On..
The AEI 5850 alarm system
magnetic sensor
Introduction
The AEI solarguard alarm system that I have had
since 2003 has been reliable, not suffering any false alarms. Some
things
were not quite as I expected: it's sold as a 7 zone system, but one of
these is permanently armed and another is the tamper zone so only 5
zones are usable for sensors. The special features of the zones apply
apply only to defined zones: zone 1 is the only one that can be walk
through on first entry, zone 2 is the only one the (rather loud) chime
can be used on so you can't have a chime on the front door. Zone 3 is
walk through after zone 1, zones 4 and 5 are left unarmed in part mode.
These constraints can be worked round when you know about them.
The instruction book (930-5850000A) was written by
someone who's first language is
English and it is reasonably usable
though its attempt to confuse the site code with the transmitting
frequency grates. There are 9 dip switches for the code but 8 and 9 are
in parallel so only 256 codes can be generated.
The reason that I have delved into this unit in such
detail was that one of mine started to run down batteries much more
quickly than the 8 months specified in the manual. Having found and
mended the leaky capacitor it seemed worth trying to make battery
life longer. This proved possible with a simple mod, detailed below.
Circuit description
Both the reed switch and the tamper switch
are
normally closed when the door is closed. The 40160 is a hex inverting
schmitt trigger with switch points of 1/3 and 2/3 of the supply
voltage. Is is permanently connected to the 12v battery. When the door
opens pin 9 of the 40160 rises and fires the 2 second monostable of C6
and R11. This pulls down pin 12 and the negative or gate of R5, D4
lifts pin 4 of the 40160 and powers up the Holtek 6010 encoder.
The 6010 reads the dip switches and transmits them
serially through Q1, the rf transmitter. It continues to repeat them
for the time that the 6010 is powered up by pin 4 and the TE/
(transmission enable) is low.
If the tamper switch is operated the 6010 is powered
up and transmits the switches but the zone code is now 7 indicating
tamper / panic to the control unit.
Low battery voltage is detected by ZD1, R3 and 40160
pin 1. When the voltage is low enough (6-7 volts?) pin 1 does not rise
above the 2/3 supply voltage trip point and pin 2 remains high during
transmission giving a low on AD8. This information is not transmitted
until the door is opened.
There are several odd bits of design here. The
external connector is connected across the reed switch meaning that
either one or the other can be used, a fact not obvious from the
manual. With D1 not fitted the reed switch has to stay open for long
enough to send the data - this is unlikely to be a problem. The C8, D8
components in the tamper channel seem to rely on the tiny saturation
current through D2 to keep pin 5 low. The function of ZD2 is obscure.
The use of 10 µF C6 on the TE/ pin of the 6010 to keep it low
during transmit seems eccentric.
Not obvious from the drawing is that L1 and C3 form
a tuned circuit which, given a reasonable Q, will develop a voltage
higher than the battery on C3. This may have led to C3 becoming leaky,
the repair of which is discussed below.
Battery life is given as 8 months. The A23 battery
used has a quoted capacity of 33 or 38 mAh which agrees with the
measured drain of 5.5 µA. The
5.5 µA is split between R12, R13 (10 M, 1.2µA each) and R9
(3.1 µA, so ZD2 does something). The 40106 takes no standby
current and the 6010 chip has no supply. 5.5µA is wasteful of
battery life: it should only be necessary to use enough current to
detect the opening of the reed or tamper switches. Each operation of
the door switch takes
4.5 mA for 2 seconds which is equvalent to 15 minutes of standby.
Modification to increase battery life
If the reed and tamper switches are operated in
series, only a single 1.2 µA current can detect both. If the npn
Q2 inverter is replaced by a pnp transistor the current flow through R9
when the door is closed can be stopped. The circuit below does both of
these things. Current drain is 1.2 µA equivalent to a battery
life of 27K hours or 3 years. Opening the door will trigger a drain of
4.5 mA for 2 seconds which is now equivalent to 1 hour of standby. So a
frequently used door will not have a 3 year battery life. But when a
door is open the drain will be nothing at all.
Q2 is now switched off by the high on 40106 pin 2.
The 100K Rmod pulls pin 10 low. There is no current out of pin 10 of
the 6010 when the door in closed as there is no supply to the 6010.
When the door is opened and pin 2 of the 6010 falls ZD2 is forward
biased and looks like an ordinary diode.
The only change to the operation of the unit is that
the tamper switch will no longer work when the door is open.
To make the mod
Remove Q2, R13.
Cut pin 5 of the resistor pack, adjacent to pin 10 of the 6010. It is
just possible to get a small pair of sidecutters under the pack.
Glue a scrap of vero board to the top of the 6010
Unsolder the right hand side of the reed switch, carefully bend the
lead without stressing the glass of the reed and solder it to the piece
of vero board.
Fit a pnp transistor:
Transistor base to Q2 base pcb hole
Transistor collector to Q2 collector
pcb hole
Transistor emitter to +ve battery
terminal.
Fit a 100k resistor from the Q2 emitter hole to J3
Link a wire from the scrap of vero to the tamper switch (non 0v side)
on the other side of the board.
A BC560 transistor was used but any small pnp would do.
Repair of rf circuit
The photo shows the repair to C3. As C3 is a surface
mount component there was no value on it and a short length of
insulated wire with a 0v connected wound round it was substituted. The
result had a reduced range, tested by setting the site code on the unit
to a non valid one and watching the rx light on the control unit. This
is
presumably due to the tuned circuit not being adjusted well and further
fiddling could have probably improved it but it was easier to swap the
unit for the one near the control so the reduced range was adequate.
The most common voltage rating of surface mount
capacitors is 50v. If L1, C3 resonate with a reasonable Q (Q is also
known
as the circuit magnification factor) the voltage on C3 will exceed 50v.
This would seem to be the likely cause of the problem given the
curiosities in the rest of the circuit.
Links
http://www.easylife.com/catalog/
http://www.aeisecurity.com/SG5850-434.html
16/6/07
Text © S C Smith
