This simple circuit will allow transmitting audio signals in an area of the approximately 100.m radius. The signal emitted by it can be tuned from its FM radio because its transmission frequency can be easily located between 88 and 108Mhz.
Its uses are unlimited, it can be used as a monitor for babies, as a small intercom, as a wireless microphone for conferences, transmit PC audio to another point of the house, or even to mount a pirate station , it is worth mentioning that it is not at all bad for some project that you have asked at school.
This circuit consists of two parts; the first stage is the amplification and coupling of the microphone signal, the second stage is a capacitor-coil oscillator circuit, this will be responsible for emitting by the antenna.
- Two 2N2222 transistors (Q1 and Q2)
- Electret microphone (Mic 1)
- 1 Electrolytic Capacitors 10uF / 25v (C1)
- 1 Electrolytic Capacitors 100uF / 25v (C3)
- 1 Electrolytic Capacitor 2.2uF / 25V (C2)
- 2 Ceramic Capacitors of 0.1uF / 50v (C4 and C8)
- 2 Ceramic Capacitors of 3.3pF / 50v (C6 and C7)
- 1 Adjustable 5-60pF capacitor (trimmer) (C5)
- 1 Resistances 1k (R1)
- 1 Resistance 15k (R2)
- 1 Resistance 6.8k (R3)
- 1 Resistance 10k (R4)
- 2 Resistors 4.7k (R5 and R7)
- 1 Resistance 2.2k (R8)
- 2 Resistance 220 Ohm (R6 and R9)
- 50 cm Wire for 0.51mm diameter (24 AWG) bridges
- 1 Battery Connector
- One 9V battery
- Two connectors or terminals (for the battery and the microphone)
P.S. The second transistor can be BF494 or BF199 to improve the sound quality.
- Cable cutting pliers
- Screwdriver (preferably plastic)
- Small saw
This time we will weld them in a much easier way, besides that it helps the coupling of the components when working with medium frequencies and noise reduction, this is due to the proximity of the components and the shortness of their terminals where they are soldered. Basically what we will do is that the bottom plate is all the negative (-) and the small pieces of copper represent the nodes of the circuit. The long copper plate represents the positive node.
The copper plate must be clean; we will sand it a bit so that when welding it adheres perfectly:
We will proceed to cut nine pieces of copper like the following:
- 6 of 4mm x 4mm
- 2 of 4mm x 12mm
- 1 of 4mm x 48mm
- The lower copper plate is 30mm x 52mm
- We start welding starting from the left side until we finish:
- Connectors for microphone and battery
- Testing the terminal for the microphone
To manufacture the coil grabs an enameled copper wire and wind it around seven more times around a pen, the function that fulfills is to emit the electromagnetic waves.
Steps for the Assembly
Solder the components of lower height as the resistance.
Then install the ceramic capacitors, the variable capacitor (trimmer), the five spindles and the transistors.
Subsequently, weld the electrolytic capacitors and the coil. Remember that on the Printed Circuit Board the terminal identified with the sign (-) in the capacitors must be located on the opposite side of the one identified with the sign (+).
Finally, solder the microphone, taking into account its polarity, the antenna and the connector for the 9v battery to the respective sprays and secure the support for the battery using the screws.
- Once you are sure that all the components have been assembled you can proceed to the test and calibration of the circuit.
- To do this, place an FM radio near the circuit, look at the dial for a silent point (without stations) and turn up the volume of the receiver to a point where you can hear the interference.
- Connect a 9V battery to the circuit and listen carefully to the radio.
- Slowly and with the help of a small screwdriver, adjust the capacitor (trimmer C5) until a similar whistling or sound is heard in the receiver, which means that the dial frequency has been tuned to the transmitter at that point.
- At that time you can speak into the microphone and you should listen to what is being spoken on the radio.
- If on the selected frequency, good reception is not achieved, repeat this procedure at another point in the FM band.
- If you prefer, instead of varying the capacitor, tune the radio until you find the point where you will find the best reception (silence).
- If after doing this, you can not tune the transmitter, you can adjust the coil that makes up the oscillator circuit by joining its turns to raise the frequency, or separating them if you want to reduce it a bit.
- This circuit works best when a battery powers it but if you want you can do it with a regulated power supply.
ACA BEGINS MY CONTRIBUTION TO THE PAGE
Our friend Diego (me), a tireless follower of this post, sends us images of his FM transmitter. Congratulations to him and all the suffering lovers of radio frequency who dare to make their equipment. 73’s to all and welcome Diego to this hard but rewarding way.
My transmitter (it came out half empty, but it was starting in the electronics):
Test and calibration of the circuit
1. Once you are sure that all the components have been assembled, we can proceed to the test and calibration of the circuit.
2. To do this, place an FM radio near the circuit, look for a point in silence (without stations) and turn up the volume of the receiver to a point where you can hear the interference.
3. Connect a 9V battery to the circuit and listen carefully to the radio.
Slowly and with the help of a small screwdriver, adjust the condenser (trimmer) until a similar whistling or sound is heard in the receiver, which means that the frequency has been tuned to the transmitter at that point.
4. At that moment you can speak in the microphone and you should listen to what is being expressed on the radio.
5. If in the selected frequency, a good reception is not achieved, repeat this procedure in another point of the FM band.
6. If after doing this, you can not tune the transmitter, you can adjust the coil that makes up the oscillator circuit by joining its turns to raise the frequency, or separating them if you want to reduce it a little.
7. This circuit works best when a battery powers it but if you want you can do it with a regulated power supply.
If you wish to have the possibility of controlling the transmitter volume, change the resistance R6 by a potentiometer, which can be approximately 10K.
To extend the life of the Battery, disconnect it when the transmitter is not being used.
If you want to apply an external audio signal such as an iPod, you must suppress the microphone and its polarization resistance R1, leaving the decoupling capacitor C1 as audio input.
The miniature FM transmitter has been designed in such a way that it does not exceed the limits of its oscillation frequency which is between 88 and 130Mhz and the field generated by the irradiations does not exceed 50mV per meter, at a distance of 15cm of the circuit.
This type of communications are governed by the rules of each country, which should not exceed certain limits, the omission of these limits, is punishable by fines and penalties.
If your circuit is assembled following the specifications described in this post, it will not exceed these limits, as any modification made to the circuit including, for example, a variation in the supply voltage, will change the range of the signal emitted.