# LED in a circuit with alternating polarities

Discussion in 'LEDs and Optoelectronics' started by croyfer, May 28, 2017.

1. ### croyfer

2
0
May 28, 2017
Hello.

I am builing a project that simplified is a dolly moving on a track.
When the dolly reaches the end of the track, it triggers a microswitch that cuts the power to the motor, and redirects the power to a LED to light up to indicate that the switch has been triggered.

My problem is that since the motor controller I am using changes the polarity of the motor to make it go left or right, I am getting positive _and_ negative voltage every other time. (depending on if the dolly is moving right or left).
I have tried to use a 12V relay to trigger and light the LED, but the problem with that is that the relay needs at least 6 volts to latch, and when the motor is running slow, the motor controller only provides 3-4 volts.

Is there anyone that has a sollution on how I can make this work? I am just starting to fiddle with electronics, and I am on a bugdet with this build, so therefore: Is there a relatively easy fix for this problem? Are there relays out there that has a latch range from 3V to 12V?

Attatched is:
1: Schematics of the circuit with the LED (and resistor) connected directly to the microswitch. The problem here is that every other time the positive and negative switches.

2: Schematics of the circuit with a relay attatched to the 2nd output of the microswitch. The problem here is that the relay need 6 volts to latch, and the motor controller doesnt provide that at slow speeds.

2. ### BobK

7,624
1,654
Jan 5, 2010
You could put the switch between the battery + and the motor controller instead of after it.

or

You could use a two pole switch.

or

You could use a second switch for the LED.

Bob

3. ### croyfer

2
0
May 28, 2017
Thank you a lot Bob!
Offcourse I should put the switch _before_ the motor controller! *d'oh*
Thank you a lot for that! Now where dit I put my soldering iron... :-D

4. ### duke37

5,224
718
Jan 9, 2011
I think you could dispense with the relay and use two LEDs connected in antiparallel where the relay is connected.
Alternatively you could use a bridge rectifier with one LED but this will drop about 2V which may not be suitable when running at slow speed.

By adding some amplification it should be possible to light a LED with a lower sense voltage.