A great project for “Fox Hunt”

All the parts, PC board and 3 button cells $8.75 You will also need an FM radio to pick up the “beeps.”
See also: Mini Tracker Tracking Transmitter

This bug can be attached to anything from a glider to a pot plant and you can track it with a radio.
Why track a pot plant?
The Beeper is the result of many requests for a mini tracking device and introduces a new world of tracking.
The circuit is very compact and consumes very little 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 the two switch contacts to keep them apart and connected to a length of cotton thread that is fixed to the floor. When the object is moved, the cotton thread pulls the plastic out of the switch and the bug is turned ON. 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 path an maybe turn up a few surprises!
Why a pot plant? Because one of our customers wanted to track the person taking her pot-plants from her front perch. The result? It was one of her neighbours!

HOW THE CIRCUIT WORKS

The circuit consists of three blocks. The first is the Phase-Shift Oscillator. It produces a sinewave and is turned on for a very short period of time by a two-transistor pulse generator made up of the two lower transistors.
At the same time, the 88MHz carrier oscillator is turned on and the tone is superimposed on the carrier to get a brief BEEP.
The two stages are coupled via a 22n capacitor. The frequency of the RF oscillator is determined by a number of things but mainly the value of the capacitor and inductor in the parallel tuned circuit.
The final building block is the pulse circuit made up of a feedback amplifier using a BC 557 and BC 547.
The circuit starts its cycle by charging the 10u via the 330k resistor. If you can’t see how this occurs, remember a capacitor and resistor in series form a DELAY CIRCUIT and it does not matter if the resistor is the top component or the lower component.
As the capacitor charges, it brings the base of the BC 557 towards the negative (0v) rail and this turns the transistor ON.
This effect is passed directly to the BC 547 via the 100n (and 1k resistor) and the BC 547 turns ON.
The voltage on the collector of the BC 547 falls very quickly and brings the 10u down with it.
The small voltage across the 10u forces the base of the BC 557 lower and turns the transistor ON even HARDER.
Both transistors become turned on fully and the energy in the 10u feeds the BC 557 until it is used up.
The 330k can no longer keep the BC 557 turned on and it turns off slightly. This effect is passed to the BC 547 where the collector voltage rises slightly and the rise is transferred directly to the BC 557 via the 10u and the circuit starts to move in the other direction.
Both transistors become fully turned off and the 10u begins to charge again via the 330k.
The circuit takes almost no current between beeps and the duty-cycle of the tone is about 10%. This makes the circuit very economical on batteries and you should get many hours of operation from a set of cells.

The block diagram above shows the 3 building blocks for the Beeper Bug. The widely differing frequencies produced by each of the blocks makes it difficult to show a combined output waveform. The diagram below shows how the Pulse generator block turns on the Phase Shift Oscillator and 108MHz Oscillator sections to produce short beeps.

Building blocks (a) and (b) combine to create the signal (the beep) shown in the diagram as amplitude lines. Signal (a) modulates signal (b) to produce a tone of approx 1kHz on the 108MHz carrier. Building block (c) turns the beep on and off. All this is shown on the diagram above but due to the enormous difference in the frequencies of the three waveforms, the figure above is not to scale.

The diagram above shows three things:

1. When the pulse generator is off, all the blocks are off and the circuit consumes almost NO current.
   When the pulse generator turns on (it turns on for a very short period of time) the voltage across it is very small and it effectively puts the top two building blocks across the power rails. This is how they are turned on.
2. The phase shift oscillator produces about 10 cycles during this period of time.
3. For each cycle of the phase-shift oscillator, the 108MHz oscillator produces about 100,000 cycles!
   During the production of the 100,000 cycles, the waveform from the phase-shift oscillator causes the 108MHz oscillator to increase frequency very slightly then decrease frequency slightly so that the end result is a tone from the receiving radio.
   When the circuit turns off, the radio picks up background “hiss” and so the result is a beep with background hiss between the beeps.

THE PULSE GENERATOR

The animation below shows another way of “seeing” the pulse generator in operation. Study the animation carefully and you will see the bar-graphs indicating the voltage on the collector of the output transistor.
When the circuit is NOT TURNED ON, the voltage on the collector is HIGH. When the circuit is TURNED ON, the voltage on the collector of the output transistor is LOW.
This is normal with all circuits like this. The voltage on the output of the circuit must be low so that most of the voltage will be across the LOAD. In this case the LOAD is the two oscillator blocks and when the pulse generator is TURNED ON, the two oscillators will see RAIL VOLTAGE.

The second diagram in the animations above shows two things:

1. The voltage across it is VERY SMALL when it is turned ON.
2. The circuit changes from OFF to ON (and back again) VERY QUICKLY. In the next page of the course you will learn about circuits that turn on and off very quickly. They are very important in electronics - they are the main building block for COMPUTERS!

CONSTRUCTION

All the parts fit on to a small PC board and by using miniature components, the size is kept to a minimum. Keep this in mind when collecting the parts. If you use old-style components, they will not fit. The safest way is to buy a kit.
All the parts must be kept close to the board when soldering so that everything is kept compact. All the resistors stand-up on the board and the easiest way to carry out construction is to start at one end of the board and fit each part as you come to it.
The overlay makes it easy to see where everything goes and the only three things you have to be careful with are: 1.The BC 557 transistor (don’t confuse it with the BC 547’s) and 2. The polarity of the electro’s.

The Beeper Bug PCB

3. Don’t forget to scrape the ends of the coil with a sharp blade to remove the enamel before tinning.
   The on-off switch can be mounded on top of the board with short lengths of wire or fitted directly. The supply comes from three button cells and these are soldered together very quickly to prevent them getting too hot.
   Finally, a length of antenna wire is soldered to the last remaining hole on the board and the project is ready for testing.

PARTS LIST

• 1 - 330R
• 1 - 1k
• 1 - 10k
• 2 - 22k
• 1 - 47k
• 1 - 330k
• 1 - 1M
• 1 - 10p ceramic
• 1 - 27p ceramic
• 1 - 1n ceramic
• 3 - 4n7 ceramics
• 2 - 22n ceramics
• 1 - 100n monoblock
• 1 - 10u 16v PC mount electrolytic
• 1 - 22u 16v PC mount electrolytic
• 3 - BC 547 transistors
• 1 - BC 557 transistor
• 1 - 6 turn 3mm dia coil enamelled wire
• 1 - SPDT mini slide switch
• 3 - button cells
• 1 - 1.7 metre wire for antenna
• 1 - BEEPER BUG PC BOARD

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 beeping.
You will not be able to detect the average current as the duty cycle is too short, but you can see it is very small by the slight movement of the needle.
Next test the current consumption and the frequency of operation. Remove the 10u and retest the current. It should be about 5-8mA and this shows everything is operating.
Switch the unit on and place the antenna as it will appear on the glider or pot plant etc.
Tune an FM radio to 108MHz and expand the turns of the coil until the tone is detected. Keep the radio away from the transmitter to prevent picking up harmonics (side tones). Replace the 10u and the project is ready for installation.

IF IT DOESN’T WORK

If the Beeper doesn’t work, you will have to determine which blocks are not functioning.
If the circuit gives out a constant tone, the pulse section will be faulty. If a clicking sound is emitted, the tone oscillator will be faulty. If no signal is picked up at all, the RF oscillator section may be faulty.
Firstly check the current consumption. If the needle “jumps” but no RF is detected, the RF may be off the FM band.
If RF is detected, you should check the value of components around the oscillator section and the number of turns on the coil.
The spacing of the turns and the diameter of the coil is also critical. The kit comes with a pre-wound coil (6 turns @ 3mm dia) and the holes on the board give you an idea of the spacing of the turns.
If you have made the coil yourself, check the dimensions carefully, they are most critical. Removing the 10u will cause the tone to be emitted continually and if a carrier is detected (blank spot on the dial) but no tone, you should check the voltage on the collector of the tone stage. It should be about 3v. A CRO will be the easiest way to see the output however an audio amplifier will be just as good.
Take the emitter of the transistor directly to the negative rail (to remove the pulse section) and check the soldering on the phase-shift components (4n7’s and 22k’s) to make sure a dry joint is not present.
If you are still having trouble, buy another kit and build just the tone section (don’t forget to take the emitter to negative). Alternatively, build the circuit on bread-board and detect the tone with an audio amplifier.
The amplifier can also be used to make sure the tone is passing through the 22n stage-coupling capacitor, to the RF stage.
As we mentioned above, if RF is not detected, the fault will lie in the oscillator stage.
The components making up this stage are the 47k, 1n, 27p, 6 turn coil, BC 547, 10p, 330R and 22n across the battery.
The capacitance of the circuit will have an effect on the frequency as will the battery condition, the earthing effect of your hand, the length of the leads of the battery (and switch) and the length of the antenna.
Once you get the circuit working you can change things around but keep everything compact at the start.
Finally we come to the pulse circuit. It is simply a high-gain DC coupled amplifier. By removing the 10u, the circuit will turn on. The voltage between the collector and emitter will be small (about 0.35v to 0.5v). If not, short between collector-emitter of the BC 557. If the circuit turns on, the BC 557 is faulty. If not, the BC 547 is faulty.
Shorting across the 10u with a jumper will turn the circuit off and the voltage between collector and emitter of the BC 547 will rise to about rail voltage.
If the circuit doesn’t work when the 10u is fitted, the problem may be a leaky electro. Try another and experiment with different values. Everything has now been checked and the circuit should be working perfectly.

OPTIONS

The kit comes with 3 button cells but you can use 2 lithium cells for a 6v supply. The output will be increased considerably and this will give a longer range. The circuit has been designed to operate at the high end of the band (108MHz) and if required to operate at 88MHz, the 27p tuning capacitor should be changed to 47p.

USING THE BEEPER BUG

The Beeper Bug can be used for many applications and its range will depend on the effectiveness of the antenna.
Obviously a long straight antenna will be the best but you can’t always get what you want when you are trying to hide things. A long fine wire will be better than a short thick one and if you want to hide the Beeper Bug in a pot plant, you can drape the wire over the branches like a thread.
If it is to be used in a glider, the main struts can be the antenna. If it is to be hidden in a box of tissues, the antenna can be taped to the inside of the box.
To get the best range, you must experiment before placing the bug on location and when it’s working perfectly, I’m sure you will be pleased with its performance.