# Logic level switch controlled via ground (0v)

simulate this circuit – Schematic created using CircuitLab

Hello, all.

My goal: to convert a ground signal pulse into a +5v signal pulse.

My question: How on earth do I do this?

Backstory: My Jeep ZJ factory doorlocks operate using 5v logic. My aftermarket alarm uses a pulsed (momentary) ground signal for both the lock and unlock feature (2 separate wires). I'm attempting to use those ground pulses to mimic the function of the native door lock/unlock switch inside the vehicle. The native circuit board inside the doorlock module uses a switched ground to "unlock" and a +5v is used to "lock" the doors. I am attempting to get the aftermarket (2 switched ground wires) to communicate with the native circuit board (1 switched ground & one switched +5v).

The amperage is tiny because these signals drive relays on the circuit board.

Initially I was going to use a 7805 to switch/reduce a constant +12v signal to +5v. I thought that I could use the momentary ground signal on the base of the 7805 so that it would only "turn on" and supply +5v when the ground/base was momentarily pulsed...which worked...but the problem is that when the system is idle (when +12v is on the input and the base is not grounded) there is +12v on the output terminal of the 7805. Therefor this will not work because the output terminal MUST be at 0v when at rest...otherwise it would always be sending a lock signal.

Is there a logic level relay that completes a circuit ONLY when a ground is supplied to it? If so, then I can use the +5v on the circuit board as a source and not have to reduce from 12v to 5v.

With an electro-mechanical relay this would be simple but these component level transistors are a little more difficult to grasp for me.

UPDATE: I just wanted to say "Wow"! I asked a question and within 15 minutes, 4 people jumped in and helped. Thank you, everyone! This place is great!

• The term "applying a ground" isn't terribly clear in a system that we presume is ground-referenced. Please include some schematics to better contextualize some of the things that are ambiguous when given in text form. Jul 26, 2020 at 17:19
• What is a "ground signal pulse"? Are you just asking for an inverter? Jul 26, 2020 at 17:20
• "The amperage is tiny because these signals drive relays on the circuit board." If there are already relays on the board, why not use these for what you need? Also, you write: "pulsed (momentary) ground signal for both the lock and unlock feature (2 separate wires)" and then *"a ground signal is used to 'unlock' and a +5v is used to 'lock' the doors". Normally, I might assume a common ground and think you have two additional wires, plus ground. But now I'm wondering if one of these wires actually is ground -- or perhaps something else. Clarify, please?
– jonk
Jul 26, 2020 at 17:26
• I can't understand your schematic. your "native door lock/unlock module" doesn't appear to be capable of doing anything; all it can do is connect +5V to itself or GND to itself, which is pointless. Jul 26, 2020 at 19:08
• But if all it does is connect high to high, it does nothing. I believe your description; I don't believe your drawn schematic. Jul 26, 2020 at 20:04

Here's the schematic, using a transistor to switch +5V for 'lock'.

• Thank you! I'll give it a shot. Regarding the transistor - I assume that the emitter is constant +5v and the collector will be my +5v output only when the base (lock signal) is grounded? Jul 26, 2020 at 19:29
• Anytime! Yes, your assumption is correct. Jul 27, 2020 at 2:12

So this is what I ended up with. I haven't connected any LEDs yet but otherwise it functions properly.

Instead of making assumptions about how the native circuit board operates, I simply created a device that completes one of two circuits based on one of two inputs.

I had to reverse the emitter and collector from the suggested design because I found that If I hooked the emitter to +5v then there was some continuity between the emitter and collector... even if the emitter was the only thing connected to anything. I surmised that this would cause issues. By using the "collector" as my +5v constant input instead, this issue was eliminated.

Now I'm researching how to turn my design into a small integrated circuit on a tiny circuit board with solder points for inputs and outputs.

simulate this circuit – Schematic created using CircuitLab

Update:

Great Success!!

Turns out I didn't need any of the above. Applying a ground directly to either CR204, or CR211 respectively, caused the locks to function as I desired...so I just soldered the alarm's door lock triggers (which are pulsed ground) directly to their respective points on the circuit board. To do that I traced out each circuit to a ribbon cable so that I could solder onto larger joints. I soldered one end of a two wire weather proof connector onto the board at the ribbon cable and then I made a small slot (with a dremel tool) in the housing for the 2 new wires to pass through.

At least the more complicated/unnecessary design functioned properly...and I learned some things...which is the most important part!