01-09-2012, 03:07 PM
MINI TRACKER
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MINI TRACKER CIRCUIT
Mini Tracker is the result of many requests for a tracking device and brings a new world of tracking to your fingertips.
The circuit is very compact and consumes almost no power. It is small enough to be hidden in anything you suspect will be lost or stolen.
By using a mercury switch or grasshopper, the bug can be “primed" for the time when it is moved and you can track it with an FM radio.
A grasshopper is a switch that is ready to go off at any time. A piece of plastic is placed between two switch contacts to keep them apart and connected to cotton thread fixed to the floor. When the object is moved, the cotton pulls the plastic out of the switch and the bug is turned ON.
One caller suffered the constant theft of pot plants from his verandah and wanted a bug to track the thief.
Another had chemicals stolen from his warehouse and the cleaner was the prime suspect. But to prove it is always difficult.
Something small enough to be placed in a case of detergent bottles was needed and that's where we came into the picture.
The most notable theft occurred from a videotaping factory where packs of tapes were leaving by the back door, some time during the night shift.
A bug like this would make an ideal detector to track down anything going astray. By attaching it to the product under surveillance, you can follow its removal and maybe turn up quite a few surprises.
The Mini Tracker transmits a very short burst of carrier which produces a blank spot on the FM dial - commonly called “silence.” Normally, a lot of background noise called “snow” is picked up by a radio when it is tuned to a frequency between the stations. The change between silence and snow produces a “click” or “beep” and this is the noise produced by the project. No actual “beep-tone” is produced - just a change in signal quality. The carrier (or silence) is emitted about twice a second.
HOW THE CIRCUIT WORKS
The circuit consists of two building blocks - both are oscillators. The first operates at a very low rate (low frequency - about 2Hz) and the other operates at approx 90MHz. The first is a square-wave oscillator with a very short “ON time,” while the other is a sine-wave. The only thing they have in common is a “feedback component,” to create and maintain oscillation. In all other respects they are different.
The first block is a 2-transistor PULSE GENERATOR and the second is an RF OSCILLATOR.
The first point we need to cover is the fact that the first "building block" is separated from the battery via a 1k resistor. This is very important as the Pulse Generator takes a very high current when it is "active." We say "high current" in relative terms as the whole circuit takes very little average current as it is active for very short bursts. But the current is high during the short bursts of operation and we need to check the current taken during the bursts, to make sure it is as low as possible.
If the 1k resistor is removed, the Pulse Generator circuit would place a 220R across the battery and this would put a heavy load on the battery during the time when the RF oscillator is operating.
This would represent "wasted power" and decrease rail voltage during the time when we need maximum output. To prevent this, we have separated the Pulse Generator circuit from the RF Oscillator with a 1k resistor.
To give the Pulse Generator its own separate "Power Supply" we have added a 22u electrolytic.
The diagram below shows the separate "Power Supply" for the Pulse Generator.
THE PULSE GENERATOR CIRCUIT IN OPERATION
The operation of the Pulse Generator circuit is more complex than first meets the eye. I don't expect you to understand its operation via this brief discussion. It will be covered in more detail in the BEC Course. It is sufficient to see the charging of the electrolytic controls the timing of the circuit and the electrolytic appears to jump from above the BC 557 to below it, as it turns the transistor on and off.
If you can "see" this effect in operation, you are on your way to understanding how the circuit works.
When the output of the Pulse Generator is LOW, the RF oscillator produces a carrier and this is picked up on an FM radio as "silence."
When the RF oscillator is not operating, the radio picks up background noise, commonly called "white noise."
The difference between these two creates the "beeps." The circuit does not actually produce a "beep."
The circuit takes almost no current between beeps and the duty cycle of the “beep” is only a few percent.
This makes the project very economical on batteries and you should get many hours of operation from two or three button cells.
The RF oscillator is turned ON when the Pulse Generator circuit is LOW and the 330R emitter resistor is effectively connected to the negative rail, via the collector-emitter junction of the Pulse Generator. The RF transistor is also turned on via the 47k base bias resistor and current flows though the collector circuit consisting of a 5 turn coil and 39p.
The capacitor begins to charge as the coil presents a blockage to the voltage at this early stage of the cycle.
As the capacitor charges, the voltage across it is detected by the coil. Gradually the coil will allow current to flow through the windings and produce magnetic flux. This all happens in less than a microsecond, however there are specific times for each of the operations and this determines the frequency of the circuit. The voltage on the collector of the transistor changes and this alteration is passed to the emitter via a 10p capacitor. The voltage on the emitter is modified and the transistor is tuned on. There are two ways of turning a transistor on and off. One is to raise and lower the voltage on the base while keeping the emitter fixed. The other is to raise and lower the voltage on the emitter, while keeping the base fixed.
The base is effectively kept rigid by the presence of the 1n capacitor and the sinewave signal produced by the parallel oscillatory circuit, turns the transistor on and off.
There is a lot more that could be discussed about the operation of the circuit but it becomes too technical at the moment.
OPTIONS
Button cells can be fitted into a micro battery box made from pieces of blank PC board. 3 cells will produce a voltage of about 3.6 - 4.5v, depending on the type of cell. You can also use lithium cells, which are 3v each. By using 2 cells (6v) the output will increase by more than 300% and this will give considerably better range. If you intend to use lithium cells, a holder will be needed.
The circuit has been designed to operate at 90MHz, and if required to operate at the high end of the band (108MHz), the 39p could be changed to 33p. Our tests showed the output at this frequency to be less than at 90MHz.
The length of the aerial can be cut to meet your requirements, and the actual length is not very critical.
TESTING
With the power switched off, connect a multimeter across the switch terminals and you will see the needle jump very briefly to indicate the circuit is operating.
You will not be able to work out the average current as the duty cycle is too short, but you can see it is microscopic by the minûte movement of the needle.
Next, test the current consumption and the frequency of operation. Connect a short length of tinned copper wire between the collector and emitter terminals of the BC 547 in the pulse circuit.
This will turn on the RF stage fully and you will be able to get a reading of about 5-8mA. This shows everything is operating.
While the link is still in place, switch the unit ON. Set up the antenna as it will be on the glider or pot plant etc.
Tune an FM radio between 89MHz and 108MHz and expand the turns of the coil to raise the frequency, until the beep is detected. Keep the radio away from the transmitter to prevent picking up harmonics (side-tones). Remove the link and the project is ready for installation.
IF IT DOESN'T WORK
If the “beep” doesn't work, you will have to determine which block is not functioning by comparing with the following symptoms:
If the circuit gives out a constant blank spot on the radio, the pulse generator section will be faulty. If no signal is picked up at all, the RF section may be faulty.
Firstly check the current consumption. If the needle “jumps” but no RF is detected, the oscillator may be off the FM band. Separating the turns of the coil will increase the frequency and maybe bring it onto the band.
You can use the Field Strength Meter or Peaker (LED Power Meter) to find out if RF is being emitted. These two projects have been designed by Talking Electronics and can be found in our price-list of kits.
If RF is detected, you should check the value of components around the RF section and the number of turns on the coil.
The spacing of the turns is also critical, as is the diameter of the coil. The kit comes with a pre-wound coil (5 turns @ 3mm dia) and the holes on the board give some idea of the spacing of the turns.