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ELM334DSC

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

Example Applications

The following pages show a few circuits that may

help you get started with the ELM334. They show

everything from a minimal circuit to a typical full

implementation

that

you

may

wish

to

try.

We

encourage experimentation, and hope that you enjoy

doing so.

The first circuit (Figure 2) shows the absolute

minimum required to use the ELM334 IC. A single

(normally closed) contact is connected between pin 6

and circuit common, while a single green LED is

connected between pins 2 and 3. We do not use a

current limiting resistor in series with the LED, as the

ELM334 has fairly high output resistance when

operated with a 3V supply, and will limit the LED

current. Two ‘AA’ batteries in series provide the 3V

source to operate the circuit.

Normally, the door closed switch is closed when

the door is, and opens when the door does (this could

be a magnetic switch, or a mechanical contact). When

the door opens then, the contact does too, and the pin

6 input goes high. With the pin 7 input low, the green

LED will turn off (see Figure 1 for the pin logic). If we

had connected a dual red/green LED, the red LED

would turn on when the door opens.

Figure 2 would likely work well if experimenting on

the bench, but really requires more circuitry to make it

more reliable for ‘real world’ operation. Figure 3 shows

the recommended additions to make a more reliable

circuit. Note that we have now shown a ground symbol

to represent the circuit common (battery negative), just

to simplify the schematic. This new circuit shows a

capacitor across the IC supply terminals, to prevent

noise generated internally by the ELM334 from

affecting the supply voltage, and thus the complete

circuit. We’ve also added a few resistors on both the

input and the output sides of the IC, mostly to protect it

from the effects of induced voltages and currents

which can cause a phenomenon called ‘latch-up’ in

some CMOS circuits. If the door closed switch or the

LED is more than a few feet away from the ELM334,

these extra resistors should be added.

Functionally, the circuit in Figure 3 operates in

exactly the same manner as the one in Figure 2.

Figure 2. Absolute Minimum ELM334 Circuit

Figure 3. A Better Minimal ELM334 Circuit

1

2

3

4

8

7

6

5

green

door

closed

3V

green

door

closed

3V

+3V

2.2K

Ω

5.1K

Ω

+3V

1

2

3

4

8

7

6

5

+3V

0.1µF

100

Ω

100

Ω