Circuit Operation

4 of 7

ELM334

ELM334DSC

Elm Electronics – Circuits for the Hobbyist

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

The ELM334 can be thought of as two inverters

that have some additional logic connected. The inputs

to the inverters are located on pins 6 and 7, while the

outputs are connected to pins 2 and 3. If pin 7 is

connected to a low level, then pin 2 goes high. If pin 6

is connected to a low level, pin 3 will go high. There

are several additions, however, that make these a little

more than simple inverters.

To make this IC more useful, we have added

‘debounce’ circuits to the pin 6 and 7 inputs. These are

similar to digital integrators - the input must be at one

level for a set time before the output can change. This

may not be necessary for simple LEDs, but if you want

to interface to a computer circuit or other logic, you will

require a clean signal such as this provides. To ensure

that the circuit works reliably even with very poor

quality switches, a debounce period of 0.5 seconds is

used.

In addition to the debouncing, there is logic

connected to these pins, so that both LEDs are not on

at the same time, etc. (the door is either closed or

open, but not both). Figure 1 shows the outputs that

can be expected for all combinations of the two inputs.

Typically, a dual red/green LED will be connected

between the two output pins, and this is what is meant

by the last column. You may connect separate LEDs

between each output and circuit common, but there is

no advantage to this, as it requires an extra wire for

remote indicators. (It is easier and cheaper to wire

separate LEDs ‘back to back’ as shown in the

Example Applications section.)

One other feature of the internal logic is a ‘lamp

test’ that is performed at each power up - the LEDs are

each turned on for 0.5 seconds so that you can be

sure that they are both working (red first, followed by

green).

The ELM334 also has a special pushbutton circuit

connected between pins 4 and 5. When a low level

input appears on pin 4, a 0.5 second pulse will be

output on pin 5. This is typically used to drive a relay

which interfaces with the garage door control circuit,

but it can also be connected to logic of your choice.

Only one pulse will be generated for each pushbutton

press, no matter how long the button is held for. The

input also provides a debouncing circuit so that a

mechanical pushbutton can be used.

That covers the basics of how the ELM334 IC

operates. The next section (Example Applications)

shows a few examples of how it may be used…

Pin 6

ClosedSw

Pin 7

OpenSw

Pin 2

RLED

Pin 3

GLED

L

H

H

Green

Dual (R/G)

LED glows

Red

Green

Alternates between red and green

for 30 seconds, then shows red

L

L

L

H

H

H

L

H

L

H

L

Figure 1. ELM334 LED Logic