Four-way remote control uses series transmission

JM Terrade, Clermont-Ferrand, France -- EDN, 1/18/2001

A simultaneous four-way remote-control system adheres to size, cost, and reduced-complexity constraints and uses a series transmission to drive parallel loads (Figure 1). You can use this system as long as the time constant of the load is much larger than the total transmission time for all data. With these considerations, this design can drive any object with four simultaneous controls as motors.

The design uses a 9-bit data packet. The emitter side of the design converts 4 data bits and a 5-bit ID code from parallel to serial. The data packet continuously transmits, and the total information arrives at the HF 433-MHz emitter. The receiver side converts the 9-bit serial data to parallel data. Then, the design compares the received ID code to the local code. The comparison result clocks the 4 data bits for the D latch. This configuration actually controls a small, battery-powered boat with two-way, remote-control switches. The switches are mom-off-mom types, which give front-stop-rear and left-center-right commands. The boat has two dc motors for propulsion and direction. The transmission uses two 433-MHz, AM-radio modules for the HF link.

Power consumption is 10 mA during emission, so the emitter circuit can use a 9V battery (Figure 2a). D₁ protects the device against polarity inversion. S₁ and S₂ are three-position, mom-off-mom switches. Only the center, or null, position is static. The user must push the switch in one direction and maintain it to keep the desired action. When released, the switch returns to its null position. With no action on S₁ and S₂, the logic levels on data inputs D₆ to D₉ of IC₁ are low due to R₃ to R₆. When an action occurs on S₁ or S₂, the corresponding data input of IC₁ is close to 5V. You can activate S₁ and S₂ at same time. Voltage-divider pairs R₁ and R₃ or R₁ and R₄ and R₂ and R₅ or R₂ and R₆ produce acceptable levels for IC₁ inputs.

Diodes D₂ to D₅ permit C₁ to charge through R₇. Then, Q₁ conducts, and Q₂ is on. D₆ acts as a power-on indicator. The voltage drop across D₆, R₉, and zener-diode D₇ results in a 5V supply for IC₁ and IC₂. C₁ continuously charges until S₁ and S₂ return to the null position. Then, C₁ discharges through R₈, and Q₁ switches off after approximately 8 to 10 sec (Figure 2b).

Inputs A₁ to A₅ of IC₁ are three-state inputs: low, high, and unconnected level. Thus, 243 combinations (3⁵) are possible. However, three-state DIP switches are expensive, and 64 possibilities are enough for many applications. If Pin 6 of S₃ provides a low level, A₁ to A₅ can be either low levels or unconnected. If Pin 6 of S₃ provides a high level through R₁₀, A₁ to A₅ can be either high levels or unconnected. This arrangement gives 64 combinations.

R₁₁, R₁₂, and C₂ form the local oscillator. The output of IC₁ at Pin 15 provides the 9-bit data packet to the HF emitter, IC₂. The HF module uses amplitude modulation. The antenna is a 17-cm wire that attaches directly to the pc board. When the power is on, transmission always occurs. After a user releases S₁ and S₂, the emitter continues to transmit the null-position information until power goes off, which takes approximately 8 sec.

On the receiver side (Figure 3a), the antenna is also a 17-cm wire attached directly to the pc board. The incoming signal arrives at the HF module, IC₁, which has a stable 5V power source. The 9-bit data packet is available at the output, or Pin 14, of the module. Just as for the emitter, DIP switch S₁ provides as many as 64 possibilities for the ID code, and the setting must be the same combination as the emitter.

The 4 data bits are available at outputs D₆ to D₉ of IC₂. When a valid transmission arrives at the receiver, Pin 11 of IC₂ goes high. But each time a user changes the position of the commands on the emitter, the Valid-T signal goes low until the new transmission is valid. Three correct transmissions are necessary. Therefore, the design needs a stable RX_OK signal, and, for this reason D₁, R₁, R₂, and C₁ create a time constant. The RX_OK signal goes low only when the transmission stops or when the ID code is invalid, which can happen if the emitter has no supply and stops emitting or if another transmitter is in the same area (Figure 3b).

The internal D latch, IC₂, clocks new output levels only when the circuit receives a new data packet. In this way, when only one transmitted bit changes, the other bits keep their previous level. When the ID code is not valid or when the HF link is lost, which implies that the distance between the emitter and the receiver is too long, D₆ to D₉ keep their previous levels. However, RX_OK goes low after 70 msec and forces D₆ to D₉ to go low.