Dimensions: The main body of the payload is cylindrical with an outside diameter of 1.735" and a length of 11.25". This is very close to the dimensions of a standard paper towel roller. A smaller cylinder, 1" in diameter and about 1.5" long is attached at one end, and contains the environmental sensors. The payload weighs just under 1 lb. The entire payload compartment is filled with hardened foam ("Great Stuff").
Casing: the payload is encased in a fiberglass cylinder custom
made and donated by Osceola Fiberglass, Osceola, MI. Two layers
of fiberglass mesh were used. The Sun will be nearly overhead
during the launch so solar heating is not expected to be
significant. The main body of the payload was painted orange to
aid in visibility in the event the payload ends up on land.
Camera: the TV camera is a small, light weight, black and white CCD camera designed for use as a security camera (Chinon CX102/43 with a 4.3 mm focal length lens). The camera is on a circuit board measuring 1.25" by 1.25". The camera operates with 7 to 14V and draws about 120 mA at 9V. The video output follows the same standards (NTSC 2/1 interlace) as a typical camcorder or VCR output, except there is no color or sound. The camera points out the top of the payload and will be pointing down on descent.
Microcontroller: A PIC16C71/04I 18 pin microcontroller circuit was developed and the microcontroller programmed specifically for the payload. The microcontroller provides the audio for the TV signal. Audio is generated through a program loop which toggles one of the output pins high and low at about a 1 kHz rate. The audio frequency is somewhat voltage and temperature dependent. In addition, the PIC16C71 includes a 4 channel, 8 bit, analog to digital (A/D) converter. The A/D channels are used to send telemetry information including the battery voltage, internal temperature, the external temperature and air pressure. The entire circuit, excluding the external sensors, is on one etched copper disk, 1.6" in diameter and it draws less than 10 mA. A small piezo-electric beeper was added for fun. The beeper sends a short audible signal approximately once each second.
Telemetry Sensors: The battery voltage and two temperatures are measured using a resistive divider. Two fixed 10 kiloohm resistors are used for the battery voltage and the temperatures measured using fixed 10 kiloohm resistors connected to a voltage reference in series with a thermistor. The thermistors have a resistance of 10 kiloohms at room temperature. One thermister is mounted on the microcontroller board and the other on one of the batteries.
Transmitter: The video and audio signals are fed to a MICROTEK ATVM-70 mini transmitter and its companion MSC-2 sound sub-carrier boards. These boards are 1" by 1.3", draw just under 100 mA, and have an output power of 80 mW. The output from the transmitter has the same format as normal TV signals and the signals can be received on a normal television using a converter. In fact, the signals can be received without a converter using a "cable ready" VCR or TV set to cable channel 59, but with the cable removed and an antenna used in its place. This transmitter operates in frequency bands allocated to the Amateur Radio Service and can only be operated by a licensed amateur radio operator.
Power Amplifier: The 80 mW signal from the transmitter is boosted to about 1W using an SAU35AH amplifier module. This module was custom mounted for this project on a 1.6" disk. A separate 1.6" disk was used for a 5V regulator for the bias voltage (VBB). The batteries are connected directly to the other two power inputs. The output power depends on the battery voltage. The circuit is, aside from small component value changes made for convenience, the same as the "test circuit" included in the engineering data sheets for this device.
Antenna: The antenna is a simple resonant dipole made from two lengths of spring wire. The dipole sticks out horizontally from the sides of the cylinder. When loaded, the two wires which make up the dipole fold along side the payload. The tuning of the antenna, power amplifier, and transmitter will supress the lower sideband signal (which is not used by commercial TVs).
Batteries: The payload power is supplied by four (4) C size Lithium batteries wired in series. Each battery has a voltage of about 3.3V with no load. The current draw from the batteries is just under 1 A, and is well over the design current for these batteries. Under load, the total voltage from the four cells is between 2 and 2.5 volts per cell, depending on battery temperature. The batteries are expected to last about 2.5 hrs. The power is activated when the payload is ejected from the rocket. These batteries will become hot to touch when operated in this manner and operation of these, or other batteries in this manner for other applications is not recommended.
All components were subjected to numerous ground tests including drop tests to simulate the high g forces expected and vacuum tests to ensure operation in the absence of air. The electrical properties of the foam filling were tested to ensure the foam would not degrade the UHF signals used.
(Close up view of electronics).
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