Mini Tracker

Description: A grasshopper is a switch that can activate at any moment. A plastic piece is positioned between two switch contacts to keep them apart, connected to a cotton thread fixed to the floor. When the object is moved, the cotton pulls the plastic from the switch, turning it ON. This type of detector is ideal for tracking anything that may go missing. By attaching it to the monitored product, its removal can be traced, potentially revealing unexpected findings. The Mini Tracker transmits a brief burst of carrier signal, creating a blank spot on the FM dial, commonly referred to as silence. Typically, a radio tuned to a frequency between stations picks up background noise, known as snow. The transition from silence to snow generates a click or beep, which is the sound produced by this project. No actual beep tone is generated; instead, there is a change in signal quality. The carrier signal (or silence) is emitted approximately twice per second. The circuit consists of two primary components, both functioning as oscillators. The first operates at a very low frequency (around 2Hz), while the second operates at approximately 90MHz. The first component is a square-wave oscillator with a very brief ON time, whereas the second is a sine-wave oscillator. Their only commonality is a feedback component that facilitates oscillation.

A critical aspect of the design is that the first oscillator is separated from the battery by a 1k resistor. This separation is essential because the Pulse Generator draws a significantly high current when active. While this high current is relative, as the entire circuit consumes minimal average current due to its brief active periods, it is crucial to monitor the current during these bursts to ensure it remains as low as possible. If the 1k resistor were to be removed, the Pulse Generator circuit would place a 220-ohm load across the battery, resulting in excessive battery drain while the RF oscillator is operational. This would lead to wasted power and a decrease in rail voltage at critical moments. To mitigate this issue, the Pulse Generator circuit is isolated from the RF Oscillator by the 1k resistor. The 22u capacitor charges through this 1k resistor, and when the Pulse Generator requires high current, the 22u capacitor supplies the necessary energy. This design principle is referred to as STAGE SEPARATION or BLOCK ISOLATION, which effectively isolates the power demands of one circuit block from the rest of the system.

The 1k resistor charges slowly relative to the circuit's operation, while the Pulse Circuit briefly draws a high amount of energy from the 22u capacitor. This results in a slight voltage drop across the capacitor, which then recharges to maximum voltage once the Pulse circuit deactivates. The schematic illustrates only the Pulse Generator circuit without extraneous components. The output from the Pulse Generator remains LOW for short bursts, activating the RF Oscillator. Conversely, when the Pulse Generator output is HIGH, the RF oscillator is OFF. A crucial feature of the time delay circuit is the "pick-off" or "detection" point, designated as point "A" in the diagram. This point is monitored by the remainder of the circuit, specifically the base of the PNP transistor. When the voltage at this point reaches a specific threshold, the circuit transitions states. Initially, the voltage at point "A" is HIGH; when it falls approximately 0.75V below the rail voltage, the BC 557 transistor activates. As a PNP transistor, the BC 557 functions as a mirror to an NPN transistor, necessitating that the base voltage be lower than the rail voltage for activation. As the 10u capacitor charges, the voltage at point "A" decreases. Although not typically advisable, the 10u capacitor charges in reverse, which can lead to operational challenges.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. A bug like this would make an ideal detector to track down a nything 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. 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 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. The 22u charges via the 1k resistor and when the Pulse Generator circuit requires its "high current, " the 22u delivers the energy.

This feature is called STAGE SEPARATION or BLOCK ISOLATION and separates the power requirements of a "building block" from the rest of the circuit. The 1k slowly charges (in relative terms) the 22u and the Pulse Circuit draws a high amount of energy from the 22u for a very short period of time.

The voltage across the 22u drops slightly and when the Pulse circuit switches "off, " the 22u charges to maximum voltage. In the diagram below, we have removed all unnecessary components and shown only the Pulse Generator circuit.

The output of the Pulse Generator is LOW for short bursts and this turns the RF Oscillator ON. When the output of the pulse Generator is HIGH, the RF oscillator is OFF. The important point on the time delay circuit is the "pick-off" point or "detection" point - shown in the diagram above as point "A. ". This point is being monitored by therest of the circuit (the base of the PNP transistor) and when a particular voltage is reached, the circuit changes state.

In our case, the voltage at point "A" is initially HIGH and when the voltage falls about 0. 75v below rail voltage, the BC 557 transistor turns ON. The BC 557 is a PNP transistor and it operates as a "mirror" to an NPN transistor, that`s why the voltage on the base must be lower than rail voltage for it to turn ON. As the 10u charges, the voltage at point "A" reduces. The 10u is actually charging in reverse and although this is not advisable, an electrolytic can be changed slightly in rev