# Trying to create a circuit that will only pass through voltage if higher than a certain level

#### bsohn

Joined Aug 26, 2019
4
OK so I have this circuit I am working on and I will describe what it is because I don't have a photo at the moment.

Anyway I have a 12V power that runs into a Normally Open side of a DPST switch and then a 3.25V power that runs into the switch on the normally Closed side of that switch/ these exit via the common so that it will send either 12V or 3.25V depending on the switch position.

The single line from the switch is connected to two triggers one for the 12V line and one for the 3.25V.

The issue I am having is that the 12V trigger of course is always being fed with at least 3.25V since when the switch is Normally Closed on the DPST 3.25V is always being fed and ONLY when the Switch goes Open does the voltage increase to 12v and the 3.25v line and trigger gets deactivated. That 3.25V always going to the 12V trigger is actually enough to Keep it active or on.

I know the simple way to do this would be to have a DPDT through and switch the ground as well but I don't have that luxury at this moment due to design constraints. so what I was hoping to do was to find out if there is a way that I can put some sort of element between the 3.25V trigger and the 12V trigger that would actually block the voltage flow to the 12V when the voltage was lower than a certain level.

I thought about a 8V Zener Diode and tried it physically in both polarity and it didn't work as I was thinking that in reverse polarity it would totally cut off the flow below 8V..

Im not super experienced with electronics but know enough to get myself into trouble but generally not fry stuff lol.. Any help would be appreciated

#### Externet

Joined Nov 29, 2005
1,592
A beefy enough zener diode that will conduct at "higher than a certain level" can energize a relay coil to let pass trough the voltage.
Or a comparator circuit that will energize such relay coil at "higher than a certain level"
Relay coil current should be observed to match method of choice. A solid state relay or a mosfet can also be considered for minimal driven current.

#### Hymie

Joined Mar 30, 2018
971
Although you have given quite a lengthy description of the problem, I’m not certain what you are trying to control/do with this 12V or 3.25V that is switched via the DPST switch.

Perhaps a circuit diagram would assist in my (and others) understanding.

#### Bernard

Joined Aug 7, 2008
5,785
Are you sure that SW is DPST, then needs to state either NO or NC. Sounds like SPDT SW?
Maybe have a 12V relay at far end , NC for 3V & NO for 12V. 4 V should not operate relay. Or 6 V relay & 6 V zener.

Last edited:

#### crutschow

Joined Mar 14, 2008
27,031
As the saying goes, a picture is worth a thousand words (and that applies to schematics).

#### bsohn

Joined Aug 26, 2019
4
OK Well It may even be hard to understand in this diagram as it is NOT to electrical convention.. But here it is.

First off TK1 and ES are the interfaces of the unit that I am working with, I have no CLUE as to what actually lays on the other side of those interfaces but I know what pins have voltage and what pins ground those voltages. The Grounds appear to be common across the board.

The Normal Operation is that TK and ES are completely separate interfaces and TK is jumper from Pin 2 and Pin 6 (or Not) and it causes a certain state for the device to be in.. TK#2 is the 3.25v line and in its normal state is jumped to TK#6

On the other side with ES thing s are a little more complicated as ES#1 is 12V and runs both to a momentary with which is used as a power button trigger Grounding Back to ES#2 and a SPDT Switch that is NO and when closed grounds to ES#4.

So what I am doing is trying to connect the state of the TK loop to the ES through the SPDT.. so that when the SPDT is normally Open it's normally Closed side of the switch puts the TK #2 to #6 loop in its normal state and then when the SPDT is switched to its closed state it Opens the NC of course Opening the 3.25voltage line through it and Closing the 12V Line causing the trigger on ES#4 to do what it needs to..

IGNORE ES #5 and ES #6 as those are a separate LED Circuit as the SPDT has an LED in it as well as the switch.

The Small SPDT between the two rounds is a state switch which should when things are correct allow the system to function in two different ways which is the whole purpose of this anyway..

The Basics of the circuit are working as I intend EXCEPT that I have found that the when the SPDT is in its open state and the 3.25V are running through the NO Side because I have the Grounds attached at TK#6 and ES#4, ES #4 is being grounded at 3.25V which appears to be enough to put that trigger into a state where the 12V Grounding out trigger is ignored when switching to the 12V so that particular function does NOT actually work as intended.

Not having that ground connected between TK#6 and ES#4 causes the correct functionality But without the ability to have a DPDT to add separate switching for the Grounds, I am unable to connect BOTH grounds at the same time.

I don't believe there is much current at all running through this system as it it just an interface system the uses switches to trigger functions. So while the full Relay suggestions would work they seem to be a bit overkill for what is going on.. BUT that leads me too the possibility that I can use a transistor switch (maybe a 2222) to do what I want allowing the connection between TK#6 and ES#4 to be opened when the TK loop goes open on the SPDT.

#### bsohn

Joined Aug 26, 2019
4
Are you sure that SW is DPST, then needs to state either NO or NC. Sounds like SPDT SW?
Maybe have a 12V relay at far end , NC for 3V & NO for 12V. 4 V should not operate relay. Or 6 V relay & 6 V zener.
Sorry yes SPDT.. and unfortunately can't get a DPDT in the switch currently needed