MINDA - The Miniature Duress Alarm system
Designed by Chapman Associates and manufactured in the U.K. by K-Tech Limited.This page is divided into a section of general information, followed by details of the range of MINDA transmitters and receivers, the transmission range of the system, the group identity coding and the re-programming of options.
General
MINDA is a family of versatile, miniature radio alarm transmitters and receivers. By selecting the appropriate transmitter, a person is able to signal discreetly to a guardian or assistant close-by or, alternatively, set up a short-range protection, anti-theft or intruder detection system. The smallest transmitters in the range are matchbox-sized and can be conveniently carried by, for instance, a VIP who may wish to call for his bodyguard, chauffeur or secretary without attracting the attention of other people around him.The small portable receiver will easily fit into a pocket or can be worn on a belt. A larger receiver (Maxi-MINDA) designed for desktop or mobile use is also available, and has a number of additional facilities. Either receiver can indicate the status of up to four suitably coded transmitters and will, if desired, emit an audible bleep when a signal is received. The portable receiver can also be supplied with a built-in vibrator for near-silent alert.
MINDA operates in the 418MHz or 433MHz radio frequency band, which the UK Radiocommunications Agency has allocated for general short-range operation by low-power security systems and car alarms. MINDA is licence-exempt when used in the UK since the transmitter and receiver modules have been built to, and are being used in compliance with, the R.A. Specification MPT 1340.
MINDA Transmitters
The original MINDA transmitter only transmits a signal when one of its buttons is pressed. This simple unit is still a current production item and has now been joined by three new 'intelligent' transmitters, offering many additional features, including the automatic transmission of confidence signals. The full range of transmitters currently available is:
Miniature key-fob transmitters
The TX-400 is a basic 'dumb' transmitter which has two status buttons
(coloured black and red) and a small red LED which lights when a button is
depressed to indicate that the unit is functioning. The antenna is a short
(13cm long) flexible wire. The TX-500 is externally similar (but with red
and blue buttons instead of red and black) but is 'intelligent', being
controlled by a very low-power microprocessor. Both key-fob units measure
61mm x 37mm x 15mm (excluding antenna) and weigh 35 grams when fitted with
battery.
Micro-switch operated transmitter
Slightly larger than the key-fob units (85mm x 45mm x 16mm - weight 50 grams
with battery), this 'intelligent' transmitter can be activated by the
operation of an on-board micro-switch or an external wired contact. Designed
primarily as an anti-theft alarm transmitter, this unit can be set up so
that an alarm is transmitted when a door or window is opened or an object
placed next to the micro-switch is moved. The antenna is a short (13cm
long) flexible wire.
Light-sensitive transmitter
Of similar size to the micro-switch operated transmitter, this 'intelligent'
unit is activated by a change in the ambient light level. This unit can be
used to send an alarm when a drawer, cupboard or other normally dark
enclosure is opened and light falls on the optical sensor.
Both the micro-switch and light-sensitive transmitters can be powered from an external 12 volt DC supply instead of an internal battery, if required. On these units, the red and yellow LEDs can only be seen when the case is opened.
Transmitter module
This module, comprising an encoder and a transmitter PCB, can be
manufactured in either a side-by-side (approx. 31mm x 26mm x 5mm) or a
back-to-back (approx. 31mm x 14mm x 10mm) arrangement. It is supplied as an
unpackaged PCB so that it can be incorporated into a housing of the
customer's choosing. The module requires that external alarm switch(es), a
short wire antenna, and a 6-12 volt battery supply be connected in order for
it to function.
Transmitter low-battery warning
Once the battery in any of the transmitters being used falls in voltage such
that it is in need of replacement, the red LED will light only dimly (or not
at all) when a button is pressed and all subsequent transmissions sent by
that unit will be labelled to this effect until the battery is replaced.
Once this signal has been detected by a MINDA receiver, its LED indicator
corresponding to that transmitter Unit Identity will start to blink out
briefly about once per second. NOTE: No such low-battery warning can be
sent by a transmitter whose battery has become completely discharged.
Intelligent transmitters - additional features
* Unit Identities
A Re-MINDA intelligent transmitter can have each of its four possible Unit
Identities (1, 2, 3 or 4) configured to signal in different ways and with
different timings to suit particular operational needs.
The Identity Coding consists of two parts - the Family Identity and the Unit Identity.
The Family Identity Code is preset when each transmitter unit is programmed and can only be changed by using the MINDA programming interface kit in conjunction with a PC.
The Unit Identity Code can be changed easily to any one of the four possible identities (1,2,3 or 4) by opening the case and moving two small internal switches. NOTE: The Unit Identity of a miniature PCB transmitter module is preset during manufacture and cannot easily be changed by an unskilled user.
If required, the Unit Identity of a key-fob style Re-MINDA transmitter can be temporarily altered by the user without opening the case. This latter option can be selected or inhibited by programming, and is also available on the miniature PCB transmitter module design.
* Confidence Signalling
A MINDA intelligent transmitter can be programmed so that for any or all of
the four identities it can transmit a regular confidence (green)
transmissions as an "I'm here and OK" signal.
A Maxi-MINDA or portable MINDA receiver can then, if required, be programmed to remain silent whenever a valid green transmission is received, and signal an audible alarm only if a red alarm is received or if no confidence signal has been received from one or more of its associated transmitters within a pre-designated period of time.
In a situation where a receiver and an intelligent Re-MINDA transmitter are being used to protect a person close-by but out of sight, the regular confidence transmission will keep the receiver silenced all the while it is present. If the transmitter moves out of radio range, its battery fails, or it is otherwise disabled the receiver will emit an alarm signal as soon as the confidence signal has been missing for the pre-programmed period of time.
This enables, for example, a VIP to be protected at night by a bodyguard in an adjoining room or a person going into a potentially hazardous situation by a backup team in a vehicle close-by. The intelligent MINDA transmitter will send the confidence signals silently and entirely automatically without disturbing the user unless he, or she, wishes to send a red alarm signal in the usual way.
MINDA Receivers
Both the portable and the Maxi-MINDA desktop/mobile receivers are entirely software-controlled by a low-power micro-processor which manages all of the housekeeping associated with decoding the received data signals, activating the appropriate status and alarm indicators, and controlling the power to the receiver for battery saving.
Status Visual Display
Each receiver can display two status indications from up to four different
transmitters, either 'dumb' TX-400 or Re-MINDA TX-500 intelligent units, or
a mixture of the two types, coded 1-4 and all operating with the same Family
Code. Pressing the red button on a key-fob style transmitter should light
the appropriate red LED on the receiver, whilst the black (or blue on the
TX-500) one lights the green LED.
A receiver will only respond to signals from those transmitters sending the same Family Code identity as the one that it has been pre-programmed to accept. This minimises the risk of interference between similar systems that happen to be operating within radio range of one another.
It is absolutely vital therefore, to ensure that all transmitter and receiver units that have to operate together are programmed correctly with the same Family Code. Any signals received from an 'alien' transmitter operating within range will not be decoded and, instead, treated by the receiver as radio interference on the channel.
Battery Economiser
The battery economiser operates by switching the receiver on for a very
brief period, looking for a signal, and if nothing is found switching it off
again. If a signal is found, the receiver locks on for long enough to
process and display any alarm signal that may have been received. When the
economiser is operating the receiver is only switched on for about two
percent of the time, thereby dramatically extending the operating endurance
before a change of battery is required.
The slight penalty for this feature is that an incoming alarm transmission must last long enough to catch the receiver during one of its 'on' periods to ensure that it is recognised and decoded. In practice this should not cause any problems provided that the transmitter button is held down for at least one second. All intelligent TX-500 Re-MINDA transmitters automatically transmit a signal long enough for this to occur, and possible problems when using a 'dumb' MINDA (TX-400) transmitter can be forestalled by ensuring that the button is always pressed for at least one second. If the battery-saver facility on the receiver is not considered to be an advantage, it can be disabled by a small internal modification.
Low-battery alert
A valid transmission received from a transmitter with a weak, but not
completely flat, battery will cause the appropriate LED indicator on the
receiver to blink out briefly about once per second. This will indicate to
the operator that that particular transmitter should have its battery
replaced at the earliest opportunity.
Jamming alert
Any signal received will cause the receiver to lock on, whether it is local
electrical interference, a transmission with a different group code, or a
wanted signal from a transmitter on your own group code. However, if a
receiver is jammed by a continuous unrecognised signal, then after a
pre-programmed period of time (supplied set to 5 seconds but can be
reprogrammed by using appropriate equipment) it produces a series of audible
beeps to alert the user and enable him to move to an interference-free
location. As soon as the interference clears the beeps will cease.
Resetting
Pressing the RESET button cancels any alert and extinguishes any LEDs that
may be lit.
Portable MINDA Receiver
This version has two user-selectable modes of operation, 'BEEP' and 'SILENT', which are chosen by moving the power On/Off switch in the appropriate direction. When in 'BEEP' mode, if a wanted signal is received the appropriate status LED will light and an audible pulsed tone alarm will be heard. The tempo of this pulsed tone is faster for a red alarm than for a green alarm, thereby facilitating the aural identification of urgent and non-urgent transmissions. When the 'SILENT' mode is selected, all audible tone signals will be inhibited and signalling is done near-silently by means of a built-in vibrator!
Maxi-MINDA Desktop / Mobile Receiver
This larger unit can be used on a desk and powered from AC mains using a suitable DC power unit, or installed in a vehicle and connected to the vehicle supply. A simple quarter-wave whip antenna can be directly attached to the rear of the receiver or a better, elevated, one connected by means of coaxial cable when greater reception range is required.
Economiser
As supplied, unless a customer has requested otherwise, a Maxi-MINDA
receiver normally has its battery economiser enabled whilst it is operating
on its own internal battery and disabled (ie. receiver runs continuously)
once external supplies are present. This arrangement can be changed, if
required, by re-programming the receiver.
External Power
If an external power source of greater than 10 volts, and of correct
polarity, is connected to the Maxi-MINDA, the receiver will automatically
use this supply in preference to its internal battery. If for any reason
the external supply fails (maybe due to mains failure) the unit then falls
back to its internal battery and continues operating normally.
Alerts
When the Maxi-MINDA is using its internal battery the alarm tone is produced
by a piezo beeper. When an external power supply is being used an additional
high power amplifier driving an internal loudspeaker is automatically
enabled.
The loudness of the alarm signals produced by the piezo beeper cannot be adjusted. If desired, the beeper can be switched off completely so that all audible alarm tones are produced only through the loudspeaker with their loudness (from silence to full-volume) being adjusted by means of the front panel volume control. Although the piezo beeper can be disabled, this should only be done when the receiver is always to be powered from an external source.
A Maxi-MINDA has a 'Green Alarm' switch on the front panel, adjacent to the 'On/Off' switch, which enables an operator to select whether or not a green alarm signal is accompanied by an audible warning signal.
External connections (on rear panel):
* BNC socket for antenna connection.
* 'Ext.DC Input' connector for an external DC supply in the range 10-16 volts (negative ground).
* 'External Speaker' connector, a 3.5mm mono jack socket, to allow an external loudspeaker (8 ohms or greater) to be used to produce the alarm tones in place of the internal one.
* PC interface connector for programming.
* Connector for programming a TX-500 Re-MINDA transmitter or RX-400 MINDA portable receiver (using the Maxi-MINDA on-board serial interface).
* A terminal strip offering:
- The drain-source output of a FET, capable of switching up to 300mA, which switches on whenever a red alarm signal from any of the four transmitter Unit Identities is recognised by the Maxi-MINDA.
- Isolated single-pole changeover contacts of an internal relay which can be activated, if required, by the switching of the above FET.
- An 'Inhibit' line to remotely inhibit any switched output without resetting the Maxi-MINDA.
Transmission Range
When using a MINDA transmitter over a line-of-sight path to a portable receiver using its attached wire antenna, a range of greater than 100 metres is likely, whereas in a busy street, hotel or station the range will be noticeably less. If a Maxi-MINDA receiver is used with a well-positioned and efficient antenna a working range of up to 400 metres is possible under ideal conditions. The useful working range of any radio system is dependent on many external factors. It is therefore recommended that, before using MINDA in a live operational situation, a few tests are carried out in the location(s) of interest to confirm consistent and reliable operation.
Group Identity Coding
Each MINDA system is supplied already programmed with a specific Group Identity. A MINDA receiver will only respond to an alarm signal if the group code of the transmitter that it is receiving exactly matches the code that it has been programmed to accept. This capability minimises the risk of interference between similar systems that happen to be operating within radio range of one another.The actual digital data message transmitted contains all of the following pieces of information, repeated a number of times:
- I am a MINDA transmitter
- My Group Identity is 'xxx'
- I am Unit Identity 'y'
- My alarm status is 'Red/Green'
- My battery status is 'OK/Low'
Re-programming of Options
Re-programming of any MINDA equipment (with the exception of the TX-400 Key-Fob which has no programmable features) requires specialist electronic knowledge, a PC and peripheral equipment. For this reason, most customers will find it more convenient to return the equipment to K-Tech for re-programming.Customers with a significant number of MINDA units and who also have their own electronic workshop, with access to a PC and peripherals, may wish to consider purchasing the special MINDA programming kit which comprises hardware interface, custom software on disk, and full instructions. Further details on this product can be obtained from K-Tech.
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