transistors
Printed From: the12volt.com
Forum Name: Car Security and Convenience
Forum Discription: Car Alarms, Keyless Entries, Remote Starters, Immobilizer Bypasses, Sensors, Door Locks, Window Modules, Heated Mirrors, Heated Seats, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=103992
Printed Date: May 20, 2024 at 11:05 AM
Topic: transistors
Posted By: adhuot01
Subject: transistors
Date Posted: April 15, 2008 at 3:55 AM
How do I use transistor to take a negative input and output a positive. I was thinking maybe I could use a 741 op-amp with the inverting amplifier cofiguration vin/vout =-r2/r1 with r2=r1 12v(-) in should output 12 v(+) correct? Or can I use an npn or pnp transistor to do this? If so how do it do it and what specific transistor should I use. I am trying to make an led that I will put in my shift knob turn on with the dash lights, which is simple, a 1k resistor and 12v led then ground it and hook up to the dash light option in the fuse box, but I also want the same led to turn on when the door is open, which operates using a ground switch. I will also need to use diodes so the doors dont turn on the dash lights and vis versa. Does anyone have a solution?
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Andy Huot
Replies:
Posted By: i am an idiot
Date Posted: April 15, 2008 at 4:27 AM
This can easily be done with a relay and a diode, if nobody else helps you with this today, I can draw you a diagram tonight. If the LED is a 12 volt device the 1K resistor is already built into it. If it is a standard LED you will need the resistor.
Posted By: KPierson
Date Posted: April 15, 2008 at 10:01 AM
You can do this with a PNP transistor, such as a 2N3906. You would wire your signal through a 220 ohm resistor to the base of the 3906. Wire the collector to 12vdc and your output will be the emitter (going through your 1000 ohm resistor to the LED). When your signal wire goes to ground it will turn the transistor on and output the 12vdc signal. ------------- Kevin Pierson
Posted By: dualsport
Date Posted: April 16, 2008 at 2:51 AM
Yep, PNP would be the ticket, might try a higher value resistor for the base. Since it's only switching an LED; a 2.2k should be fine, since the current gain is probably at least 20.
The base current would only be about 5mA, which would still be able to switch 100mA.
Posted By: KPierson
Date Posted: April 16, 2008 at 5:27 AM
Yeah, that wouldn't be a bad idea. The 220 ohm value is the value I use for 5vdc systems. It would still work, on 12vdc but a higher value wouldn't hurt. I usually try to limit the base current to around 25mA; a 560 ohm resistor will acomplish that. For your small LED load, dualsport is correct, you could go much, much higher. The beta of that transistor, I believe, is around 50.
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Kevin Pierson
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