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Comparitor Circuit Needed

Printed From: the12volt.com
Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=4926
Printed Date: June 30, 2025 at 9:29 AM


Topic: Comparitor Circuit Needed

Posted By: studman
Subject: Comparitor Circuit Needed
Date Posted: October 29, 2002 at 1:24 AM

Here's the basic setup:

I have a dimmer switch that is a Variable Resistor (VR).  When the VR is in "clicked/full brightness" position (meaning that the lights are at full brightness), 0.77 volts or less is outputted from the VR.  When the VR is not in "clicked/full brightness" position (meaning that the lights are dimmed), 1.4 volts or more is outputted from the VR.  Basically I need a comparitor circuit (and a relay to invert voltages) that will take 0.77 volts or less and activate a 12 volt circuit.  If the VR outputs 1.2 volts or more, then the 12 volt circuit is not activated.

VD = Voltage Dimmer

VO = Voltage Out

When VD <= 0.77, VO = +12v

When VD > 0.77, VO = No Connection

Can someone help me design this circuit?  Any help would be appreciated.




Replies:

Posted By: PLAYER69
Date Posted: October 29, 2002 at 9:44 PM
What are you trying to accomplish with this?  There might be another way to do it.  The way you want to go about it would be pretty hard to get it to do what you want.




Posted By: go2pac
Date Posted: October 29, 2002 at 11:38 PM

The dimmer output should be negative. You must have tested the wire referenced to ground instead of referenced to the parking light. But anyways...

A transistor turns on typically at about 0.6-0.7 volts. Since the minimal voltage you got is 0.77, you will need to make a voltage divider so that less than 0.6 volts is at the base of the transistor. Given that, you can use a NPN transistor that will output a negative signal which you can use a relay to invert that signal to a positive or turn on a PNP transistor to supply +12volts. I would draw one but i'm tired tonight.



-------------
Karl

Pacific Accessory Corporation
Mobile Audio Interfacing Equipment
Connecting You to Your Music Since 1976





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