Simple RF Sensor without Battery Circuit

This article shows you how to build a simple tester to see if a radio transmitter is working without needing any batteries.

The tester itself gets its power from the radio waves the transmitter is sending out and lights up an LED to show it is working.

What is a RF Sensor without Battery:

A wireless sensor that detects radio frequency signals without the need of a traditional battery or other external power source is known as an RF sensor without a battery.

Instead, energy harvesting techniques are often used by these sensors, enabling them to draw the necessary power from RF signals present in their environment.

The application of these sensors can be advantageous in a variety of fields, including industrial automation, environmental monitoring, and the Internet of Things.

Circuit Working:

Parts List:

Below mentioned are the details about RF Sensor without Battery Circuit working:

The circuit utilizes a simple length of wire or a pull up alternative antenna as the receiving antenna.

This antenna captures the RF signals emitted by the transmitter under test.

When the RF transmitter is switched on, it emits radio frequency signals.

The receiving antenna captures these RF signals and induces a small voltage across its terminals.

The induced voltage is then fed to a diode, typically a germanium diode, which rectifies the alternating current AC RF signal, converting it into a direct current DC signal.

This rectification process is crucial for extracting a usable DC voltage from the RF signal.

The rectified DC signal charges a capacitor, storing the accumulated charge.

This capacitor acts as a temporary energy storage device.

The stored energy in the capacitor is used to power the LED indicator LED1.

When the voltage across the capacitor reaches a threshold, it triggers the LED to illuminate.

The LED used in the circuit must be a high brightness type LED.

The choice of this specific LED is crucial for efficient illumination driven by the relatively low voltage derived from the RF signal.

To test an RF transmitter the antenna of the circuit is brought into close proximity or touching distance, of the RF transmitters antenna.

This ensures optimal reception of the RF signals.

If the RF transmitter is emitting RF signals the induced voltage from these signals is sufficient to charge the capacitor and trigger the LED LED1 to turn on.

The illuminated LED serves as a visual indicator of the successful detection of RF signals from the transmitter under test.

Formula:

Principle: Rectification of Diodes

The circuit is essentially used as a simple diode rectifier, this is the idea that matters:

A diode stops current flow in one direction while permitting it to flow in the other with little resistance.

The diode equation provides a general understanding of its behavior:

I_d = I_s (exp(V_d / (n * k_B * T)) – 1)

here,

• I_d: Diode current
• I_s: Diode saturation current (a small leakage current)
• V_d: Voltage across the diode
• n: Diode ideality factor (typically between 1 and 2)
• k_B: Boltzmanns constant
• T: Temperature (kelvin)

How the formula works:

The feeble AC voltage coming from the antenna is mostly rectified by the diode D1.

The diode permits current to pass during the positive half cycle of the AC waveform, charging the capacitor C1.

Current flow is inhibited by the diode during the negative half cycle.

By reducing the pulsing DC voltage from the rectified AC waveform, the capacitor C1 serves to create a more stable DC supply for any possible future circuitry.

Note:

The intricate link between voltage across a diode and current flow is represented by this formula.

Though it serves as a basis for comprehending diode behavior, it is not directly utilized for design in this straightforward circuit.

How to Build:

LED Selection:

• Choose a high brightness type LED LED1 for efficient illumination driven by the detected RF signal.

Antenna Setup:

• Prepare a simple length of wire or utilize a pull up alternative antenna sourced from an old TV or CB equipment.
• This will serve as the antenna for the circuit.

Circuit Housing:

• Construct the circuit inside a compact metal or plastic box.
• Ensure the chosen material provides adequate protection and containment for the components.

Circuit Connection:

• Connect the LED1 to the circuit in such a way that its illumination is directly driven by the detected RF signal.
• The absence of an external power source emphasizes the circuits unique design.

Testing Procedure:

• If you have an amateur radio or CB transmitter that requires testing, switch it on.
• Position the antenna of this circuit in close proximity to the RF transmitters antenna ensuring a touching distance.
• Monitor the LED1.
• If the RF transmitter is emitting RF signals, you will observe the LED1 turning on, indicating successful detection.

Conclusion:

To conclude, the circuit harnesses the energy from the detected RF signals to power an LED providing a simple and battery free method for testing RF transmitters.

The combination of a minimalistic design diode rectification, and energy storage in a capacitor makes this circuit an ingenious solution for hobbyists and enthusiasts seeking a simple RF transmitter tester.

References:

Schematic of a battery-less selfpowered RF transmitter.