Voltage drop

Projects General Guidance
1

Hi all,

I'm working on a small project but have no idea if I can do this?
I'm using a ready made module with the Arduino but to get the module to activate I have to reduce a pin that has a constant 3v on it to 0v for it to activate.

For example, a module in a smoke detector allows the smoke detector to contact a care unit if it sounds.
There is a pin on the module that is giving out a permanent 3v. if I connect that pin to the negative of the battery it activates the module.

How can I use Arduino to put a negative to that pin for a second that is needed then give back the voltage? It shows that it has to drop down to 0v for it to activate.

Any help is appreciated.

:slight_smile:

2

It's a little "dangerous" with an unknown circuit but an NPN transistor with an [u]open collector connection[/u] to the 3V input should pull it down.

You'll need a resistor in series between the Arduino-output and the transistor base (to limit the current into the base). 1K will probably work in this unknown circuit.

The logic will be reversed. When the Arduino output is low the transistor is off and it has no affect, leaving the 3V as-is. When the Arduino output goes high the transistor turns-on. Current flows between the collector & emitter "pulling down" the 3.3V input.

3

That sounds like a plan.

I will have a look down that root and see if I can get it working.

Thanks for your help.

4

Could you give me a little more help on this?
I have tried every way I can but cannot get it to activate.

I put the 3.3v from the module to the collector.
Pin2 connected to 1k resister then to the base.
Wasn't sure what to connect to the emitter so tried every thing.

Any more help please.

Thanks, I'm new to all this but could save lives if I could get it working.

5

You will need to connect the emitter to smoke detector ground AND Arduino ground.
An opto isolator may be safer.
schemeit-project.png

schemeit-project.png

6

Signal voltages in smoke alarms are normally referenced to mains neutral hense the need to use mains rated cable for alarm interconnects.
As mentioned previously, without circuit diagram which will be nigh impossible to obtain, one is operating in dangerous territory.
That plus the use of Arduino in life dependant areas has been brought up many times previously.
There are dedicated alarms out there designed for low voltage or voltage free situations so I'd suggest you look into that as a safer alternative all round.