Application of multifunctional remote control PT8A977 / 978

Abstract: The main performance, internal circuit structure characteristics and control logic relationship of the five-function wireless remote control circuit PT8A977 / 978 with excellent performance are studied. The typical application circuit of PT8A977 / 978 and its working principle are introduced in detail.

1 Introduction

The toy remote control car adopts wireless remote control technology of servo motor. The basic requirements of remote control circuit design are high performance, low cost, stable operation, flexible control, simple circuit, and strong anti-interference ability. Generally, two miniature DC servo motors are used to drive the toy remote control car to realize the forward, backward, left, right and acceleration functions of the toy remote control car. The competition in the toy remote control car market is becoming more and more fierce, and the electrical performance of the toy remote control car is also increasing. The wireless remote control circuit design of the toy remote control car determines the overall performance of the toy remote control car. In this paper, a design scheme of a miniature toy remote control car circuit with excellent performance is given. The scheme is mainly designed and completed using the PT8A997 / 978 integrated circuit controller.

2. Wireless remote control circuit of remote control car

Ordinary toy remote control cars generally have the basic functions of forward, backward, left turn, and right turn. These functions can be completed by two miniature servo motors, respectively. The motor has no speed regulation function. The principle block diagram design of the wireless remote control circuit is shown in Figure 1.

The circuit consists of two parts: wireless transmission and wireless reception, in which the wireless transmission is composed of coding circuit and RF transmission circuit. The type of the integrated circuit used in the coding circuit is PT8A997. The circuit has 5 coding functions, of which F / B is used to control the forward and backward of the servo motor; L / R is used to control the left and right turn of the servo motor; turbo is used to accelerate.

Figure 2 and Figure 3

The decoding circuit of the wireless receiving circuit part can be completed using the PT8A978 integrated circuit chip. The demodulated RF signal is amplified and filtered in the PT8A96, and then the baseband signal is obtained. After the system samples the signal, the decoding logic can extract the F / B, L / R and the function bits from the received signal, and output the corresponding forward, backward, left-turn, right-turn and acceleration functions used by the control circuit. level. In order to meet the safety needs of toy remote control cars, an overload protection circuit should also be designed for the servo motor.

Figure 4

3 Main performance structure of PT8A997 / 978

PT8A97 / 978 is a supporting wireless remote control codec integrated circuit, they all have 5 pins, corresponding to 5 kinds of codec / decode functions. According to the difference of outline packaging, PT8A97 / 978 has three serial numbers respectively, and its series products are listed in Table 1. PT8A97 / 978 has the complete control function of the remote control car, their working voltage is 2.5 ~ 5.0V, when no function key is pressed, the chip will automatically power off, the on-chip oscillator stops working, thereby reducing the working current. The use of the encoder / decoder is very simple, and only a few external components are needed to form a complete practical circuit.

Table 1 PT8A977 / 978 series product introduction

3.1 The performance structure of the coding circuit PT8A97BP / 97BW

The internal structure and outline packaging of the coding circuit are shown in Figure 2 (a) and Figure 2 (b) respectively. It can be seen from the figure that the inside of the encoder is mainly composed of an input circuit, an encoding circuit, an oscillation circuit, a timing generator circuit and an output circuit. The input circuit has five input pins, which correspond to the five function buttons forward (backward), backword (backward), right-ward (rightward), leftward (leftward), and turbo (acceleration). The coding circuit in the chip sends digital codes to the SO and SC output pins. The digital codes correspond to the defined function keys. The SO code output terminal is used for wireless remote control, and the SC code output terminal is used for infrared remote control. A counter in the on-chip timing circuit can make the PT8A9777BP / 777BW have an automatic power-off function. Its pin PC output can be used to control the on and off status of the external working power supply. Pressing any function button will immediately activate the chip. The encoding format and word format output by the encoder are shown in Figure 3 (a) and Figure 3 (b) respectively. In the coding format, W1 represents the function code, and W2 represents the start code. The pin function descriptions of PT8A997 / 97BW are listed in Table 2.

Table 2 PT8A977BP / 977BW pin description

3.2 Functional structure of the decoding circuit PT8A978BPP / 978BW

The internal circuit structure and outline packaging of the decoding integrated circuit are shown in Figure 4 (a) and Figure 4 (b) respectively. The decoding integrated circuit is much more complicated than the encoding integrated circuit. It is mainly composed of three groups of amplifiers, signal sampling and error detection, decoding circuit, control logic circuit, oscillator, timing generator, latch, and output circuit. PT8A978BPP / 978BW has 5 output pins, each with 5 functions. The received signal is amplified by a three-stage amplifier, and then the signal sampling, error detection and decoding are performed to control the operation of the remote control car. The relative error between the operating frequencies of the oscillators encoding and decoding the two chips must be less than ± 2.5%. Figure 5 shows the specific encoding and decoding timing. The pin functions of this decoder are listed in Table 3.

Table 3 PT8A978BP / 978BW pin description

4 Typical circuit of PT8A9777 / 978

4.1 Application of PT8A9777PP / 1977BW in transmitting circuit

The typical application circuit of PT8A97BPP / 97BW in the transmitting circuit is shown in Figure 6. The circuit uses 9V battery power supply, the working voltage of transistors Q1 and Q2 are both 9V, and the working voltage of integrated circuit chip is 4.7V. When the switch S1 is closed, the light-emitting diode LED1 is turned on to emit light, indicating that the working power is turned on. The 9V voltage is directly added to Q1 and Q2. At the same time, the voltage regulator circuit composed of R2 and D1 can provide the chip with a working voltage of 4.7V. Resistor R1 is used to determine the oscillation frequency of the internal oscillator OSC of the encoder; key switches L and R are used to control the left and right rotation of the remote control car; key switches F and B are used to control the forward and backward movement of the remote control car. Transistor Q1 and L1, X1, C3 form a capacitive three-point carrier oscillator, the operating frequency of the oscillator can be 27MHz or 49MHz. The coded digital signal output from the SO pin of the encoder and the carrier signal output from Q1 are simultaneously added to the base of Q2, modulated by Q2, and filtered by L3, C8, and C9, and then transmitted by antenna L4. When the carrier frequency is 27MHz, the component parameters of the RF circuit are: C3 = 47pF, C8 = 180pF, C9 = 47pF, R3 = 120kΩ, R5 = 15kΩ, L1 = L2 = 2.2μH, X1 = 27.145MHz. If the carrier frequency is 47MHz, the component parameters of the RF circuit are: C3 = 22pF, C8 = 100pF, C9 = 30pF, R3 = 68kΩ, R5 = 22kΩ, L1 = L2 = 1μH, X1 = 49.860MHz.

Image 6

4.2 The application of PT8A978BPP / 978BW in the receiving circuit

The typical application circuit of PT8A978BPP / 978BW in the receiving circuit is shown in Figure 7. This circuit uses a 6V power supply voltage. When the power switch S1 is closed, the 6V voltage is directly applied to the two sets of full-bridge drivers of the servo motors M1 and M2. Then, it is reduced to 3.3V by R10 and Zener diode D1 to serve as the working voltage of the RF receiving circuit and decoding chip. After the wireless remote control signal is received by the RF circuit composed of the antenna and Q1, it will be sent to the VI1 input pin of the decoding chip. After the received signal is decoded by the chip, it can output function signals from the pins 7? Left ?, 6? Rich ?, 11? FORWARD? And 10? Buckward? To drive the two full-bridge motor drivers, so as to drive the bridge road The transistors are turned on alternately to control the forward and reverse rotation of the servo motor. Now take the servo motor M1 as an example: when the output of the FORD pin of the decoding chip is high and the output of the Buckward pin is low, Q2, Q6, and Q5 are turned on, while Q3, Q7, and Q4 are turned off, and the power in M1 The armature current is from right to left. At this time, M1 should rotate forward; conversely, when the output of the FORD pin of the decoding chip is low and the output of the Buckward pin is high, Q2, Q6, and Q5 are turned off, and Q3, Q7 , Q4 is on, and the armature current in M1 runs from left to right. At this time, M1 should reverse. When the carrier frequency of the receiving circuit is 27MHz, the component parameters of the RF receiving circuit are as follows: C1 = 10pF, C2 = 47pF, C3 = 47pF, C4 = 3300pF, the model of Q1 is C1815-Y, R1 = 150kΩ, R2 = 680Ω, L1 = 7T, L2 = 8.2μH; and when the carrier frequency is 49MHz, the component parameters of the RF receiving circuit are as follows: C1 = 15pF, C2 = 25pF, C3 = 10pF, C4 = 2200pF, Q1 model C380-0, R1 = 180kΩ, R2 = 820Ω, L1 = 5T, L2 = 3.3μH.

Picture 7

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