Example Applications (continued)
Figure 4 carries the circuit of Figure 3 a little
farther, creating an alarm type circuit. We have used a
dual red/green LED this time, and also connected the
green LED output to the pushbutton input. In this way,
a momentary opening of the door switch will create a
pushbutton input, when the green LED goes off. Once
the green LED is off, it will remain that way, even if the
door contact should close (as internally a pushbutton
input always forces the green LED off, and the red
LED on). Note that there will be a 0.5 second pulse at
the Control output (pin 5) when the door first opens -
this might be used to trigger an audible alarm.
now added a switch to indicate that the door is fully
open. The position sensing switches (possibly
magnetic reeds) are connected to the 5.1KW pullup
resistors in order to provide a full logic swing input to
the ELM334 as they operate. The 2.2KW series
resistors provide some protection for the chip as the
wires to the switches are likely to be lengthy, and
susceptible to induced voltages and currents. After
processing, the appropriate voltages appear at pins 2
and 3, driving the LEDs through the 150W current
limiting resistors. Since the supply is now 5V, we have
increased the LED resistors slightly in order to
maintain roughly the same current.
This monitor circuit could be used for a door of any
type, not just a garage door. Perhaps a shed, or a
storage bin. A contact such as from a thermostat could
also be connected to warn that a temperature has
gone above or below a set limit. To reset this circuit
requires turning the power off then on.
The control portion of the circuit may appear to be
a little odd-looking at first. To understand its operation,
note that one of the two driven LEDs is always on,
whether flashing or solid. Due to the connection of the
two NPN transistors then, one of the NPNs is always
biased on, keeping the PNP on, and pin 4 of the
ELM334 at 5V. When the remote pushbutton is
pressed, the LED circuit is shorted out, and neither
NPN can conduct. The PNP thus shuts off, and pin 4
of the IC drops to 0V, its active level. With the PB input
active, a pulse is output at pin 5, causing the relay to
pick up for 0.5 seconds.
The circuits so far have used a battery to supply
power. The problem with this is that a pair of AA cells
will only last a week or two in such an application. To
avoid always having to monitor the monitor, it would be
good to power the circuit from a different supply that is
derived from the main AC service. Figure 5 on the next
page shows a circuit that assumes you are able to
obtain 12V from a source (most likely an AC adapter).
This allows generating a 5V supply for the ELM334
while also providing a higher voltage that is suitable for
driving a relay (from the Control output).
Although this circuit was designed to monitor
doors, there are likely to be many other applications
that it can be adapted to. Monitoring thermostats, or
light levels, or water levels, for example.
Operation of the LED portion of Figure 5 is very
similar to the previous circuits, except that we have
+3V
3V
0.1µF
+3V
100W
1
2
3
4
8
7
6
5
5.1KW
2.2KW
green
red
100W
door
closed
Figure 4. A Remote Monitor with Memory
ELM334DSC
Elm Electronics – Circuits for the Hobbyist
6 of 7
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