• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

>>>Dose this circuit realy work?

Not open for further replies.

Someone Electro

New Member
Dose this circuit realy work?

Its sepose to transmit a FM modulated signal.

Its just looks to simple for thansiting FM (FM transmiters are complicated)

Ive sean the shematics for a 1 kW FM transmiter :shock: (Are you going to send i signal to the moon?!!!)

Its perfect to test transmiters in the basement (you dont have to seak a free freqency)



Active Member
It should work. The output power will be too low and you won't get range more than a few tens of feets.

panic mode

Well-Known Member
It's perfectly fine. Just don't forget to bring audio signal through capacitor to the base of transistor. You can also reduce 1k resistor in emiter circuit for 330 or even 220 Ohm to get better range. If you cannot find variable capacitor in 4-40pF range, just put fixed capacitor of ca. 15pF (12-18pF will do). This capacitor should have good thermal stability to reduce drift. To ensure that frequency doesn't interfere with local station(s), just compress or stretch the coil little bit (change the distance between first and last turn of coil). Components are not critical, you can change any of them (in reasonable range). To test oscillator, power the circuit and put it close to FM receiver. Slowly change receivers frequency untill you find spot with perfect silence (no noise). Lift oscillator circuit ca 1/2" or 1cm above table surface and let it drop on table. You should hear loud click form the receiver. (vibrations of the coil upon impact changed frequency of the circuit - thats FM). When you bring audio signal to input of the oscillator circuit, transistor section Base-Collector works like Varicap diode. Audio signal changes potential of the base and this change affects capacity of Base/Collector section of the circuit. This (parasitic) capacitance is part of oscillator circuit (thanks to 0.01uF capacitor between transistor base and top of the oscillator coil). Changing either capacitance or inductance of the oscillator circuit while oscillator is running is changing carrier frequency of the oscillator. Changing frequency of the oscillator is called FM. Simple, isn't it...

Someone Electro

New Member
It didnt work! :cry:

The power suply is a batery(its a litle run down but it givs good power)

The input is the output from my computers soundcard

Here is the ranking what i think ti cod go wrong(1st i belive most its wrong)
1st place:
I think its the coil!
it seas 1/4 of a inch (i use cm)
I made it 0.9 cm (i used a a big pencil). Is it rignt?

in 2nd place:

I used ceramic(those that look like orange thin pils) capacitators

Did i get the capacitator codes right?
0.01 uF = 103
4-40 pF = 30
4.7 pF = 4

in 3rd place the transistor:

Im shure which is the B,C,E(on the PCB that i gothit of was marced whitc is the B,C or E)
I used:
i C19
i 012
i H

Last place:
A lose conection


Plese dont tell me that my transistor is a PNP!Or i will cry :cry: :cry: !


Active Member
Have you checked the free-running oscillator frequency on CRO? May be the component values you have chosen is giving frequency which is out of range of your FM tuner. Thats why you are not able to get any reception.

Someone Electro

New Member
Oops! :oops:

The transistor is an PNP !

I got a NPN :

I tested it (wean i delivrd positive in the base the light got on)

Meaby amyone got a datasheat for it?(i want to know how much mA can it handle)

How cod i put a PNP in ?!!!!!!!!!!! :evil: :evil: :evil: :evil: :evil:

Someone Electro

New Member
I it stil didnt work

I made the coil 8 thurns oround a 9 mm paper circle (the wire is thin and it just dint stay in the coiy shape).I thik it shod be orund a 7 mm circle

Im not so shure abaot the capacitators!(the codes for the smal vaues)

Ithik the freqency is of the FM band this time!
Not open for further replies.

EE World Online Articles