What does the project present?
The DIY project presented is a mini FM transmitter, easy to build. Any standard receiver with FM band can be used as a receiver. The assembly is ideal for surveillance of a room, surveillance of children, having a emission radius up to 400m.
Electronic diagram of the miniature FM transmitter
Constructive details
Coil L1 has 8-10 turns, from reuses made of similar old coils, wound on an insulating core with the approximate diameter of an ordinary pencil. C4 is a 4-40pF semi-adjustable capacitor.
Adjust the receiver (radio) to a "clean" place at the bottom of the band. With a plastic screwdriver adjust C4 for clearer reception.
Experiment with patience! Most components do not have critical values, so you can change them to see the effect!
The capacitors are of ceramic type, preferably of class 1%, or equivalent. Any capacitors can be used except electrolytic or tantalum capacitors.
If you want to use this outdoor installation, it is recommended to choose capacitors that are less sensitive to temperature changes.
List of required components
R1, R4, R6 = 10 KOhm
R2 = 1 MOhm
R3 = 100 KOhm
R5 = 100 Ohm
R7 = 1K
C1, C2, C3 = 0.1 uF
C4 (semi-adjustable capacitor) = 4 - 40 pF
C5 = 4.7 pF
Q1, Q2 = 2N3904
L1 = 0.1 µH
1 x Electret Microphone
1 x Copper wire antenna = 20-25 cm
1 x 9V battery
Bibliography:
https://cidigg.blogspot.com/
Dominte Claudiu (Romanian translation)
I am not convinced that it works without a 20-30 pF capacitor in parallel on the 1k resistor from Q2.
Possible to work with some transistors and not with others.
About what frequency zone does it emit?
The assembly emits between 88-108 MHz.
Be careful that it has many harmonics and in this range it bothers many receivers.
Before '89 I woke up with the militia at the door because of toys of this kind. I built them on the EST strip and sold them. My father chose with a warning, I with the rest, the belt, Gods and saints 🙂
From what I understand, circuits like this rely on the Early effect to generate a signal that contains * and * the FM component, but in a minority proportion.
Then the selectivity of the antenna and later of the receiver intervenes. The problem is that it is not an energy efficient modulator that probably pollutes electromagnetically.
Many nonlinear circuits are capable of outputting the FM component somewhere.
Before '89 these things were forbidden but everyone used them. You were caught because of a so-called friend, it was awful.
My luck was that I was a minor. After the Revolution, some of these Chinese things appeared on the market, in the shape of a small flashlight, but they disappeared from the market instantly. Now we call on Alliexpres.
I built it in the high school years at about 17 years old and it was in a tin box with an inner chassis that came from some electronic modules.
At the bottom were two 4,5 V ELBA batteries in series and at the top was the electronic assembly, a telephone headset as a dynamic microphone, and a telescopic antenna.
Well, I was impressed by the exceptional clarity of the transmitted signal. We were listening in a Darcle radio on FM how the sparrows were chirping outside and the crystal could be heard…. Worth doing!
With a toy of this powered at 12v I transmitted from Vf.Cozia on the frequency 93.50 Mhz approximately 18-20km. The emission power was ~ 280mW.
An amplification stage must be added so as not to be influenced by the antenna frequency.
Theoretically, BF 214 .. 215 would not work?
Maybe the cutting frequency is lower at BF 214. Maybe BF 200?
It works and BF214 I tried in '83, AF126 was better for 1,5 volt power supply.
2N3904 is a general purpose npn transistor with ft = 300 MHz. BF214 is an RF transistor and has ft = 150 MHz. BF200 is also RF but with ft = 325MHz. Before we start with the assumption, it is good to document ourselves.
I made this montage 40 years ago and I was very pleased with it… (I had copied the diagram from a device brought by a friend from Israel). 😁 The machine was made industrially and was the size of half a pack of Kent cigarettes. I don't remember if he had a brand (year 1983?)
BC 547 at the microphone and ZTX320 at the oscillator. You go 1km easily.
http://www.kitsrus.com there they have the assembly with those transistors and they also give the diagram with the agreement of the antenna.
Does anyone want to know how the Q2 oscillator works? It's a Colpitts. Q1 is polarized by R6 in direct current. Alternatively, the base is massaged by C3, which can be even larger. Thus works the common base in rf. C5 is critical, it sets the condition for oscillation….
Good, it also works at 1,5v if you remove the resistor from the oscillator transistor emitter. I made about 50 pieces of this, the coil is on a diameter of 6mm 3 turns and in series with the antenna put a 1pf cond so as not to affect the frequency change when you put your hand on it by capacitive effect. A crystal oscillator is ok, stable and does not run in frequency…