# simplest way to invert + to -?

Discussion in 'General Electronics Chat' started by Terje HÃ¸egh SÃ¦verud, Aug 8, 2009.

1. ### Terje HÃ¸egh SÃ¦verud Thread Starter New Member

Aug 8, 2009
3
0
Hi all. I am putting an accessory in my car that is a mirror with lights that are to turn on with my dome lights. So I took my dome light wire for a signal, but it is + and the mirror lights turn on with -, so the mirror lights were working opposite the dome light.

What is the most simple way to change the + to - so the mirror lights will work the correct way? (but not with a relay because that is too loud in the cabin)

I was thinking to use a logic IC NOT gate, or a transistor inverter NOT gate. Which is more simple for a car environment?

My mirror lights takes .43 amps. Is there a logic IC made that will work for me?

Thanks you all

2. ### russ_hensel AAC Fanatic!

Jan 11, 2009
825
57
It is unlikely that anything in the car runs on negative from ground, as the battery is only + from ground. Could you post what you think the circuit is and why you think it is negative. ( it may be switched on the ground side, but that does not make it run on negative current )

3. ### Terje HÃ¸egh SÃ¦verud Thread Starter New Member

Aug 8, 2009
3
0
The mirror lights need "-" to work. The mirror is a Gentex 50-GENK30. The car uses CANBUS and the signal I can get from the dome is "+" so I need to invert it.

It works like with a relay, trigger to 86, "-" output from 87, ground 85 and 30. I just can not use a mechanical relay because they are too loud.

Thanks
Terje

4. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
In addition to Alberto's suggestion - you will need a resistor between the +v and the transistor's base in order to limit the base current.

1.2k Ohms should be sufficient for your application.

5. ### t06afre AAC Fanatic!

May 11, 2009
5,936
1,229
I also think a N channel FET would do the job nice in this case. Where in Norway do live Terje?

6. ### Terje HÃ¸egh SÃ¦verud Thread Starter New Member

Aug 8, 2009
3
0
I come from by Stavanger.

Should this gate driver work? FAN3223TMXCT, datesheet http://www.fairchildsemi.com/ds/FA/FAN3223C.pdf .
Or this MC34167T? datasheet http://www.onsemi.com/pub_link/Collateral/MC34167-D.PDF .
^Could I hook up either of those directly without other parts?

If I did a transistor and resistors, a TIP31 NPN Transistor, and my math tell me 10k ohm 5 Watt Base resistor and 30 ohm 5 Watt Collector resistor? Trigger into Base, output to Collector. Do I not need Collector resistor?
Thanks
Terje