# 12V Voltage Sensing Switch (?)

Discussion in 'Electronic Basics' started by Ben Lessani, Mar 7, 2004.

1. ### Ben LessaniGuest

Hi, I've been dabling with my car central locking, making my attempts
to integrate the alarm into the system. When I lock the doors remotely
with the key, the door pins go down (naturally), but when I press an
additional button on the key, it deadlocks the doors. There is a wire
(lets call it JOE) which is always at 12V unless the doors are
deadlocked, when it is at 6V. I am not sure of the current running
through this circuit, but I know its not powerful enough to switch a
relay. I was thinking I could use a transistor type circuit to achieve
what I need. I can use a relay in its opposite state, so when no
voltage is supplied, the circuit is complete, but when a voltage is
applied - it opens the circuit. I was thinking I could put a resistor
from JOE to the BASE of a transistor, to reduce the voltage down to 6V
as standard, then when the doors are deadlocked, the voltage will drop
6V, taking the voltage at B to be 0V. I've drawn some diagrams below
to try and help explain what I'm trying to say. The wire does not have
enough current to close a relay, so I need a transistor to close the
relay. But I want to resist the voltage input at the resistor using
R1, so that it is normally at 6V, this means that when I deadlock the
doors, and the voltage drops, the input at the transistor is 0V,
leaving the 'switch' open, leaving the relay open.

12V
|
State 1 (JOE AT 12V) |
8 |--|-----
8 | | RELAY CLOSED
8 |--|-----
C |
/
12V 6V |/
----| R1 |----B-|
|\
\
E |
|
|
GND

********************************************

12V
|
State 2 (JOE AT 6V) |
8 |--|-----
8 | \ RELAY OPEN
8 |--|-----
C |
/
6V 0V |/
----| R1 |----B-|
|\
\
E |
|
|
GND

Could you please email me with a response at

Delete the Q's if you want to email me, everyone else is doing it, so
I thought I might join in the fun .

Thanks V. much

Ben Lessani

2. ### valentin tihomirovGuest

These are series diodes that are normally used to shift voltage down. Then
you can use an opto coupler for sensing presence of voltage to switch more
powerful circuit.

3. ### Ben LessaniGuest

I've put all this text in above, but what I'm trying to ask is whether
this will work, and if it does, what transistor do I need (model
number please), and what value resistor for R1 do I need?

Thanks V. Much
Ben Lessani

4. ### Ben LessaniGuest

Whats the laymens term of this.

5. ### Mikal HodvikGuest

Ben,

You're on the right track, but you really need a zener diode in series to
the base, to get a crisp decision threshold. I suggest an 8.2V zener, with
the cathode to JOE, and R1 at about 390 ohms. It's wise to shunt base
leakage current to ground with something like 10K ohms. The transistor could
be a TIP31, or something smaller if you're not using a power relay. Also add
a reverse-biased rectifier diode (1N4001, etc.) across the relay coil, to
prevent damage from inductive kickback!

Mikal Hodvik