Someone anonymous left me a comment in this post asking that, since I had analyzed the transmitter, I also described the receiver. The comment I deleted, for the lack of care of its editor, but the request seemed right. A typical receiver of a cheap car made in China does not have much crumb. This I present is one that cost me between 3 and 4 euros (for those who are more familiar, about 4.5 USD).
Current Radio Control circuits
There are three types of low-end radio controlled cars. Of course they do not have to be cars, the external form can be any. What matters is the circuit. Of course we talk about radio control at 27MHz, there are other controls that work with infrared but I will not talk about that.
As I say, in RC models today we find only three types of circuits. Because the manufacturers are the same and barely change the schemes. The scheme depends on the channels that the car has. Channels are the independent actions you can perform.
Diagram of a channel: These are the most basic and only have a button on the remote. They are the typical ones that nothing else turn them on the car goes forward. When we press the button it goes backwards and at the same time it rotates, to continue advancing as soon as we release the button. The circuit is very simple: a transmitter in the control and a receiver tuned in the car. As soon as the receiver picks up the command signal, it switches the address. Often the signal is not even modulated.
Scheme of two channels: These have three states: forward, backward and stopped. They have two push buttons, one for forward and one for backward that can be independent or joined in a lever. The transmitter is an oscillator that can emit two tones of different frequencies (250Hz and 1000Hz), we already described the operation in this input. As for the receiver, the scheme is usually based on the integrated RX-3 from Silan. That is going to be the one we describe today.
Scheme of five channels: They are the cars with functions of back-forward-turbo and left-right. In this case it is no longer comfortable to use different frequencies for each option, so digital modulation is used. Both the transmitter and the receiver use dedicated integrated. The TX-2B and the RX-2B respectively. We are not going to talk about them today
Of course there are many more schemes. But these and their variants are the most common you will find in the bazaars. For the mid-range and modeling, especially in airplanes, other not so simple circuits are already used.
Two-channel receiver
This is the receiver of an RC car with two channels: forward / backward and stopped in the absence of signal. First let's look at the plate to get an idea:
We could reproduce the circuit from the tracks, as we did with the transmitter. But it is very boring, in addition in the datasheet of the RX-3 comes a scheme proposed by the integrated manufacturer. It is to be hoped that ours does not deviate too much and in fact it is very similar, deleting some components to save costs.
I have colored some sections so you can see them better (click to enlarge). Let's see how it works.
Section A: Radio frequency stage.
It seems that it is a regenerative receptor. The feedback is done through the 5.6kΩ resistance. These circuits apply positive feedback almost to the point of oscillating with the input signal. For how simple they are, they have very good sensitivity and selectivity characteristics. They have known each other since the earliest days of radio. The first patent is from 1914, with valves, of course.
The transmission reaches the antenna, passes through the tuned tank circuit and is amplified with the transistor. One of the diodes of the transistor also acts as an AM detector. Detecting and re-amplifying the tone with which the carrier is modulated. This type of design was used much earlier, when the cost of the transistors was very high. And that cost less than valves. The first transistor radios that came out proudly announced 6 transistors. Today the remote control that we analyze has 7, and the computer with which I write and read has several millions of miniature transistors inside.
The extracted audio tone goes to section B to be amplified
Section B: Audio amplification.
The integrated RX-3 incorporates two inverting amplifiers ready to use. The outer pins connect with what would be the equivalent of the inverter inputs.
The resistors and capacitors that make up this section are the feedback networks of both amplifiers. The first of them has an amplification of about 30dB which is greatly reduced for high frequencies by the effect of the 500pF capacitor in parallel with the resistance.
The second stage is configured with a gain of 10dB. All this grossly without counting the losses by the coupling capacitors, in series with the input resistors, which separate the direct current and only let the alternating current pass through.
The entire amplifier stage has a gain of 40dB. The detected tone is applied to pin 4 of the integrated. This is the demodulated signal input. When a 1000Hz tone arrives at this pin, pin 11 will be set high -forward- and the car will move forward. On the other hand when a tone of 250Hz arrives, the pin 9 -backward- will turn on and roll backwards
Section C: Bridge H.
When we apply tension to an engine it turns in a certain direction. If what we want is that we rotate one way or another at will we have to use a special arrangement of transistors to feed it. This circuit is called bridge H.
When the integrated applies voltage to the pin 11 -speed- the transistor Q9 goes to conduction. With it as a cascade reaction they also switch Q11 and Q13, grounding the left terminal of the motor and supplying positive voltage to the right one. And the engine will turn in one direction.
On the other hand, when pin 8 is activated -return- transistor Q8 is activated which in turn activates Q12 and Q10. Under these conditions, the left terminal of the motor would receive positive voltage while the right terminal is connected to ground. Just the reverse of the previous situation, and the engine will turn in the opposite direction.
There are variants of this scheme. In the scheme there are 5 NPN and 1 PNP transistors. However on the plate we have there are 4 NPN and 2 PNP. There are multiple possibilities but the idea is the same.
Section D: Food.
Finally, section D is the circuit power. There is not much to emphasize here. There are components that are missing in the commercial plate, for example the diode D1, which prevents against inversion of the batteries, they have saved it. As well as some filtering capacitors.
We see that the part that feeds the stage A is decoupled by a resistance of 100Ω and a capacitor. It serves so that no residual RF signal can leak into the power line and interfere with the integrated one.
In some circuits this part is not well designed, and the RF is coupled with the power supply, it can also pass through the parasitic capabilities between the tracks for example. In many cases of erratic behavior, especially with micro controllers this is the problem
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