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ELM334 Datasheet(PDF) 4 Page - ELM Electronics |
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ELM334 Datasheet(HTML) 4 Page - ELM Electronics |
4 / 7 page 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 |
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