Simple RF Sensor without Battery Circuit

This article teaches how to construct a simple battery free tester for checking whether a radio transmitter is functional.

The transmitters radio waves provide electricity to the tester which flashes an LED to indicate if it is working.

What is a RF Sensor without Battery:

An RF sensor without a battery is a wireless sensor that detects radio frequency signals without using a conventional battery or another external power source.

However these sensors usually use energy harvesting techniques which allow them to gather the required power from RF signals found in their surroundings.

These sensors are useful in a number of applications such as the Internet like things, industrial automation and environmental monitoring.

Circuit Working:

Parts List:

The following information refers to the workings of an RF sensor circuit without a battery:

The circuits receiving antenna is either a pull up alternative antenna or a basic piece of wire.

The transmitter being tested generates RF signals which are picked up by this antenna.

Radio frequency signals are released by the RF transmitter when it is turned on.

These radio frequency waves are picked up by the receiving antenna which then creates a little voltage between its terminals.

A diode usually a germanium diode receives the induced voltage and changes the alternating current AC radio frequency signal turning it into a direct current DC signal.

The process of taking of a useful DC voltage from the RF signal depends on this rectification process.

The collected charge is stored in a capacitor that is charged by the rectified DC signal.
The purpose of this capacitor is to temporarily store energy.

The LED indication LED1 is powered by the energy stored in the capacitor.

The LED turns on when the voltage across the capacitor hits a certain threshold.

An LED of the high brightness variety should be used in this circuit.

For effective lighting powered by the relatively low voltage obtained from the RF transmission this type of LED design is essential.

To test an RF transmitter the circuits antenna is brought within touching distance of the RF transmitters antenna.

This allows the best possible RF signal transmission.

If RF signals are coming from the RF transmitter the voltage generated from those signals is enough to charge the capacitor and turn on the LED1.

The successful detection of radio frequency signals from the tested transmitter is indicated clearly by the lighted LED.

Formula:

The below mentioned formulas are for Simple RF Sensor without Battery Circuit:

Principle: Rectification of Diodes

The main point is that the circuit works well as a simple diode rectifier:

While allowing current to flow in one direction with little resistance a diode prevents it from flowing in the other.

A general idea of its behavior is gathered from the diode equation:

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

here,

• I_d is the diode current
• I_s is the diode saturation current for a small leakage current
• V_d is the voltage across the diode
• n is the diode ideality factor which is between 1 and 2
• k_B is the boltzmanns constant
• T is the temperature in kelvin

The formulas functioning:

The diode D1 mainly rectifies the weak AC voltage coming from the antenna.

The capacitor C1 is charged when the diode allows current to flow through it during the positive half cycle of the AC waveform.

During the negative half cycle the diode stops current flow.

The capacitor C1 helps to generate a more stable DC supply for upcoming circuits by reducing the rectified AC waveforms pulsating DC voltage.

Take note:

This formula shows the difficult relationship between current flow and voltage across a diode.

This simple circuit does not directly use it for design but it provides a basis for understanding diode behavior.

How to Build:

To build a Simple RF Sensor without Battery Circuit follow the below mentioned connection steps:

Choosing an LED:

• For effective illumination powered by the detected RF signal use an LED with a high brightness.

Tuning the Antenna:

• A pull up alternative antenna from an old TV or CB device can be used or a simple piece of wire can be easily made.
• This will act as the circuits antenna.

Housing for Circuits:

• Build the circuit within a small box made of plastic or metal.
• Make sure the material you choose provides the components adequate restriction and protection.

Connection to the Circuit:

• Connect the LED1 to the circuit such that the detected RF signal controls its lighting.
• The circuits unique design makes clear by the lack of an external power supply.

Method of Testing:

• Turn on your CB transmitter or amateur radio if it has to be tested.
• Make sure the circuits antenna is touching the RF transmitters antenna by placing it near to it.
• Keep an eye on LED1.
• After successful detection one can see that LED1 turns on if the RF transmitter is sending out RF signals.

Conclusion:

The circuit provides a simple and battery free way to test RF transmitters by using the energy from the detected RF signals to power an LED.

This circuit is a clever answer for hobbyists and beginners looking for an easy RF transmitter tester since it combines energy storage in a capacitor with a basic diode rectification construction.

References:

Schematic of a battery-less selfpowered RF transmitter.