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With LM1872

For this circuit we will use the companion of integrated circuit transmitting LM1871 of National that is the (receiving/decoder). He himself is in charge to receive the RF from the antenna and to decodificar it so that he is interpreted by the servo ones of the standard type.


Servo connected directly to an exit of the LM1872

General description

The LM1872 is a receiver and decoder of Radio frequency (RF) complete designed for applications of radio control. The device is used so much in frequencies of 27MHz, 49MHz or 72MHz to control several types of toys like cars, boats, tanks, robots, airplanes and trains. The design of the superheterodyne controlled by a crystal at the same time offers a good sensitivity and selectivity. When it works altogether with transmitter LM1871, this it provides four independent information channels to him. Two of these modulated channels with in wide of a pulse analogically (PWM) and the other two are digital channels (ON/OFF) with a capacity to handle until 100mA. Each one of these channels can be convertiar to the other type of channel with some external circuit. This way they are possible to be gotten to obtain 6 totally analogical channels or 6 totally digital channels. Single some external components make lack to complement to the LM1872 which by itself includes a local, mixer oscillator, detector of IF (Frecuencia Intermedia), AGC (automatic Control of gain), controller of synchronism of exit and all the logic of the decoder.

Characteristic

  • Four independent information channels.
  • Completely integrated
  • Minimum external elements for its operation.
  • Rank of operation of 50KHz to 72MHz
  • Superheterodyne design with much selectivity and sensitivity.
  • Operation with 4 batteries of 1.5V
  • Exelente rejection to the noise sources
  • Handling in the exits of up to 100 mA
  • Controlled by crystal
  • It internally has the interface adapted to work directly with servo the commercial ones.
Tension of operation: Minimo=2.5V; Nominal=6V; Maxima=7V
Current of consumption: maximum @27mA.

Description of the Circuit

For the transmission of analogical information the codification system/decoding (LM1871 and LM1872) uses the system of modulation by the wide one of pulse or PWM, but known in the atmosphere of the R/C like PPM. In the application of this type of modulation the RF carrier is interrupted in small fixed time intervals (to see “metric ton” in the figure of down). Each one of these intervals this followed by a pulse of wide called variable (tCH); of this form the multiple time intervals are defined in series (metric ton + tCH).
The synchronization is at the same time obtained thanks variable but long that tCH called tSYNC which produces that t'SYNC of a timer is finished to the time that this in the receiver and that serves to return to zero an accountant who is in charge to count the received channels.
The total sequence of pulses including the synchronism pulse constitutes a single cycle of the signal radio base control, this cycle is called “frame” and it is recognized like simple period of frame (tF).

Diagram of signals in the LM1872

The transmitting circuit done with the LM1871 this equipped to transmit up to 6 channels, the circuit that was used here with the LM1872 can receive 2 analog channels and two digitalises (but ahead side like expanding up to 4 and 6 analog channels). The decodifica receiver the RF signal detecting the flank of fall or negative flank of the pulse, this signals when happening through three binary splitters called in the fig of above like “flip-flop To, B and C”. Soon examining each one of the exits of these splitters separately they are possible to be identified and to be recovered up to 6 independent channels. In fact single both first they are those that are decodifican and the rest single serve to determine the digital state of the 2 other digital exits. For the identification this is used the result of an accountant who counts the amount of transmitted channels. To see the following figure.

Waveform in the transmission based on
transmitted channels and their decoding like channels on/off

The receiving circuit

The receiving circuit is a superheterodyne receiver of the type “simple conversion” with AGC, which uses for the mixture 455KHz and with the transforming that set out they obtain 58dB of gain. The active digital detector in addition adds 30dB of gain by on a common silicon diode, giving like total result one ganacia of the system of 88dB.
The same gain with other transforming can be obtained more or less. The rank of operation frequency can go from 50KHz to 72MHz being able to use any intermediate band of work.

A short antenita of 30 or 60 cm is the one that is used typically, the same one approximately has a low radiation resistance 4 from 0.5 to Ohms and 3pF to 5pF of associate capacity. This antenna is reconciled to the mixer circuit by means of a tank of high Q formed by C3 and T3. This tuned tank maintains outside reception to transmissions of high power that come from sources of high diffusion like commercial FM or TV.
When the operation it is in 49 or 72 MHz the CB interference (CB) highly is also diminished preduce a good rejection to the frequency typical image of the heterdyne ones.

The function of automatic control of gain (AGC) is only applied to the signal of intermediate frequency (IF), this system allows to avoid errors of decoding by means of the control of the gain of the signal. This is a very good characteristic because if were not the saturaria receiver with the signal level when this near the transmitter and perderia very easily the signal when it is remote of the transmission source.

For the description electronic but detailed to see the page Nº 8 of the
.

The circuit decoder

The intention of this circuit is to recover the codified information of the radio frequency carrier and to thus turn it a signal that can understand the corresponding actuator according to the channel, like also the information that serves like determining the digital state of an exit of these characteristics. The nucleus of the decoder is the binary accountants called flip-flop To, B and C of the diagram. The exit of the detector (already demodulated) feeds the entrances on the binary accountants and the one on the timer on synchronism. When the RF carrier throws the first pulse the descendent flank advances the accountant a state, during the “state under” (metric ton) the capacitor that conforms the synchronism timer it maintains deactivated thanks to the Q12 transistor. Soon the carrier introduces the flank positvo of the next pulse begins to receive the wide pulse modulated in (tCH) then the capacitor of temp. of synchronism it begins “to rise” to try to arrive at limits of (V+/2), but in just a short time that gives tCH him it does not reach to obtain it. Then when tCH finishes the negative flank comes from the pulse and the cycle is repeated of which the accountant advances a resetea pulse and the synchronism timer because the capacitor remains deactivated, and soon this is repeated for the second analog channel.

In order to decodificar previous analog channels both digital floodgates NAND of three called entrances G1 and G2 examine the train of pulses that gives back the binary accountant in such a way that it allows them to identify the periods of time that represent those channels. When decodificar this way the wide total of the decodificado pulse is equal to the sum of the fixed time of the state under (metric ton) but the variable time of the high state (tCH). Finally a stage of power in Darlington is in charge to give to this same pulse each repetitively 20ms, but with greater capacity of power to feed servo a standard one dorectamente.

After both first analog channels an amount comes from pulses corresponding to the other channels. This amount varies from one to four, each one of 500us of fixed duration; which determine the state of the digital exits in the receiver. A very interesting data is that until the group of pulses finishes (tF), the decoder responds to these pulses so as as if they were analog channels but it does not give any exit. When finishing the train of pulses of data the transmitter sends the pulse of synchronism (tSYNC), and like this pulse (that also is variable because “absorve” the variations of tCH so that ctte stays tF) he is much more great that the other pulses the period of the synchronism timer is located in 3,5ms. It is therefore that reset in the accountants only goes off in the period of time of the synchronism pulse and not before.

The receiving circuit/decoder and its plate of assembly

In the figure that follows it shows one of the typical circuits that can be used so much for 27MHz or 49MHz. If the propose antenna uses the reception systems c*on can be obtained a good decoding for signals of 10uV and 12uV for 27 and 49MHz respectively.

Circuit for two analog channels + two digitalises

List of Components

Description
Value for 27MHz
Value for 49MHz


R1 (It disconnect of motor)
R2 (Resistance of the Timer)
R3 (It disconnect of the Mixer)

C1 (OL Bypass, optional)
C2 (Tank of the oscillator)
C3 (Tank of entrance of ant.)
C4 (Bypass of polarization)
C5 (It disconnect of motor)
C6 (Timer of sincro)
C7 (It disconnect of the mixer)
C8 (AGC)
C9 (Bypass of FI, optional)
C10 (Bypass of feeding)
C11 (Bypass of the Local Oscillator)

L1 (Coil of OL)

T1 (mixer Trafo of FI)




T2 (Trafo of filter of the FI)




T3 (Trafo of impedance of ant)

X1 (Crystal)

D1 (protection diode)


20 Ohms
=< 470K
200 Ohms

0.01uF
43pF
39pF
0.1uF
100uF x 10V
=< 0.5uF
Between 0,01 and 0.1uF
0.1uF
0.01uF
Between 0,01 and 0.1uF
0.001uF

9esp; 0.8uH

Pins (1y2) 131 esp
Pins (2y3) 33 esp
Pins (1y3) 164 esp
Pins (4y6) 5 esp

Pins (1y2) 98 esp
Pins (2y3) 66 esp
Pins (1y3) 164 esp
Pins (4y6) 8 esp

9 esp (Pri); 3 esp (sec)

Parallel type to 3er Sobretono

1N914 or 1N4148


20 Ohms
=< 470K
200 Ohms

0.01uF
24pF
24pF
0.1uF
100uF x 10V
=< 0.5uF
Between 0,01 and 0.1uF
0.1uF
0.01uF
Between 0,01 and 0.1uF
0.001uF

6esp; 0.4uH

Pins (1y2) 131 esp
Pins (2y3) 33 esp
Pins (1y3) 164 esp
Pins (4y6) 5 esp

Pins (1y2) 98 esp
Pins (2y3) 66 esp
Pins (1y3) 164 esp
Pins (4y6) 8 esp

1,5 esp (Pri); 6 esp (sec)

Parallel type to 3er Sobretono

1N914 or 1N4148

References
Primary winding
Secondary winding
OL (the Oscillating Premises)
AGC (Automatic Control of Gain)
Bypass (Capacitor that serves to eliminate undesired RF)
All the capacitores are of type “ceramic NPO” or anyone of lost losses exepto C5 that is electrolytic.
The resistance are to 1% and R2 agrees that he is preset to fit the T'SYNC.


Disposition of the components in the printed circuit


Vista of the mounted components

This sensitivity has been like but adapted empirically for the applications of the type terrestrial vehicle of toy. Less gain reduces to too much the reach rank and but gain increases the noise generated by the interferences.

As to increase the reach of reception?

Nevertheless for applications that they require but of 50 meters can be caused that the antenna is but long that 60 cm. Are some ways to alter the sensitivity of the receiver; one is diminishing the relation of turns in the transformer of entrance T3, when doing this gains gain when capturing but signal but loses Q in the transformer. Another one of the things that can be made to improve the gain of the system is to change the point of taking of signal in the coil of the transforming T1 so that this but moved away to the feeding pin or but also the relation of turns can be diminished since it becomes in T3. For example passing of a relation of turns in T3 of 32:1 to 5:1 in 49MHz experiment an increase of sensitivity of the double (passage of 6uV to 12uV). The Q of the mixer I get worse, but in main lines they stayed 3dB of bandwidth (BW).
The derivation in the primary winding of the transformer of FI, T2, also can be fit like improving the gain, this serious one doing it but far from the feeding like T1, but aui it is necessary to have certain well-taken care of because it can cause that the loopback of the AGC oscillates.

On the bandwidth (BW)

With respect to the bandwidth, to be able to receive the signals correctly it is good that the possible thing stays but small. 3.2KHz and 3dB of BW of the circuit are but that sufficient like so that they pass 500us of metric ton, even reaches it stops like calming a little the electrical noise and the interferences of the adjacent channels. In 49MHz the assigned frequencies to R/C estan separated in channels of single 15KHz one of the other in case two frequencies of these esten in use, the channels deberian to choose themselves with a separation of 60KHz. In case 3 are used frequencies then do not podran to be separated but of 30KHz one of the other, and if they were the 4 or 5 frequencies the serious separation of 15KHz, therefore it is that it looks for that the bandwidth is the possible minimum. In 27MHz the R/C frequencies even estan to 50KHz of others, but the problems are the transmissions of BC that are separated to each other to 10KHz (also 5KHz) therefore are one of the saturated bands but of interferences of phantom RF. The circuit responds attenuating to the signals of the other frequencies to 34dB to 15KHz and 56dB 50KHz.

Adjustments of synchronism

The synchronism timer (t'SYNC) must have the “prepared cut” in such a way that he is but long that the pulse of channel (tCH) but long, but but short that the pulse of synchronism (tSYNC) but short. Using the components of the proposed circuit the timer has the cut to 3,5ms which worked perfectly within the synchronism pulse that is of 5ms.

On the general stability

The circuit consists of numerous capacitores of bypass for RF that act as “to stabilize” the circuit before the precencia of spurious, but all are not totally necessary to assure the necessary and good stability perfomance. An economic version but (it is justified in the case of producing it in series) can not use the capacitores C1, C9, C10 and C11.
While but clean and small it is less capacitores the mounted plate where this the circuit of bypass is necessary. In the case that the plate demonstrates to be unstable they can increase the values of the capacitores C7, C9 and C10 to 0.1uF to improve it. But it does not have because to worry if it is used the circuit and the plate proposed in this document because it is possible even to be used with certain stability until 72MHz.

Consumption in the digital exits

These exits can provide an exit of maximum current with 100mA and has a resistance of 7 saturation of Ohms. Through the emitters they can grant 100mA max to 1V like being able to exitar to a transistor NPN or a SCR. Unfortunately this type of consumptions can take to that the LM1872 is burned if one does not consider that the maximum dissipation of he himself is of 1000mW. A very good one practices serious to measure that the digital exits have limited the current 100mA. If greater capacity makes lack of current they can add the two exits connecting Pin 7 with Pin 9.

Intermediate frequency

The intermediate frequency (FI) is used in 455KHz and there are no other possibilities within standard because the integrated circuit this limited internally in a FI handling 1MHz at the most and 50KHz like minimum.

Adjustments of the receiver

The procedure of adjustment of the circuit is quite simple and fast because the stages estan well separated and they do not have greater interaction to each other.
The first oscillator is fit touching L1 whereas <=10pF in Pin 2 is moderate with an end of low capacity. During the adjustment side in the oscilloscope a maximum tip and soon a steep diminution. It is necessary to fit L1 in such a way that the point is just before the moved away maximum tip of the steep diminution.

Soon to be able to correctly fit T1, T2 and T3 a signal of originating RF of the transmitting circuit with the LM1871 must be injected in the antenna (made in the first part of this I articulate). This is because the cirstales that are used in this type of circuits have an error of the 0,01%. In 49MHz a deviation of 5KHz can cause that both circuits “do not see” because does not enter the passband filter of FI that is of 3,2KHz. The signal must be injected in the antenna to segurar itself of a correct load of the tank of entrance T3.

The adjustments are much more easy with an automatic control of gain (AGC) he himself this available one just by to send to mass Pin 16. Measuring the amplitude of the FI in Pin 15 it is possible to be used as a good guide of adjustments. In this point the value does not have to exeder 400mVp or the D2 diode began “to trim”. Another thing to consider is that the capacity of 10pF of the end of test in Pin 15 produces that the frequency of work of the FI tank runs 2KHz. Unless it is not possible to be used an end of test of smaller capacity is recommended to take the reading from he himself value in the pin without use of the secondary one of T2. If the amplitude in Pin 15 is diminished relatively to a factor of 8.25 podria to be used an end of even 50pF without noticing changes in the frequency of the tank.

The method I practice to catch the signal is not to extend the antenna to him to the transmitter and to move away it less in such a way that in the receiver they are possible to be obtained 400mVp or in pin 15 of the CI. Then it is when T3, T1 and T2 for a maximum signal adjust.

Operation to 72MHz

The band of 72MHz is very spread to be used in the controls of airplanes. The FCC allows levels of power much greater than in 27MHz. Elevated frequencies as 72MHz is not problem for the LM1872, he himself in if he is stable and it has good sensitivity and selectivity in these frequencies. The circuit that follows for which it has been having a sensitivity to less detect signals of 2uV in his antenna, which this designed with a resistiva impedance of 100OHMs in its quarter wave. Also it has the filter of FI with good 3,2KHz of bandwidth.
In order to avoid problems of it connects when the antennas of the receiver and transmitter estan one of the other closely together operating to high power, is due to give much importance to the relation ignited/extinguished in the carrier of the transmitter. A stage can be used the LM1871 like exitador of low power and be fed on RF power with an amplifier in class C which turns out to be an efficient and cheap exit for high power.

Circuit for 72MHz

Table of components for 72MHz
Description
Value for 72MHz
R1 (It disconnect of motor)
R2 (Resistance of the Timer)
R3 (It disconnect of the Mixer)

C1 (OL Bypass, optional)
C2 (Tank of the oscillator)
C3 (Tank of entrance of ant.)
C4 (Bypass of polarization)
C5 (It disconnect of motor)
C6 (Timer of sincro)
C7 (It disconnect of the mixer)
C8 (AGC)
C9 (Bypass of FI, optional)
C10 (Bypass of feeding)
C12 (Tank of entrance)

L1 (Coil of OL)

T1 (mixer Trafo of FI)




T2 (Trafo of filter of the FI)




T3 (Trafo of impedance of ant)

X1 (Crystal)

D1 (protection diode)
20 Ohms
=< 470K
200 Ohms

0.01uF
22pF
24pF
0.1uF
100uF x 10V
=< 0.5uF
Between 0,01 and 0.1uF
0.1uF
0.01uF
Between 0,01 and 0.1uF
160pF

4esp; 0.2uH

Pins (1y2) 82 esp
Pins (2y3) 82 esp
Pins (1y3) 164 esp
Pins (4y6) 30 esp

Pins (1y2) 82 esp
Pins (2y3) 82 esp
Pins (1y3) 164 esp
Pins (4y6) 8 esp

  2 esp (Pri)4 esp (sec)

Parallel type 5to Sobretono

1N914 or 1N4148

Expansion to 4 analog channels

For those applications that they require but of two analog channels the LM1872 can be expanded for being able to recepcionar and to decodificar 4 channels with an appropriate external circuit.
The reception decoding of 4 channels is obtained thanks to that a new pulse of synchronism in means of the plot is created, this new pulse seudo-synchronism flame (tps), this pulse transmits our friend LM1871 obvious.
This seudo synchronism produces in the receiver a premature detection of the aim of the plot starting off the old plot in 2 parts (to see following figure). The idea is to transmit channels 1 and 2 in first half of the plot and channels 3 and 4 in second half. The electronics associated to the LM1872 is in charge to obtain the data through the two only analogical exits that the LM1872 has. This identification becomes thanks to the aid of one of the digital exits. In the measurement that the digital channels respond both to the account of channels detected in the transmission of small groups, obtendra a digital exit because in the second group muentras are three channels that in the first single group are two.

Diagram of signal in the decoder.

There are two alternatives with regard to the external or associated circuits to the LM1872 for the detection of 4 channels. First of them it uses an integrated circuit that is a MUX of 4 entrances that is in charge to rutear the pulses in the correct form with the aid of digital channel B.


Circuit to extend up to 4 analog channels

The other circuit is much more economic and not for that reason he is less efficient, this it walks very well and it is simple for the design of the plate. The diodes with the asterisk come up that the servo one has an incursion in inentendibles values by and in several cases they can be eliminated.

Another method to obtain 4 analog channels

Expansion to 6 analog channels

A greater capacity of decoding can even be obtained up to 6 analog channels with an auxiliary decoder. The LM1872 is necessary, a simple a comparator and shift register entered serial and exit parallel of 8 bits to obtain the decoding of 6 digital channels.


Circuit with a shift register, also can take control of 4017 or 4015

The comparator is a simple transistor that reconstructs the signal that gives the detector to him of the called LM1872 Q1 in the figure of above. The comparator done with a single transistor reconstructs the signal that gives the detector to him from the synchronism timer and it injects it reconstructed to the entrance of clock of shift register (MM74C164). The registry receives channel “one” in clock and soon it qualifies one of his exits giving him exit to the information without modifying the wide one that it is the sum of tCH + metric ton with a smallest retardation of 10uS as it is observed in the following diagram:


Detection of the 6 analog channels by means of signal of clock in the accountant

The retardation of 10uS is very important because thanks to we made sure that a 1 or not noise in the reception is the high state of the channel which provocaria an erroneous reading.

Conversion of analogical digitalis exits

An analogical exit digitalis with the aid of an integrated circuit can be turned and very cheap easy to obtain that he is an investor CMOS or also call floodgate “NOT” (CMOS) connecting it as it is in the following circuit:


Circuit to turn an exit of analogical (1ms to 2ms) digitalis

The internal resistance of 10K and the capacitor of 0.15uF fix a constant of time of 1ms which enters tre 0.5ms and 2ms of the transmission of the channel.  For pulses of but of 1ms the first investor it gives a zero momentary one uan time each complete plot. These repetitive unloadings of C2 avoid that it reaches the value of V+/2 because the constant of time of R1 C2 is much greater (70ms) than the total time of the plot (between 20ms and 30ms). Then as the investor has a zero in his entrance the Q1 transistor has a stop in its exit and feeds the load (LOAD) that can be a motor brushless for example.
For pulses of less of 1ms the first contrapolariza investor the D1 diode and this produce that C2 happens of limits feeding the entrance on the second investor producing a zero in the exit with the consequence that the load lets be fed. For loads that they demand but current they can use the investing rest (4 by chip come) in parallel with Q1 and for light loads Q1 without problems can be eliminated totally.

When a single one of the exits of the two analogical ones is only nesesario; the following circuit with CI the LM555 offers simplicity combined with an exit that can handle to loads of 150mA the logic of the circuit is the following one:


Intelligent detection of the state

The fall of the pulse of analog channel 1 is used to resetear the timer and to prepare it for the comparison with the wide one of the pulse of analog channel 2 with a propose constant of time of 1,1ms by the internal resistance of 10K with the capacitor C1. After pulses of wide the greater one to 1,1ms C1 it reaches to happen of certain level causing that the 555 feed the load. The feeding of the load while this activated the 555 takes place a cycle of work of 95% whereas this extinguishing is of the 100%. This does not imply any kind of problems whereas the load is motor, solenoids, lamps and horns.

Replacing servo by bridge of transistors and a motor of CC

The following circuit can serve like replacing some servo expensive functions of the commercial ones like the one to cause that a car goes for ahead, stops and goes in reversa. It has a capacity of 100mA with 4 transistors and 5to (Q5) is a protection. Supposing that it is sent erroneously feeding to both branches of the bridge (it can happen if it appears some noise because the car I move away too much of the transmitter) desabilita the Q5 transistor the right branch whereas this activated the left.


Circuit of control of a motor of CC through the standard exit

One of the main problems that it has east type of circuit is that the noise of the brushes of the motor causes errors of all type. In order to attenuate this type of noises in any case the networks of capacitores and resistance of but down can be used. Or to use another type of motor as it can be