ELM334
Example Application
Figure 1 shows the ELM334 in a typical circuit that
provides both monitoring and control. An unregulated
12V supply is used to drive the relay coil and is stepped
down to 5V for the IC. The regulation isn’t essential for
this type of circuit, but it is a convenient means to
reduce the 12V, while providing some filtering from
motor noise, etc.
Operation of the LED portion of this circuit is
straight-forward. The position sensing magnetic reed
switches are connected to the 5.1KΩ pullup resistors in
order to provide a full logic swing input to the ELM334
as they operate. The 2.2KΩ 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 150Ω current limiting resistors.
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 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
high level is output at pin 5, causing the relay to pick up.
Although this circuit was designed for a very
specific purpose, there are likely to be many other
applications that it can be adapted to. Monitoring
thermostats perhaps, or water levels…
+5V
+12V
5.1KΩ
+5V
78L05
0.1µF
0.1µF
+5V
2.2KΩ
‘open’
1
8
7
6
5
green
150Ω
2
3
4
5.1KΩ
2.2KΩ
‘closed’
red
Remote LEDs
& Pushbutton Control
150Ω
+5V
+5V
10KΩ
1N4001
+12V
12V Relay
To the
motor
control
2N3906
10KΩ
2N3904
5.1KΩ
5.1KΩ
10KΩ
2N3904
2.2KΩ
2N3904
Figure 1. Typical Monitoring and Control Circuit
ELM334DSB
Elm Electronics – Circuits for the Hobbyist
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