CricketSat Manual and Assembly Instructions
History:
The CricketSat evolved from the desire to provide a real working data telemetry device at very low cost to those interested in experiencing actual data gathering and recovery methods. The device had to be easy to construct with a low parts count; yet provide the essential capabilities for true data measurement, generation and telemetry transmission.
The Name:
"It looks like a cricket!"...chuckled Jeneene Hartshorne, high-school science enthusiast, and daughter of Lloyd Hartshorne, a mentor at Stanford University's Space Science Development Laboratory. She was referring to the 'V' dipole antenna sticking out from one end of a tiny printed circuit board that was the very first model of the yet unnamed circuit. Professor Bob Twiggs, director of the laboratory, said..."Okay...it's a CricketSat!"...and that's about how it got the name.
Function:
The CricketSat is a complete, fully functional temperature data telemetry package. The transmitter, despite its low power of +10 dBM (10 mW) near 434 MHz, when used with a resonant antenna, can achieve considerable freespace distances. Calculations will show that even antennas with 0 dBi gain on both ends (based on receiver sensitivity of 1 uV), would result in a distance of almost 39 km! (24.23 mi.)
Terrestrial coverage is difficult to determine, due to the infinite variances involved. But, with the same antennas and receiver sensitivity as above, nearly 19 km. (11.81 mi.) is possible with
unobstructed paths. It doesn't seem possible, but under the best conditions, figures don't lie.
With gain antennas, such as a beam, dish or helical, impressive ranges are achievable.
To those knowledgeable in RF it is understood that the transmitter should have more output filtering if using gain antennas for transmit.
For airborne use, a basic antenna on the CricketSat transmitter and a gain antenna on the receiver to extend the range would be easiest.
How the CricketSat works:
The 555 chip is configured as a resistance-controlled oscillator. A thermistor is included for sensing temperature. The thermistor is a temperature dependant resistor, whose value is 10,000 ohms (10 Kohms) at 75 degrees F. When the temperature goes up, the resistance goes down, and vice-versa. This change in resistance causes the frequency of the oscillator to change.
Based on inherent differences in component tolerances, calibration table values will vary, depending on the components used. If any component changes occur, re-calibration is necessary. Remember, except for operational testing, unless you calibrate your unit, data is, for the most part, meaningless. Calibration procedures are included in this document.
The output of the 555 oscillator is fed to the input of a UHF transmitter, which transmits the data, either as an audio tone, or as slow on/off signal carriers. This same output is also fed to an LED, which will light at the output's rate, or frequency, providing a visual and/or optical signal.
Data Recovery:
You have a choice of an audio tone frequency of about 1000 Hz (1 kHz) or of approximately 2 pulses per second (2 Hz). Components for both data rates are included.
The 1 kHz rate will provide an audio tone at that frequency and will be heard on a receiver tuned to the transmitter's frequency.
The CricketSat also can provide data without a transmitter. An LED included in the circuit will flash at the pulse rate and would allow reading the telemetry visually without a transmitter. Obviously the 1 kHz rate would provide a steady glow from the LED and would be impossible to visually count the number of pulses by eye. However, with a lightwave receiver you could optically receive the data rates. Optical lenses would increase ranges. The LED could even be exchanged with an infrared type for daylight use. It's left to your imagination and ingenuity in this application.
The 1 kHz rate provides a much higher temperature resolution. But, it also requires a means of measuring the higher frequency in order to calibrate and decode the data. An audio frequency counter will be needed to do this. It can be in the form of an audio transducer next to a receiver speaker, or a simple interface to a computer running a frequency/event counter program. In the field, one could even just record the tones on a portable recorder and then feed that audio into a counter at another location.
The 2 Hz rate can be used to visually count, or in the case of using the transmitter, audibly hear and count the number of pulses in a given time period. In this most basic method, perfectly usable telemetry can be recovered with nothing more than a watch with a second indicator and a pad and pencil. Temperature resolution is correspondingly lower; but certainly useable.
Assembly Instructions:
Parts List:
C1.1uF - (marked 104) for 1 kHz, 47uF - for 2Hz
C2.1uF (marked 104)
D1LED
L1, 2100uH. molded choke (marked - wide silver band,brn,blk,brn, narrow silver band)
R1Thermistor (10 Kohm @ 75 deg. F), blue with white dot on tip
R23 Kohm (org,blk,red)
R3750 ohm (for 9VDC)included, (use 330 ohm for 5-6VDC, 1000 ohm for 12-15VDC)
TRXUHF transmitter module
U1555 integrated circuit
Misc.8-pin DIP socket, printed circuit board, 9V battery connector
CricketSat Printed Circuit Board layout (component side)
IMPORTANT:
Components should be mounted as shown in the board drawing. Keep referring to the layout drawing to make sure you have the orientation of U1, D1 and TRX, the transmitter module, correct. Mounting these components the wrong way will damage or destroy them, and you will have to replace them on your own. The notch shown on U1, indicates Pin1 is the first one to the left, or a dot by a pin indicates Pin1. The smaller solid box on TRX is the round metal can on the module. You must mount TRX this way! The flat spot on the base of the LED is the cathode. The schematic symbol, next to the holes for D1, show the cathode's lead should be in the left hole, the one next to the symbol.
ALSO: Before you begin to solder…remember, use just enough to insure a good connection. Too much might bridge to a nearby trace or pad and cause a short circuit. A short could cause considerable damage to your components. The solder connection should look bright and shiny with a smooth flowing look. Soldering is a skill, so practice if you need to. Keep the soldering iron tip clean using a wet sponge to wipe it. Keep the tip tinned by melting excess solder on it and let it be for a moment. When you wipe the tip on the sponge you should see a clean bright surface. That’s the condition you should have for proper soldering. Do this as needed
Start by mounting the low profile components, such as the 8-pin DIP socket. Although the DIP socket can be mounted either way, its best to mount it with the small notch as shown. That way you are reminded of which way to insert the chip. Solder only one pin first. Then holding the socket with a finger, touch up the soldered pin while gently pushing the socket flush with the board. CAUTION: components and pins get hot! Watch your fingers! Now finish soldering the rest of the pins. (NOTE: you do not have to use the socket for U1. You may want to solder the 555 chip directly in the holes for it. The socket was provided as a convenience if wanted.)
C2 is next. Mount it flush to the board. The leads can be bent to hold the components in place when turning the board over to solder them. Solder one lead first, then, using a finger to hold the component, touch up the solder connection to allow the component to settle against the board. Solder the remaining lead. Remember…things get hot from soldering.
Mount C1. This will be either .1uF for 1kHz, or the 47uF polarized capacitor. If you're mounting this component, note the "-" marking, indicating the lead that goes in the hole next to Pin 1 of the 555 chip. Make sure you mount it correctly.
Mount L1 and L1 vertically, as shown in the layout, by bending one lead around and parallel to the component body. Space the bent lead so it and the other lead enter their assigned holes.
Solder the unbent lead first, touching up the solder point while holding the top of the component and lightly pressing it to seat the body against the board. NOTE: try pressing the component against your work surface while making and/or touching up the connection. Saves fingers from heat! Easy does it!…don't press hard. You might lift the solder pad from the board!
Insert R2 at its location. Use the same technique in mounting this vertical component as in L1 and L2.
Don't cut the leads of R1, the thermistor. Mount it at its location with as much lead length as possible. This is to make it easy to just dip its body in water for calibration. Remember, you can gently bend the leads with needle-nosed pliers if needed to reduce its height above the board.
Install R3. Remember to install the proper value based on the operating voltage you plan to use. The value included in the kit is for 9VDC operation. You can get the other values, if needed, at your local electronics parts store. Their values are listed in the 'Checkout and Test' section. Do not attempt to operate the circuit at 9 to 15VDC with a resistor for 5 to 6VDC. Although the LED can operate safely, why let excessive, un-needed current pass through it? The current you save with the proper value of R3 increases your circuit's operating time…and its good engineering practice.
Install the battery connector if desired. Remember, this connector can also be used with various battery packs which have the mating connector on them. Available in AAA, AA, C and D sizes, they're great for long-term operation. Solder the red lead to the "+" hole and the black lead to the "-" hole.
ANTENNAS:
You must have a 'load' on the transmitter's output. The RF (radio frequency) energy generated by the transmitter must go somewhere. Without this 'load' the energy will have to dissipate at the output stage of the transmitter. Causing this to happen will most likely destroy the transistor in the output circuit (almost instantly!). The 'load' can be an antenna or a 51 ohm resistor across the antenna terminals. The resistor can dissipate the RF energy, and will protect the transmitter's output circuit. The antenna does the same except it radiates the energy (the resistor will radiate, but at very small distances compared to the antenna). The transmitter would like to see about 50 ohms of resistance, so the 51 ohm resistor will work very well.
A simple, effective antenna is a 'V' dipole. Cut two lengths of wire small enough to fit in the holes as shown on the board layout drawing. Each wire will be 6 inches long. Use solid wire, sizes 20 to 26 will do. If you can find someone who can spot weld a solderable tip on some spring steel wire, that will be the best kind of material. The steel wire can be repeatedly folded or compressed and will spring back to its original position without breaking. Solid copper wire will work as long as you don't have to do this.
You might ask…"why a 'V' dipole?" The reason for this is that a dipole, in it's standard configuration as two wires 180 degrees from each other's ends, present about a 72 ohm load (average) to the transmitter when connected at the point where the two wire's ends nearly meet. Arranging the two wires so they are at a 90-degree angle from each other results in a load of nearly 50 ohms. Since the transmitter's output is rated for 50 ohms, this would be the way you should arrange the two wires at the antenna terminals for maximum transmitter efficiency and power transfer.
Some have said the transmitter can withstand high mismatches and still work. Although a single 12" wire can be an effective radiator, it has a high ohmic value (referred to as impedance); and the specifications call for a 50 ohm load at the output of the transmitter supplied. You are welcome to try whatever antenna configuration you wish; but it's believed the 'V' dipole will be a very efficient radiator for its size and makeup.
Remember, if you decide to use gain antennas, effective low-pass filtering should be applied to reduce existing harmonic content present at the transmitter's output.
Congratulations! You're done!
Take a break and come back to admire your work!
Before proceeding to the next section, it is very important that you check your work with a thorough visual inspection. Make sure all solder connections and component orientation and values are correct with no solder shorts. If possible, please have someone else check the board also. This step can avoid a big disappointment if you apply power and damage your CricketSat because of a wrong component or short on the board.
CHECKOUT AND TEST:
Operation:
ALWAYS BE ABSOLUTLY SURE YOU ARE CONNECTING THE BATTERY TEMINALS CORRECTLY…*BEFORE* YOU TOUCH THE CONNECTOR!!
YOU WILL DAMAGE AND MOST LIKELY DESTROY THE CHIP AND TRX!!
If you are concerned about the above…a simple safety measure is to connect a diode in series with the red lead from the battery connector. Solder the end of the diode with the band in the "+" hole. Solder the other lead of the diode to the red wire. You will lose 0.7V, but its a small price to pay for not potentially ruining the circuit components. Any silicon diode will do, even a 1N914, as the current draw is small.
CAUTION: You must have either a resonant antenna or a 51 ohm resistor connected to TRX's antenna terminals before applying power!
The CricketSat was designed to run on 9VDC with the components supplied. It will run on as low as 5VDC (use 6VDC with the above diode), or up to 15VDC. NEVER exceed 15VDC.
CAUTION: When using the CricketSat with the transmitter...NEVER exceed 12VDC, or you will
blow the transmitter!
TIP: For the lightest payload weight, two or three 3VDC lithium flat cells in series work great. In fact, if you're planning to fly the CricketSat in a balloon, lithium cells are the way to go as it gets really cold way up there! Alkaline cells will fail on you. Make sure you insulate the cell pack (heat shrink tubing works nice...DON'T USE BLACK!) so they don't short against something.
IMPORTANT:
At 5 - 6 VDC: change R3 to 330 ohms.
At 12 - 15VDC: Change R3 to 1000 ohms.
Calibration:
The method for temperature calibration is simple. Get any thermometer that can be placed in a glass of water, some ice and some warm water.
Important! The thermistor's leads must not be immersed in the water. Don't cut the leads of the thermistor if possible. They can be bent to lower the height of the thermistor if needed. If you are careful, you can just touch the tip of the thermistor to the water's edge for a reading while holding the CricketSat by the board. Don't allow your fingers to touch the thermistor, or it's leads, to avoid false readings. Use insulated tweezers to hold the thermistor if you decide to extend the leads for a detached mounting.
With the thermometer in the water, record the temperature. Then obtain a count from the CricketSat. For the 2 Hz rate, take a 15-second count at the very minimum. 30-second counts give more resolution, and show the rate of change characteristics of the sensor itself and the circuits. The 1 kHz rate will deliver enough information in far less time. Correspondingly, add ice and/or warm water, noting changes in readings and related counts from the CricketSat. The more readings you take and record the better calibration and reference chart you get for interpreting your data. Take the time to do your best, it's worth it!
Schematic diagram of the CricketSat
Summary:
Use care when assembling the CricketSat. Wear eye protection. Warn others to avoid you while cutting component leads. Work in a ventilated area.
TAKE YOUR TIME!...ENJOY THE ASSEMBLY!...and remember to make a good calibration chart!
ALWAYS SAFETY FIRST! The CricketSat is great fun along with gaining scientific knowledge and experience...but never at the cost hurting, or worse, you, your friends and others.
HEY!...tell us how you used your CricketSat! We'd love to hear from you!
ERRATA SHEET
CricketSat Manual and Assembly Instructions
Reason:
A change to the data line bias, on this board release, is required to improve and correct the modulation on the transmitted signal. NOTE: This is required if you are using a transmitter on your CricketSat.