Re: 12v Sensing Switch


lessani@xxxxxxxxx wrote:
Hi guys,

I asked this question a long time ago, and I only got back to finishing
the project recently. My problem was that I needed to create a circuit
that turned another on or off, depending on the voltage state of a
given wire, 12/6v.

So, I came up with the following,

12V
+
o----.
| |
Relay _|_ - 1N4001
Coil |_/_| ^
| |
o----'
___ |/
JOE ---|<--|___|--+--| TIP31
8.2V 390R | |>
Zener .-. |
| | |
10K| | |
'-' |
| |
o----'
|
===
GND

Which works absolutely perfect (ish). It works fine, but when I
installed this into my car, the 12v/6v wire that is meant to trigger
the circuit has 0A running through it - it is more a clock pulse thing
I think. So, how can I make a circuit that will trigger the above
circuit given two states,

12v Circuit Off
6v Circuit On

Given a wire that can be in two states, 12v 0A and 6v 0A. I'm thinking
I might have to use a Zener to reduce the voltages to 6v\0v and then
some kind of IC like a 555 which the clock input to switch a J-K, or
something along those lines.

Help is very appriciated and the simpler the better!

Thanks guys,
Ben

Hi, Ben. You can get a "12V = OFF, 6V = ON" by using a PNP transistor,
like this (view in fixed font or M$ Notepad):

|
| +12V +12V
| | |
| .-. |
| R2| | |
| | | |
| '-' |
|Vin ___ | |<
| o-|___|- -|
| R1 |\
| |
| .---o
| | |
| 1N4001 C|
| - C|RY1
| ^ C|
| | |
| | |
| === ===
| GND GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

If you use a PNP darlington, you can drive a decent-sized relay with R1
= R2 = 10K. The relay turns on when the input voltage goes down to 6V,
drawing half a milliamp of current from the base of the PNP.

If you want to interface with some digital logic, just use a standard
PNP like a 2N3906, and use a 1K load resistor instead of the relay and
the diode. You will then have a logic signal you can interface to CMOS
logic (you can't use this with TTL) -- a logic "1" (12V) at the
collector when the input is 6V, and a logic "0" (GND) when the input is
12V. 4000-series logic will work with your 12V supply, which should
make things easier.

I'm not clear on what you're trying to accomplish with this signal.
You need to carefully define what you need to do.

Good luck
Chris

.

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