Chapman Associates - 'MICRON' Miniature Command Radio System

Designed by Chapman Associates and manufactured in the U.K. by K-Tech Limited. 

Overview

‘MICRON’ is a very small radio-controlled switching system, fully CAA approved for security use on aircraft, in which the miniature Radio-Switch has the capability to acknowledge automatically any valid commands received from an operator using a hand-held Command Unit. It operates at very low power in the licence-exempt UHF band, is battery powered, and all functions are managed by intelligent low-power microprocessors.

Main features

• The ‘MICRON’ Radio-Switch produces virtually no RF radiation when receiving, which makes it ideally suited for use in locations where it may be positioned close to other very sensitive electronic equipment, and also greatly facilitates its compliance with ETSI-ETS 300 220 and similar regulations.
• The basic Radio-Switch unit is only 52mm x 36mm x 12mm and weighs just 13 grams (excluding antenna and connecting leads). When fitted in a plastic case (optional), its dimensions become 57mm x 41mm x 20mm.
• The Radio-Switch consumes only 250uA (average) when using battery economiser.
• Two different output configurations are available, each capable of switching an external DC load of up to 500mA, as detailed below:- 
  (a) A miniature latching relay which provides one fully isolated pair of changeover contacts. This latching relay only requires a short pulse to ‘flip’ it from one state to the other and then retains that position, even if power is removed, until it is ‘flipped’ back to the previous state. Two versions of the Radio-Switch using the latching-relay are available, one for use on a DC supply voltage of 4.5-9 volts and the other for 8-16 volts. 
  OR 
  (b) A solid-state high-side power switch. This version switches the ‘MICRON’ DC supply voltage (which can be anywhere in the range 5-16 volts) to its output lead whenever the Radio-Switch is commanded to the ‘ON’ position. It is totally silent in operation but does not provide the fully isolated switching offered by the latched-relay version. Its switched output voltage will be 1.0-1.2 volts less than the DC supply voltage and, although it is protected against reversal of the DC supply, its output circuitry is not protected against damage that could be caused by a prolonged current overload.
• All units use a very secure Family Identity code which is workshop re-programmable, together with a Unit Identity code (1, 2, 3 or 4) which can be set or altered by the user.
• Up to four Radio-Switches, all programmed with the same Family Identity code but different Unit Identities, can be independently controlled by the same Command Unit.

The Radio-Switch

The Radio-Switch has no On/Off switch. When deployed, and using the battery economiser mode, the receiver is normally switched on for around 120 milliseconds every 2 seconds (i.e: a 6% duty-cycle) until a valid code is recognised, after which it then 'locks-on' and decodes the data received. This means, however, that any transmission from the Command Unit has to last for at least 2 seconds to ensure that it will be successfully 'captured' and decoded by the receiver.Once a valid transmission has been recognised by the Radio-Switch microprocessor, as soon as that transmission ceases the Radio-Switch changes to ‘transmit’ mode and an acknowledgement signal is sent. This comprises a short sequence of long or short tone ‘beeps’ which, when received by the Command Unit, inform the operator as to the new status of the output switch (ON or OFF). NOTE: The answer-back signal can be inhibited by removing the small RED jumper link.Following receipt of any valid command, the Radio-Switch stays ‘awake' for a further 10 seconds in case additional commands are to be sent. Any transmissions made during this period can be of quite short duration since the receiver is already 'awake'. Ten seconds after receiving the last valid transmission, the receiver reverts again to battery economiser mode.The Radio-Switch (if operating continuously) draws a current of around 4mA but, by using its battery economiser facility, this can be reduced to an average of around 250uA. If receiver battery power economy is not an important consideration and a faster response is required, the economiser can be completely disabled by fitting the YELLOW shorting-link.Visible indicatorsTo enable the operation of the Radio-Switch to be easily monitored, three small LED’s are mounted on the PCB. They indicate the following:-Green LED Lights (quite dimly, in order to minimise unnecessary current drain) whenever the Radio-Switch receiver or transmitter is actually being powered. When using the battery economiser this LED should flash at regular intervals until a valid signal is received. It should be continuously lit if the battery economiser has been over-ridden. Yellow LED Lights (dimly, as described above) whenever a signal bearing the same Family Code (but no necessarily with the correct Unit Identity) is being received. Red LED If fitted, this LED lights when an ‘ON’ command has been received and stays lit while the Radio-Switch is powered until an ‘OFF’ command is received. 

The Command Unit

The Command Unit is housed in a black ABS case 146mm x 91mm x 34mm and powered by an internal 6 volt battery pack, comprising 4 x MN1500 (AA) batteries. It is designed to be hand-held and very simple to use. A quarter-wave whip antenna is attached to it by means of a BNC connector.On the front panel of the Command Unit is a rotary switch which provides On/Off switching and Identity selection of the Radio-Switch to be controlled. By pressing one of the two large push buttons, one Red and the other Black, the transmitter can be activated to send the appropriate ‘On’ or ‘Off’ command to the distant Radio-Switch. Whenever the transmitter is operating the two-colour LED at the top of the front-panel will light up red.As soon as the large push-button is released, the Command Unit’s receiver will be powered for a few seconds, the LED will change to green, and the acknowledgement from the Radio-Switch (if sent) should be heard on the built-in loudspeaker. Volume can be adjusted by means of a rotary control.In order to listen on the radio channel and check for possible interference, the receiver can also be switched on by holding down the small push-button adjacent to the Unit Identity selector switch. The LED will light up green when this is done also.When operating the Command Unit, if the LED fails to light, or is lit dimly when a button is pressed, the internal batteries probably need to be replaced.Even though the Command Unit has an On/Off switch (part of the Unit Identity switch), its quiescent current in ‘standby’ (not transmitting or receiving) mode is less than 150uA so, even if it was not switched off after use its battery drain is minimal.