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Turning an LED Off when voltage is applied

Patrick Blackburne

Jul 24, 2015
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I have a 12vdc supply and need to send V- through a normally closed alarm contact (reed switch) so that when the switch is closed (circuit is made) an LED will be turned off. When the switch is open (circuit is broken), the LED will be turned On. I need to do this with multiple reed switches and multiple LEDs, each operating independently. I thought I could accomplish this with a Hex Schmitt Trigger MM74C14N (6 in / 6 out) but it doesn't seem to work the way I need it. I have tried both 12v and 5v on the supply and the inputs. In both cases it does turn the LED off and on as desired, but it affects all the inputs and outputs. So for instance when I remove trigger voltage from one input, all the other inputs and outputs have voltage and all 6 LED's are on. They are operating collectively, not independently. In testing, I hooked up an LED to output 1, nothing on the other inputs/outputs, but when trigger voltage was removed from input 1 , all the other inputs and outputs also have voltage at their pins. Its the same case of collective, not independent operation. I don't know enough about this IC to know where I have gone wrong or what I should do to correct this if this IC will work the way I need this circuit to operate. Any help is appreciated.
 

Gryd3

Jun 25, 2014
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I have a 12vdc supply and need to send V- through a normally closed alarm contact (reed switch) so that when the switch is closed (circuit is made) an LED will be turned off. When the switch is open (circuit is broken), the LED will be turned On. I need to do this with multiple reed switches and multiple LEDs, each operating independently. I thought I could accomplish this with a Hex Schmitt Trigger MM74C14N (6 in / 6 out) but it doesn't seem to work the way I need it. I have tried both 12v and 5v on the supply and the inputs. In both cases it does turn the LED off and on as desired, but it affects all the inputs and outputs. So for instance when I remove trigger voltage from one input, all the other inputs and outputs have voltage and all 6 LED's are on. They are operating collectively, not independently. In testing, I hooked up an LED to output 1, nothing on the other inputs/outputs, but when trigger voltage was removed from input 1 , all the other inputs and outputs also have voltage at their pins. Its the same case of collective, not independent operation. I don't know enough about this IC to know where I have gone wrong or what I should do to correct this if this IC will work the way I need this circuit to operate. Any help is appreciated.
Drawing a diagram will help us to know how it's currently wired.. we can make suggestions on that.
 

duke37

Jan 9, 2011
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If you have a significant load, you could put a LED with resistor across the switch, When the switch is open there will be voltage for the LED but the load will have to supply the current.
 

BobK

Jan 5, 2010
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Did you leave the other inputs unconnected? They must all be connected to a valid logic voltage.

Bob
 

Gryd3

Jun 25, 2014
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If you have a significant load, you could put a LED with resistor across the switch, When the switch is open there will be voltage for the LED but the load will have to supply the current.
;) Details from the op would help.
I thought of this... but we had another user run into issues with doorbell lights wired in the same way.
 

Patrick Blackburne

Jul 24, 2015
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It appears this will not work as I had hoped. It was my misunderstanding that the Hex Schmitt Trigger IC was compatible with CMOS but would operate as well with 12V input, therefore producing 12v outputs that were reverse state so an LED would be off when 12v - was applied to the input and the LED on when 12v - was removed from the input. Not the case. This IC appears to work only with a 5V + input.

The problem I am trying to solve with the simplest circuit and number of parts is creating an interface between 16 normally closed alarm contacts and a monitor panel that has an LED for each alarm contact. The monitor panel and its LED's are pre-wired to operate on 12v, each LED triggered on by 12V -. The Hex IC with its 6 in -6 out's would have been perfect if it had worked the way I had hoped. (3) Hex IC's and I would have been done. Now to trigger the Hex IC, I can easily drop the voltage to 5V using a resistor before the inputs, but the outputs would be 5V +. I would still need something to convert that 5V+ output from the Hex IC to 12v - to turn on the LED's. Maybe going with transistors is the better move if they can more easily accomplish the task of either replacing the Hex IC to work with a 12v - input and output or used after the Hex IC to take 5V + and switch that back to 12V -. Maybe there is an IC that can accomplish this directly and I am just unaware of it. Any suggestions on simplest method here ?
 

duke37

Jan 9, 2011
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CMOS will operate happily on 12V.
The 40106 hex trigger will work here. If the polarity is wrong, you can change the load or use two gates in series then you get three leds/chip.
It should be possible to run the leds directly (with resistor) from the 40106 if they do not take much current.
 

BobK

Jan 5, 2010
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Would you answer my question? Are you putting a valid logic level (0 or 12V) on each input at all times. If you are not it will not work correctly. A circuit that supplies 12V to the input sometimes and an open circuit at other times will not work. The off state must be 0V, which is not the same as an open circuit.

Also, post a schematic if you want further help.

There is no reason why we should not be able to make this work.

Bob
 

Alec_t

Jul 7, 2015
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You have a choice:
1) Use a CD40106 with a 5V or 12V supply but high-brightness low-current LEDs (since standard CMOS outputs aren't intended to source/sink more than a few mA), or
2) Use a 74HC14 with a 5V supply (not 12V) and either that type of LED or the standard LED (drawing e.g. 10mA or so).
With either arrangement you will need pull-up resistors on the gate inputs, to prevent them floating and to pull the input high when the switch opens.
 

Patrick Blackburne

Jul 24, 2015
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The chips I have are stamped with two identifiers. The first is MM74C14N , the second is CD40106BCN. These do appear to work with 12V inputs. The LED's are standard 3mm with 10K resistors. I have tested the LED panel directly with both 5V and 12V and the LED's light up fine, so I am not opposed to this working at either voltage. I agree the problem with the IC points to lack of pullup resistors so the inputs don't float. As well you point out the necessity for 0 volts which is likely a problem. Not sure what size pullups to use or how to create a zero volt state at this point so I'll have to do some reading on those subjects.Thanks for the insight.
 

Patrick Blackburne

Jul 24, 2015
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The use of pull-up resistors did correct the problem I was having. The IC is now working as I need it. Thanks for the insight.:)
 
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