PICAXE 433MHz data transmitter & receiver
The 433MHz data transmitter circuit is shown at left, while below is the receiver circuit. Note that the transmitter supply should be regulated for accurate ADC readings.
This design uses a pair of PICAXE08M2 microcontrollers and low-cost 433MHz ASK transmitter and receiver modules to provide a wireless remote data monitor with LCD readout.
The transmitter comprises a PICAXE08M2 that monitors DC voltages via potentiometer VR1. The micro’s internal ADC converts the DC voltage at pin 5 to a digital signal with 10-bit resolution and sends the data, along with a station identifier and checksum, to a 433MHz ASK transmitter module (Jaycar ZW-3100). The data is transmitted at 2400 baud and the period between transmissions can be set in software in one-second increments up to over 18 hours. The initial rate is 15 seconds.
During the break period, transmission is off to reduce power consumption. The transmitter and micro are powered from via two alkaline cells giving a nominal 3V. However, for accurate analog readings the DC supply should be regulated as the digital count will vary as the battery voltage changes. Standing current is of the order of 1mA and less than 10mA during the short transmission bursts.
The receiver comprises a PICAXE08M2, a 433MHz ASK receiver module (Jaycar ZW-3102), an MCP23008 I2C port expander (available from www.futurlec.com) and a 2-line LCD display.
The PICAXE monitors the data from the 433MHz receiver module. When a valid data packet is received (station identified correctly and checksum correct), the LED connected to port 4 (pin 3) is flashed and the received data is processed and sent via I2C protocol to the 8-bit I2C port expander which interfaces to a 2-line 16 character LCD display. The display shows a text message, the data and a packet counter to confirm that all is working correctly. The LCD module is operated in 4-bit mode.
The 433MHz receiver has no muting facility. Hence, during periods of no-signal, the AGC sets the gain to maximum and there is a high level of noise on the data line. To ensure reliable reception, the transmitter is switched on for a short duration to allow the receiver AGC to operate and then briefly low again with the receiver AGC still active, thus eliminating output noise before the data stream is sent.
The software can be easily modified for other purposes, for example to send text messages, water level monitoring etc. A number of transmitter and receiver pairs could operate simultaneously using different station identification and slightly different transmit periods. A PICAXE14M2 with software changes could be used in lieu of the PICAXE08M2 to avoid the need for the port expander.
The range is at least 200 metres outdoors and 25 metres indoors. As the operating frequency is in the Industrial, Scientific and Medical band that does not require licencing, other devices such as door bell, garage door and keyless car entry transmitters could interfere with operation. However, these transmitters are generally not continuous and a data packet would only be lost if one of these other units was transmitting physically close to and concurrently with the data transmission from this circuit.
Data reception reliability is high due to the use of station ID and checksum. Data security is low as the transmit packet can be monitored by others. A simple encryption routine has been implemented that would puzzle anyone who had the time and inclination to eavesdrop on the data stream.
A 3-way pin header (ICSP SKT) and two resistors provide a simple PICAXE programming interface on both the transmitter and receiver circuits.
The software (433MHz Tx-Code.bas and 433MHz Rx-Code.bas) can be downloaded from the SILICON CHIP website.
Hope Valley, SA. ($60)