulator fitted, if it has this can by
easily bypassed. In its simplest form a variable dc voltage can be applied to
the motor to vary the speed. If this is not available a wire-wound variable
resistor, its maximum resistance should equal that of the motor, in series with
the motor and the supply will suffice.
Monitor part
The next part
of the electronics is the LED cluster and LED driver board. They are identical
to the design given previously for the simple monitor. The sync pulse separator
circuit and automatic motor synchronisation circuit are not needed. The video
input is connected directly to the detector head, which is a separate
unit.
The head assembly Fig 5. Construction of the head assembly. There is enough room
within the unit to mount the head amplifier fig 4 and the LED driver electronics
fig 2. The dome sensor is mounted centrally on a piece of insulating material
directly in line with the lens in order to focus correctly. Figs 6 and 7. Two views of the head assembly under
construction. The dome sensor is mounted in the centre
of the lens housing on the axis of the lens to ensure correct focusing. Fix it
on a piece of copper clad fibreglass PCB. There is enough room within the unit
to mount the head amplifier. Patrick de Scheemaecker designed this pre-amplifier
circuit. Good construction techniques should be employed producing this module.
As the overall gain of the unit is relatively high, a compact unit will reduce
susceptibility to electrical noise. An onboard regulator ensures that the
circuit is isolated from fluctuating supply voltages. The potentiometer is used
to regulate the signal going into the amplifier. This is analagous to the action
of the iris in a camera which controls the light reaching the film.. The video
output from this unit (which does not contain any synchronising signals) is fed
directly into the LED driver circuit, which was featured in the last section. If
the picture is seen in negative, the wires to the dome sensor must be
exchanged.
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Once the
lens assembly is complete the next module to build is the head assembly (Fig.
5.). The main case is available for those in the UK from Maplin. The figure
shows the general arrangement of the unit. Contained within the box are all the
electronics with the exception of the power supplies. A lens mounted in this
unit focuses the image onto the photo sensor; a screwed rod facilitates
adjustment of the lens from the outside of the unit.
To minimise
internal light reflection within the unit the internal exposed metalwork must be
sprayed matt black, small spray cans of this are available from most hardware
outlets. Due to the high gain of the head amplifier it is recommended that
care be taken in construction of the head amplifier. Using RF techniques on
copper clad fibreglass board will reduce the problems associated with poor
insulation on standard stripboard.
The LED driver electronics is similar to
that used in the NBTV monitor, the sync detection circuitry may be discarded, as
this is not required on the camera/monitor. There are two sockets mounted on the
rear of this assembly. The first is for the 12 volt dc input to the unit whilst
the second is the output to the display LEDs. Please note the latter needs both
connections to be isolated from the chassis.
The Head amplifier
The circuit for this unit is shown below.
As the camera/monitor is rather large
and not portable when working, a zoom lens is very welcome. Les Robotham and
R.Pedro have produced a home built design for such a lens which follows in the
next chapter.