Figure 1 : Wiring diagram of the micro-receiver UHF.The receiver that we propose in this paper is suitable for different transmitters operating on 433 MHz. In addition to receiving their broadcasts, it can also record on a tape. Receptor activation is controlled by the squelch internal hybrid module, the heart of this assembly.
The advantage, compared to a conventional receiver is obvious. There is no need to be constantly listening to the receiver to see if something happened on the telephone line supervision or in the capture zone of the microphone. Indeed, each signal received by the receiver will trigger a recording that can be listened to anytime.
To provide easy study of the pattern shown in fi gure 1, it must be broken down into four basic parts, which are the hybrid radio receiver, the relay circuit, the BF in amplifier and power supply.
The radio receiver hybrid
The first floor is a CMS module, referenced FM RX-Audio. It is produced by the company Aurel. This is a complete receiver, tunable fi xed, with a superheterodyne circuit, stabilized SAW oscillator capable of demodulating the FM radio signals, it picks up via an antenna connected to pin 3.
At the output of the demodulator is connected to a preamplifier audio indicator, which makes available the audio signal on pin 10.
The second intermediate frequency amplifi er is connected a sensor field (Field Strength Detector), which can inspect to the intensity of the carrier as well as activate an output based on the level of squelch.
This level sensor can control the electronic switch (solid-state switch) connected, internally, the hybrid module and is available between pins 18 and 19.
This electronic switch will inhibit the output as the carrier will not exceed a certain level (muting) or activate a relay, when a carrier exceeds the required level, afi n put in service any device (remote) as a tape and / or amplifi er.
This is the role that e to the static switch, the squelch threshold is set by the trimmer R8.
At rest, when the reference voltage supplied to pin 15 by the cursor is above the trimmer potential obtained by rectifying the IF signal (intermediate frequency), the controlled switch is open and the pins 18 and 19 are isolated to one another .
The relay circuit
Once the carrier arrives, so the radio signal from the transmitter, the amplitude of the intermediate frequency is such that it exceeds the threshold potential by R8, so that the internal comparator actuates the electronic switch, putting the pin 19 in communication with the pin 18.
Now, the voltage of the circuit can join the relay coil, which is then activated, turning on the audio amplifier and the output indicator for the start of the tape.
It is easy to understand what has been said, considering the following: the first inverter relay, normally closed (NC), sets short-circuiting the terminals of the volume potentiometer R6, preventing the audio signal reaches the entrance to the amplifi er U3.
The other contact of relay RL1, normally open (NO) and corresponding to the output REM (remote) can control the tape recorder.
To activate the relay, the contact opens and allows NF commissioning of audio output with volume imposed by R6 and the NO contact (REM) is closed, thereby activating the tape recorder.
Note that the trimmer R7 adjusts, as desired, the noise level, afi nd'obtenir recording loud and clear without any distortion.
The LF amplifier
It is a small stage built (U3), which allows us to listen to headphones when a message transmitted by the transmitter is sensed by the receiver. This amplifier is realized with the integrated circuit now classic, LM386N National Semiconductor, which can deliver up to 1 watt on a 8 ohm load. Here, considering that helmets only tolerate a few hundred milliwatts, the resistor R4 limits the output current, In order to manage an audio signal clear and undistorted.
The integrated circuit U3 is connected to a few passive components, necessary for its proper operation. It found, among other things, the network C6/R3 for stabilizing the voltage gain to about 150 times and the network R5/C10, indispensable to offset the output impedance varies with signal frequency and to avoid dangerous self -oscillations always possible with high gain.
The power
To function, the unit needs 12 to 15 volts, applied to the terminals marked + and - VAL. The diode D1 protects the entire assembly of a still possible polarity reversal.
U1, the traditional 7808, is capable of supplying 8 volts well stabilized, with which we feed the audio amplifier and the LED indicator lights, which indicates the presence of voltage.
As for the RX-FM audio, the 3.3 volts needed are provided by the zener diode DZ 1, whose current is limited by the resistance R1.
The capacitor C2, fi lter the 3.3 volts of noise and other potential pests.
Figure 2 : Installation diagram of components of the micro-receiver UHF.
Figure 3 : The prototype ready for operation.
Figure 4 : Design, scale 1, the printed circuit of the micro-receiver UHF.Component List
R1 .... 220 Ω
R2 .... 270 Ω
R3 .... 100 Ω
R4 .... 4.7 Ω
R5 .... 10 Ω
R6 .... 4.7 kΩ pot. flax. for this
R7 .... 4.7 kΩ trimmer horiz.
R8 .... 4.7 kΩ pot. flax. for this
R9 .... 560 Ω
C1 .... 470 uF 16 V electrolytic
C2 .... 100 uF 25 V electrolytic
C3 .... 220 nF multilayer
C4 .... 2200 pF ceramic
C5 .... 100 nF multilayer
C6 .... 10 uF 63 V electrolytic
C7 .... 1000 pF ceramic
C8 .... 10 uF 63 V electrolytic
C9 .... 100 uF 25 V electrolytic
C10 .. 100 nF multilayer
D1 ... 1N4007
D2 ... 1N4007
DZ1 .. 3V3 zener
LD1 .. Green LED 5 mm
U1 .... Regulator 7808
U2 .... Audio FM RX module Aurel
U3 .... LM386 AF amplifier
RL1 ... relay 12 V 2 RT for this
Miscellaneous:
1. Support 2 x 4-pin
1. 2 pole terminal block
2. buttons to pot.
1. UHF antenna
1. cable with 3.5 mm stereo jack (mounted mono)
1. cable with 2.5 mm jack mono
1. 3.5 mm mono jack for chassis
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