Electrical Engineering/Relay Switch


QUESTION: I recently asked you a question pertaining to an automotive issue and you were very helpful so I was hoping you could assist me with another electrical issue.

I am trying to maintain some features of my daytime running lights. The two things I want to achieve are
a)lights switch on when car unlocked.
b)lights are always on when engine is running.

I am thinking that this can be accomplished using a relay switch but I need to allow two different sources to switch the relay (one hardwired to the side marker light, and the other wired directly from the alternator). The setup would also need to allow the separate sources to provide current simultaneously and still keep the switch on. Is this possible using a single relay and if not is there an alternative option?

Thanks in advance for any advice!!

ANSWER: If I understand your question correctly, you can operate the relay from two different inputs by using two diodes.  Assuming that you have two wires (A and B), with either one or both of them having 12V present when you want the relay coil to be powered: if the coil has a 100 ohms coil, for example, 12V/100 ohms = 0.12A.  So any standard 1A diode  with 50V or higher rating(such as 1N4001, 1N4002, etc.) would be OK. You can probably find them at Radio Shack.  Any coil resistance of 100 ohms or higher is OK.

Connect wire A to the anode of diode 1, connect wire B to the anode of diode B, connect the cathodes of the two diodes together and to one lead of the relay coil.  Connect the other lead of the relay coil to ground.  Then the coil will operate when 12V is present on either of the wires (or both of the wires).

Hope this helps!


---------- FOLLOW-UP ----------

QUESTION: Diodes of course! Would a relay even be necessary if the voltage is already correct? For instance, could the diodes be wired as you have suggested but both cathodes be connected directly to the positive lead of the running light? I assume that the diodes would prevent any issues that may arise from connecting the different leads together and since ground is inherently shared there should be no issues. Is this correct? Would there be any issues with using the lead from a 12v 5w bulb (wire a) and a new lead connected directly to the alternator of the vehicle (wire b)? Also, would I need a diode with a higher resistance than what you have listed?

The relay allows a lower switching current than the current of the lamp.  You could do it without the relay if the diodes have a sufficient current rating.  If the bulb is 5W, for example, that would be 0.42A, (i.e.  5W / 12V = 0.42A ) but the the starting current is higher, so a 3A diode would be needed.  If the power of the lamp is not 5W, use a 5 times factor to select a current rating for the diodes.

I don't understand your alternator question.

Hope this helps!


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Dave Nyce


I have been an electronics engineer for many years. I can answer questions on analog and digital circuits and my specialty is sensors.


I am the inventor on 27 US patents, and also some foreign ones. Developed sensors for many years. Licensed private pilot (airplane and rotorcraft), have HAM radio license. I'm not an expert in computer networking.

AAAS, Certified Control Engineer, (former UL Advisor for Intrinsic Safety), Benefactor member of NRA. Life member of the following: Experimental Aircraft Assoc., US Parachute Assoc., National Trapper's Assoc., Apex Masonic Lodge #584, Scottish Rite of Raleigh, NC, Academy of Model Aeronautics, Grass Roots North Carolina, NC Rifle & Pistol Assoc. Member of the following: Aircraft Owners & Pilots Assoc., US Hang Gliding & Paragliding Assoc., Tripoli Rocketry Assoc., Apex Historical Society, The Planetary Society, USA Volleyball, Shriners of North America, York Rite Masons, National Space Society, Benevolent and Protective Order of Elks, and the Precision Aerobatics Model Pilots' Assoc.

Books: "Magnetic Displacement Sensors" section in: Measurement, Instrumentation, and Sensors Handbook, CRC Press, 1999. ISBN 0-8493-8347-1; "Magnetostrictive Sensors", "Hall Effect Position Transducers", & "Strain Gage Accelerometers" in: Instrumentation and Control, a Mechatronics Handbook, John Wiley & Sons, Inc. NYP; “Magnetic Level Gauges” chapter in: Instrument Engineers’ Handbook - Measurement & Analysis, 4’th Edition, CRC Press, 2003; Author of the book “Linear Position Sensors, Theory & Application”, John Wiley & Sons, 2004; “Electronic Transmitters”, “Linear & Angular Positioning of Machinery”, and “Inerting Systems” sections in: “Instrumentation Engineers’ Handbook – Process Control, 4’th edition, (2005); “Hazardous area classification and management”, and “HART Networks” sections in the book: “Instrumentation Engineers’ Handbook – Digital Process Networks and Software, 4’th edition, publication in 2011 Magazines: Magnetostriction-Based Linear Position Sensors, SENSORS magazine, April, 1994: Tank Gauging Advances, Fuel Technology & Management, January, 1997: Magnetostrictive Position Sensors, Measurements and Control, September, 1998; A Moment in Positioning, PTDesign, February, 1999; Magnetostrictive Position Sensors (update), Measurements & Control, September, 1999; Position Sensors for Hydraulic Cylinders, Hydraulics & Pneumatics, November, 2000; Magnetostrictive Linear Position Sensors, Fluid Power Journal, April, 1999; Sizing & Applying Magnetostrictive Linear Position Sensors, Motion Systems, Feb., 2002; Position Sensors in Medical Applications, ECN, May 15, 2002; Featured in “Level Sensors Go Floatless”, Machine Design, May 8, 2003; "Guitar Man" feature article, The Pelican Post, Oak Island Press, Oak Is., NC, Winter 2005; The LVDT: A Simple and Accurate Position Sensor, SENSORS magazine, August, 2005; "Model Airplane Day!", FLYING MODELS magazine, September, 2006; “Kids Having Fun!”, Half-A Flyer magazine, January, 2012

BSEE, MBA, Management by Objectives - Honeywell, Total Quality Management - MTS Systems Corporation, Looking Glass Management Workshop - Center for Creative Leadership, Motion Control Systems - Western Michigan University, College of Engineering, Organizational Excellence - University of Cleveland, Finance for Executives - Sloan School of Business, MIT.

Awards and Honors
Vaaler award, EDN magazine, "Inerting for Safety", 1987 Listed in Who's Who in Engineering, in the South, in the World. "Total Quality Management" medal awarded by MTS Systems Corporation 1991 "Best Sequel" award for the video production: "For Engineers Only" at the MTS national sales meeting, Las Vegas, 1998 (written and directed by David S. Nyce) Voted "Most Effective Leader" at Center for Creative Leadership: Looking Glass, Greensboro, NC 1995 Silver Award for New Technology at SENSORS EXPO, in Chicago, 2001 for SEF Liquid Level sensor MTS Circle of Innovators award, 2003 Elected Master of Masonic Lodge #584 , Apex, NC, 2005 "Gold Honour Award" for outstanding service in York Rite Masonry, by the York Rite Sovereign College of North America, August 22. 2007 Board of Directors: WaaRev Sensors, and the Apex Historical Society Maynard Pearson House Plaque hanging in the Masonic Fellowship Hall, for Outstanding Service and Dedication to Apex Masonic Lodge #584, Apex, NC

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