Thanks to the use of a circuit allowing vocal activation (vox), this small transmitter UHF, consumes a current lower than 2 milliamperes at rest. The stage HF uses a module Aurel TX-FM Audio, which guarantees a range ranging between 50 and 300 meters according to the environment.
The two most important characteristics of microphone-transmitters of the professional type are consumption and the power radiated by the stage HF. Of course, stability in frequency and the quality of modulation are not to neglect. The microphone-transmitters, must consume very little while generating a sufficient power to guarantee a range of a few hundred meters.
It reflects on a statement ... and there is a solution
In virtually all cases, micro-transmitters are powered with a battery, which limits the available energy.
To ensure good battery life, it is necessary that the consumption is as low as possible.
To achieve this goal, we must think outside the box, which led us to, among others, the following choices:
- Increasing the operating frequency while reducing the RF power.
- use a system of relatively narrow band modulation.
- activation of the circuit only in the presence of an audio signal ...
In practice, the only possible way is to use a stage voice activation (vox) commanding a stage HF to 433 MHz in the presence of an audio signal.
When a person speaks into the place where the transmitter installed, the device goes into emission.
Obviously, to ensure that the transmitter does not turn off lights and continuously, the system has a monostable stage that holds the transmission for 20 seconds after each strobe.
As we use a resettable monostable type, during a normal conversation, there is no interruption of the transmitted signal. The circuit becomes idle if no sound is received for more than 20 seconds.
With this system, we obtained consumption at rest of 2 milliamperes, against a consumption of about 15 milliamps issue, which would, without the vox, permanent consumption of the transmitter.
In practice, this results in a significant increase in battery life.
Assuming that the micro-transmitter is activated for one hour per day for food and we used an alkaline battery of 600 mA / h, the circuit will operate for approximately 10 days, 2 days only cons for a free version of vox.
A significant difference!
A vox and a RF module
The system vox, is the element that characterizes mainly microémetteur UHF presented in these pages. For the rest, the assembly is classic, thanks to the use of an RF hybrid module, the TX-FM Audio Society Aurel.
For receiving the radio signal generated by this transmitter, you can use a receiver equipped with a hybrid module RX-FM Audio, perfectly adapted to operate in conjunction with the module used in this micro-transmitter. Of course, you can also use any receiver receiving UHF FM (see Figure 6).
This system provides a range of between 50 and 300 meters, depending on the obstacles between the transmitter and receiver, parasites in the area, the type of antenna used, etc..
The transmission frequency is exactly 433.75 MHz, obtained through a quartz oscillator, thus particularly stable. Modulation is FM type, with a maximum deviation of ± 75 kHz.
Activate with a vox, a hybrid module TX-FM Audio is very simple, because this module has a digital control input (pin 2), through which it is possible to switch on and off the circuit .
With a logic high (5 to 12 volts), the device operates normally, cons with a logic low (0 volts) the hybrid module is inhibited and its consumption is zero.
The switching time is very short, only 100 microseconds.
Using a hybrid module TX-FM Audio and surface mount components (SMT) for the remainder of the assembly, we have succeeded in developing a micro-transmitter with particularly small dimensions, easy to use in different applications and , above all, very simple to assemble.
All the most critical section, that of the high frequency part, is already fully functional and does not require any adjustment or development.
After this long introduction, now see more closely the electrical circuit of the microwave transmitter.
Le schéma
Figure 1 : Electrical diagram of the microwave transmitter controlled by voice.The audio section uses an electret microphone (MIC) with its polarization resistance (R1).
The output signal at a low level of this floor capture the audio signal is applied to the first preamplifier stage contained in the hybrid module, specifically one that has as an input pin 4 and pin 6 as an output.
This section ensures a gain of about 20.
The signal thus amplified, available on pin 6 is passed through the voltage divider consisting of R7 and R8, into the second stage preamplifier, also present within the hybrid module, whose entrance is on pin 7.
This stage is characterized by a lower gain than the previous stage, it is here only about 5.
For both amplifier stages BF content in the hybrid module, the external divider excluded, the maximum gain is 100.
We must consider that if our divider has a ratio of 10 to 1, in our case the total gain of the low frequency floor is 10.
This level of amplification can be modified to increase or decrease the sensitivity of the micro-transmitter, depending on the type of microphone used.
Indeed, all microphone capsules do not present the same sensitivity.
To change the gain of the stage, it can affect the value of resistor R8. By increasing the value of this component, the gain of the stage low frequency increases proportionally.
To function properly, the hybrid module TX-FM Audio, needs no other external components, apart from the antenna to be connected to pin 15.
In other words, if we were just a micro-transmitter reduced to its simplest form, the circuit could be finished!
In addition we could also eliminate the capacitor C2 and use the link capacitor contained in the hybrid module.
Obviously, pin 2 (enable), which controls the startup, should be connected to the positive supply line.
In our case, for cons, the pin is controlled by a simple vox circuit constituted by the transistor T1 and two of the four gates of U1.
Normally, the entrance gate U1B (pins 8 and 9) is at high level by the resistor R6, so the output of this gate (pin 10) has a low level, inhibiting the operation of the hybrid module. Instead, the output of gate U1A has a logic high level, because we have a low logic level on its input pins (5 and 6).
In reality, the level is about 2.3 volts, close to the switching threshold, but the door sees as a low level.
This voltage depends on the type of polarization of the transistor T1: pins 5 and 6 are in fact connected to the collector of this component. By carefully choosing the values of R2 and R3, it is possible to accurately determine the voltage present at rest on the collector of T1.
This floor works, too, as LF amplifier.
The signal produced by the microphone, in addition to reaching the entrance of the hybrid module, also reached the base of T1 through capacitor C1.
Lets see what happens when the microphone picks up audio signals, even very low.
Figure 2 : To receive the signal from the microwave transmitter described in these pages, it is possible to use a receiver designed to operate on frequency 433.75 MHz, based on the module RX-FM Audio Aurel and integrated in a small box that can be advantageously placed in a pocket. Depending on the environment, the scope of the system can vary between 50 and 300 meters. This receiver will be described shortly in the journal.
Figure 3 : Layout diagram of the components.
Figure 4 : Design of printed circuit scale 1.Component List
R1 = 10 kW
R2 = 2.2 MΩ
R3 = 22 kW
R4 = 10 Ω
R5 = 10 Ω
R6 = 470 kΩ
R7 = 22 kW
R8 = 2.2 kΩ
C1 = 100 nF SMD
C2 = 100 nF SMD
C3 = 10 uF 6.3 V SMD tantalum
D1 = 1N4007
T1 = BC847B Transistor NPN SMD (SOT 23)
U1 = Integrated MC14093BD SMD
U2 =-module Aurel TX FM Audio
ANT = Antenna given (17 or 34 cm)
MIC = preamplified electret microphone
Miscellaneous:
A jack 9 V battery
1 PCB ref. S317
All resistors are 1 / 4
Watt CMS.
Operation
The signal is amplified by transistor T1 and is superimposed on the voltage of the collector. In practice, the collector voltage varies up and down relative to the supply voltage of rest.
If the variation up allows the DC component from reaching the switching threshold of the door, the latter, even if it is for a court time t, changes state.
Pin 4 goes to logic zero, causing the switching of the second door and activate the hybrid module.
When the first gate changes state, the capacitor C3 is charged immediately, now at the bottom entrance of the gate U1B, which allows the hybrid module to remain active.
Due to the presence of diode D1, the capacitor can be discharged only through the resistor R6.
We sized the values of R6 and C3 to allow the circuit to remain active about 20 seconds.
Obviously, if during this time the door was U1A switch again, as happens in the presence of other signals from the microphone, the capacitor is recharged again, reactivating the timing of the monostable.
The sensitivity of the circuit, in fact, the audio signal level required for its activation, depends, in this case, the dc level of the collector of T1, more than the amplification of the audio signal.
In fact, over this potential is close to that required for the commutation of the door, the greater the sensitivity, so that even a very low audio signal is sufficient to switch the door.
By acting on R2, it is possible to change the collector voltage of Q1 and thus the sensitivity of the vox. By decreasing the value of resistance, the voltage drop on the collector and sensitivity as well. On the contrary, an increase in the value of R2, increases the sensitivity.
Do not increase this value too, so that the collector voltage exceeds that threshold, which could keep the vox permanently active.
Figure 5 : All components of the micro-transmitter, mounted on a printed circuit adapted to small dimensions. The components are all the type for surface mount (SMT) and, thus, they are welded copper side. The printed circuit thus produced is attached to the hybrid module TX-FM Audio to form a sandwich, which, when fully closed, allows to minimize the dimensions of the device.
Figure 6: To receive the signal from our micro-transmitter, the receiver further discussed in Figure 2, it is also possible to use a UHF receiver, or even a scanner tuned to the frequency of 433.75 MHz. While allowing a good quality sound signal emitted by the microwave transmitter, these devices have a very narrow bandwidth compared to the bandwidth of our accomplishment. The signal from the TX-FM audio is frequency modulated with an excursion (Δ) of ± 75 kHz. The power emitted is slightly less than 10 dBm, equivalent to 10 mW on a 50 ohm load.
Figure 7 : Pinout IC MC14093BD used in the circuit of vox.
Figure 8: The module TX-FM AUDIO Aurel is ideal for applications such as streaming audio HI-FI, alarm via radio (helpline) and remote control(DTMF).1 = +12 V 6 = OUTPUT BF 15 = ANTENNA OUT 2 = TX ENABLE 7 = INPUT 2 BF 3, 5, 9, 13, 16 = GROUND 4 = INPUT 1 BF
Figure 9 : The module RX-FM AUDIO Aurel is a superheterodyne receiver with FM. Its wide bandwidth makes it ideal for applications in the hi-fi audio systems and portable receivers.1 = +3 V (±10 %) 15 = SQUELCH 19 = + SQUELCH 3 = ANTENNA IN 18 = OUT SWITCH 2, 7, 16, 20 = GROUND
The practical realization
It only remains now to deal with the practical realization of the micro-transmitter.
As you can see from the pictures, all components used in the wiring, with the exception of the diode D1, is the type mounted on the surface and, thus, they are welded on the copper side of the (tiny) PCB.
For this operation, use a soldering iron, low-power, fitted with a fine tip and kept clean at all times.
The most critical component to develop is surely the CMOS integrated circuit.
For this, we recommend you solder only one pin of the integrated circuit, to enable precise positioning. Once the component is properly seated, solder all the other pins.
To connect the microphone, use a piece of shielded wire. If the cable length does not exceed 10 or 15 inches, you can use two pieces of wire ordinary twisted.
The circuit thus prepared, is connected to the hybrid module TX-FM Audio to form a sandwich as shown in Figure 5.
The circuit requires no adjustment or development, except what has been said for the resistors R2 and R8.
As an antenna, use a piece of stiff wire of 17 or 34 inches long (1 / 4 or 1 / 2 wave).
To verify the circuit operation, it is necessary to use a UHF portable receiver or a receiver for reception of this frequency.
Using a multimeter, measure consumption at rest and in the transmission, which must be followed by 2 mA and 15 mA.
If the sensitivity of the vox does not satisfy you, you will act, as we have explained above, the value of R2.
To change the microphone sensitivity, it is by acting against the value of R8.
No comments:
Post a Comment