Page 1 Copyright 2005 e-Gizmo Mechatronix Central
You always wanted wireless control of your project with a PC. e-Gizmo now brings you your buildingblock module-
8/16-bit RF Wireless Remote Output that you can control using your PC!
Designed and Written by Henry Chua
I met a lot of students and enthusi-asts who build or are trying to buildgadgets that they hope can be re-motely controlled by RF (Radio Fre-quency). Because of the unavailabil-ity of any locally-built RF remote cir-cuit boards, the moneyed ones sim-ply purchase this stuff from overseassuppliers, spending 12,000 pesos,perhaps a bit more.
Others resort to buying RC (RadioControlled) cars, tearing it apart, tak-ing only the much needed RF remoteparts. A cheap RC car with two outputfunctions can cost as little as 600 pe-sos. This seems to be attractive enough to jump intothis solution, except that:
Unless you are an experienced electronics geek, find-ing the correct output and matching it with your circuitcan be very tricky.
Most R/C toys operate on 27MHz to 49MHz frequen-cies. If you are planning to use a microcontroller or mi-croprocessor with your wireless, then you have a prob-lem. As those who already tried this found out,microcontroller circuits generate all sorts of RF noiseinterference within these frequency bands. This severelyaffects the reception ability of the receiver circuit, re-ducing the control distance range to only a couple ofmeter or so. Of course, a wireless controlling distancewithin your arm's reach will not look impressive at all.
Our wireless kit is designed to be free from these prob-lems. It operates at 433MHz unlicensed ISM (IndustrialScientific Medical) frequency, far from the interfering sig-nal frequencies coming out of your microcontroller cir-cuits. It is easy to use, fully documented. All available
Frequency: 433MHz nominal430-439 MHz
Output Power:0dBm (1mW)
Harmonics:2nd < -15dB3rd up < -25db
Sensitivity:8uV @ 2.4Khz 80%mod
RF Bandwidth (-6db):2.4 MHz typ
Adjacent Frequency Rejection fo +/- 5MHz:- 55dB
Control Distance:> 100ft in open space
Copyright 2005 by e-Gizmo Mechatronix CentralAll rights reserved.
No part of this publication may be reproduced in any form without the written consent of e-Gizmo Mechatronix. Content subject to change without prior notice.All informations contained herein are believed to be correct and reliable.
Before using this document, you must agree with the following terms and conditions:1. e-Gizmo Mechatronix and the author cannot be held liable for any damage that may occur with the use or misuse of any information contained in this document.
2. You are allowed to reproduce this publication and the product it describes for personal use only. Commercial reproduction is prohibited!
8/16-bit Wireless Remote Output Page 2
Figure 1. Complete schematic diagram of the transmitter unit.
Power to the transmitter is directly drawn from the RS-232C lines. With pin 4 (DTR) output permanently stay-ing at the V+ side, the transmitter can be switched ONand OFF through pin 3's (TX) output of the serial port.In other words, the serial data output itself turns ONand OFF the transmitter, effecting a Amplitude Modu-lated system. D1 keeps the voltage to the transmitterfrom exceeding 5.1V, at the same time, limits the volt-
Figure 2. The receiver circuit functional block diagram
I/Os are fully explained with some interfacing examples.The transmitters plugs into one of your PC serial comport. It is Visual Basic friendly, 10 lines of code is enoughto operate it.
The wireless kit consists of two subsystems, the trans-mitter circuit, and the receiver/decoder circuit.
The circuit schematic of the transmitter is shown on Fig.1. The circuit is remarkably simple, thanks to the RFmodule UC1817. I used this module because (aftersome modification) of its good frequency stability overtemperature and time, although it contains no saw filtercomponent at all. Of course, a 433 MHz saw filter-stabi-lized oscillator circuit would give much better frequencystability, but you'll need a load of luck to find one in thelocal hobby market.
CONN DSUB 9-R
age to -0.6V in case pin 4's output suddenly decides togo negative.
The receiver circuit is a bit more complicated, it will makemore sense if we describe the circuit in its block dia-gram form as shown in Figure 2. The complete sche-matic of the receiver is shown on Figure 2a on page 3.
Page 3 Copyright 2005 e-Gizmo Mechatronix Central
CK1 2 3
11 12 13 1 2 3 415 16 17 18
8 9 10
8/16-bit Wireless Remote Output Page 4
We all know the function of the antenna; it grabs what-ever radio signal wanders in its reach. Because of itsfinite length, it favors radio signals with frequencies thatfalls within half wavelength, which, by design, is433MHz.
The receiver is more picky. It will work only on 433MHzsignals, totally ignoring all others. The very weak signalfrom the antenna is greatly amplified, at the same timedemodulated in this block, resulting in the recovereddata signal from the receiver output. The receiver blockconsists of components Q1, R1 to R4, C1 to C7, L1, L2and D1.
The recovered data signal however is still too weak andis mixed with all sorts of noise. The amplifier stageboosts the signal to a more usable level, using to itsadvantage its inability to amplify high frequency noise,thereby improving its signal to noise ratio. Then, thispreconditioned signal is fed to a pulse shaper wheredata signal is reconstructed into nice digital signals themicrocontroller can now understand. The amplifier cir-cuit consists of U2A and associated components, thepulse shaper is formed by U2B and associated partswired as Schmitt trigger.
The microcontroller then assembles the received datainto an 8-bit data format. As wireless transmission isvery prone to error due to unaccountable external influ-ences, extra bits are transmitted and received as a formof error checking. If everything matches the error-check-ing code, then the received data is fed to its 8-bit outputport. A successful data transmission is indicated by aflashing LED indicator in the receiver board. Data trans-mission format is discussed in detail in the Program-ming section. The microcontroller section is based on aZ86E02 microcontroller U1. U3 converts the DC supplyinput into a stable 5V source to supply the whole circuit.
Part of the circuit, the receiver circuit in particular, worksat a very high RF frequency (UHF 433 MHz). The choiceof components becomes very important. At this fre-quency, the PCB layout becomes very much part of thecircuit. Each component is carefully laid out to minimizeunwanted interaction against each other. Merely repo-sitioning these components can have an astonishingeffect on the performance of the circuit. Many peoplewho do not understand this suspects that RF design isengineering mixed with witchcraft, or maybe the otherway around.
Figure 3. Component side of the receiver module.
Figure 4. Bottom side of the receiver module. Fourcomponents are soldered on this side; U1, U3, Q1, and
Receiver Board Assembly
Use a PCB layout pattern which is a faithful reproduc-tion of the pattern shown in the downloads. If you choosenot to, you may be buying yourself into trouble youmust be certain you know what you are doing.
Treat each component with reasonable care. The ICsare particularly susceptible to damage due to ESD (Elec-trostatic Discharge) and must be handled properly. Sol-dering workmanship is very important, this project mustbe built by a soldering iron master!
General PCB Assembly procedurewww.e-gizmo.com/ARTICLES/ProjectB/Assembly.htm
Page 5 Copyright 2005 e-Gizmo Mechatronix Central
Mount and solder the components in the following or-der : resistor, capac