Detecting low voltage input for replicating window drop

I have created a small program to move a window down when the door is opened and up when closed.
The program i have created works fine on the breadboard, using a normal button and using transistors triggering relays to control the motor.

I was hoping to pull a signal from the actual door latch though and this is where i have come into an issue.

There is a door ajar signal wire which connects to ground when the latch is open and closed (an internal switch). Unfortunately it is only reading 0.7v when closed and 0v when open, which isn't high enough to trigger the digital input pin on the arduino. I can't change the voltage source on this signal wire as it is already part of another system.

Some ideas i was thinking of were;

Acquiring a low voltage coil relay
Trying to amplify 0.7v to above 1.5v
Triggering the base of a transistor

If any of those ideas seem to be in the correct direction or if you know of a better way of solving this, i would very much appreciate it to know.

Pull up resistor to 5V - its an open-collector interface I suspect.

That would be the best way if it works. If it does not work, I might try a comparator. Much easier than amplification or finding a relay.

i have been asked about the other system

With regards to the 'other system', i have followed two wires into a lock. The two wires connect and disconnect when the lock is engaged or disengaged, respectively. One wire is connected to ground, the other, i believe, connects to a module which reads whether the door is open or closed.

I detected the 0.7v by testing with a multimeter between the wire going into the module and ground (the metal work of the door rather than the opposite wire).

The ground wire i did a continuity test with the metal work of the door.

I did a continuity test between these two wires, it would bell out when the lock is engaged.

I could draw it out but it is two wires, one to ground, one to a box, and a switch between them.

Hope this helps.


Using a comparator might be the way - I see you have another system already sensing this wire, no doubt
using a transistor, hence the 0.7V.

For 5V use a 1k:10k divider to create 0.45V to one of the comparator inputs, the signal wire to the other,
comparator output to an Arduino INPUT_PULLUP pin... The LM339 is the commonly used quad comparator
of choice, get a few, they are very cheap and available DIP or SMT. Comparators need decoupling just like
logic chips. Beware the funky pinout of the LM339, BTW.