Low voltage trigger

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=75848
Printed Date: May 21, 2024 at 2:32 PM

Topic: Low voltage trigger

Posted By: debeerd
Subject: Low voltage trigger
Date Posted: April 07, 2006 at 12:59 PM

I'm using the Pulsed to Steady output setup, it works fine when using 12V as the trigger.

The trigger voltage is between 0.4V and up to about 7V somewhere.

0.5V will not trigger a relay, what can I do to increase the trigger output but also keep it below 12V ?

Replies:

Posted By: KPierson
Date Posted: April 07, 2006 at 1:11 PM

Sounds like you need a transistor. You can switch on a transistor with very low voltage. Look in to a 2n3904 NPN transistor!

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Kevin Pierson

Posted By: debeerd
Date Posted: April 07, 2006 at 7:12 PM

Cool, how would I wire the transistor ?

I've never worked with those before.

Posted By: debeerd
Date Posted: April 07, 2006 at 7:34 PM

I think I have worked it out after some searching around...

1 will be the trigger,

2 will be 12V,

3 will be to the relay to switch it on.

???

Posted By: firstrax
Date Posted: April 07, 2006 at 8:48 PM

Pin 1, Emitter, out to relay

Pin2, Base, Trigger

Pin3, Collector, 12v

Your going to need a base resistor otherwise it will turn into a LET. Depending on the off state of the trigger you may also need a bias resistor to keep it from oscillating. You may need a low saturation transistor or a fet. 0.4 volts is right on the edge of the 3904s saturation voltage.

posted_image

Datasheet: https://www.st.com/stonline/products/literature/ds/8850/2n3904.pdf

Posted By: KPierson
Date Posted: April 08, 2006 at 7:32 AM

Don't forget to add a diode across the coil of the relay to protect the transistor.

What exactly are you trying to do? Where does the trigger come from?

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Kevin Pierson

Posted By: debeerd
Date Posted: April 09, 2006 at 8:29 AM

The trigger is from the coil, rev counter.

Using the pulsed to steady output for the fuel pump.

So when the motor is off and the ignision is on then the pump won't run all the time.

I've done the pulsed to steady relay and it workes great, but the trigger is too low to set off the relay.

posted_image

So I'm going to add the transistor setup to it.

What size resistors do I need ?

Posted By: debeerd
Date Posted: April 11, 2006 at 4:21 PM

Another item needing help, Can one lower a 12V signal to 9V ?

This is for the HAL pickup in the distributor.