ELM365
Example Application
Figure 1 shows the ELM365 as the controller in a
typical home security system.
The series resistors on pins 4, 6, and 7 have been
included in this design for protection from electrostatic
discharge, or induced currents. Although the circuit will
function correctly without them, they are recommended
for the added protection that they afford.
Series connected magnetic switches monitor door
positions, and connect to the delayed loop input. Several
window switches are connected in series to the instant
loop input, and panic switches are provided in the
bedrooms to cause an alarm when pressed.
The circuit supports two outputs, and both have
been included with this design. The pulsed output
drives an alarm buzzer, while the constant output can
drive yard lights, etc. Both outputs use a conventional
transistor circuit to energize a 12V relay coil.
Four LEDs are included in the circuit to show power
on (circuit armed), alarm, and loop status. In addition to
status, these LEDs also provide loop current for switch
wetting, and assist with the input pullup function.
Although not essential, it is recommended that the LEDs
be installed.
Power for the circuit is from an AC supplied 12V
DC source, with only temporary backup provided by a
9V battery. Both inputs are diode coupled, passed
through the key switch, and reduced by the 78L05
regulator to 5V for the logic.
+5V
Panic
Switches
2.2KW
+12V
2.2KW
+5V
+5V
1N4001
Alarm
Constant
Output
2.2KW
300W
Instant
Loop
300W
5
6
7
8
4
3
2
1
2N3904
1.0KW
+5V
+12V
Delayed
Loop
Power
1N4001
300W
Pulsed
Output
2.2KW
300W
1.0KW
Key Switch
2N3904
(Arm/Disarm)
Normal
12V Supply
+5V
0.1µF
78L05
0.1µF
Optional 9V
Backup Supply
Figure 1. Home Security System
ELM365DSB
Elm Electronics – Circuits for the Hobbyist
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