LED RF Power Meter

Friends, following picture is of a Simplest radio frequency(RF) indicator and meter that i have got yet.



PURPOSE OF CIRCUIT:

Circuit is used to indicate whether or not the radio frequency Tx circuit in your hand oscillates.
If LED on the board is bright means your circuit is oscillating means you ROCK.


Need for the circuit:

Anybody who had messed with rf circuits will readily agree with me that whenever a rf circuit is turned on, the very first shout made by mind "is oscillator part of it working?"
(and next will be a prayer " O'God may this INDUCTOR be blessed not cursed!" , at least in my case :-)

Let suppose you are working on FM- Tx or AM-Tx ,they will always contain a sound preamplifier and oscillator section. preamplifier section may be tested with headphones or Power amplifier(like PC speakers) but how to test the oscillator section?
You may think of Radio receiver.
But whats your transmitting frequency ? what if you have low power Tx or frequency much below or above than radio could receive ? Tuning each band in search of your transmision will really be cumbersome,specially when you even dont know that whether transmission has begun or not?
You may readily be disappointed and think "sala inductor kharab hai".

Here comes usage of above circuit. No matter what frequency it is, if your oscillator part is working LED will glow.
This , at least, makes you optimistic about your circuit that yes it working, now i have just to vary coil spacing or value of tuning capacitor to get desired frequency.



Working of circuit:


Basically circuit is designed to charge 100nF capacitor .
Circuit starts working when 5cm lead is attached to RF emitting source(like antenna,collector of oscillating transistor etc).

Now whenever positive cycle comes it creates a push in electrons present in wire which is transferred to capacitor through upper diode & for negative cycle a pull is produced which again makes electrons to flow in same direction thus charging 100nf capacitor.
LED is exactly connected to capacitor as if it is a charged battery.

100pf capacitor is used to create dc isolation.

Circuit Modification:


When i finalized circuit and tested my FM-Tx, LED was glowed very weakly.
When i removed 470 ohm resistor,Led strength increased,but not satisfactory.

Accidentally when i touched cathode of LED, it became magically bright because it got better ground through my body.
Be careful brighter the led will draw more power from your transmitting circuit.


Moreover:


The circuit can work as strength meter if put your digital multimeter's test leads at terminals of capacitor.
That is multimeter will indicate voltage built across capacitor. higher voltage greater strength of you transmitter.