2 potentiometers, one circuit

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micrors4racer

Copper - Posts: 88

Joined: August 18, 2009

Posted: August 19, 2010 at 6:38 AM / IP Logged

I have a rev-limiting board in my car that helps limit it to 6.5k rpm using a 3 wire potentiometer. I want to add another potentiometer set lower to around 3k rpm so that when my ebrake is up (using that ground switch built in that normally turns on that little BRAKE light on your dash) it by passes the 6.5k potentiometer and uses the one set for 3k rpm and when the ebrake is released it goes back to the original potentiometer set at 6.5k rpm.

I was told a DPDT relay would work with this but i'm unsure of how to wire it up.

i am an idiot

Platinum - Posts: 13,667

This member consistently provides reliable information

Joined: September 21, 2006

Location: Louisiana, United States

Posted: August 19, 2010 at 7:35 AM / IP Logged

I need a diagram of how the pots are connected to the vehicle to be able to help you.

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: August 19, 2010 at 8:20 AM / IP Logged

A SPDT relay should work as well, assuming the module just needs a voltage input.

I would assume one side of the pot is connected to voltage, one side is connected to ground, and the middle leg goes to the rev limiter. If that is the case tie the middle leg of the existing pot to pin 87A of a standard automotive relay. Connect your second pot in the same fashion and connect its middle leg to pin 87. Connect pin 30 to the input on your box.

Connect pin 85 to ground and connect pin 86 to your brake light circuit.

Kevin Pierson

oldspark

Gold - Posts: 4,913

Joined: November 03, 2008

Location: Australia

Posted: August 19, 2010 at 10:01 AM / IP Logged

You might even be able to use the switch to connect (else short) the 2nd pot if they are grounded... (and 3kRPM is less resistance than higher RPM).

micrors4racer

Copper - Posts: 88

Joined: August 18, 2009

Posted: August 19, 2010 at 5:01 PM / IP Logged

Thanks guys, Kpierson I will try that out once I get a hold of a SPDT relay. As far as the diagram, there is none because this rev limiter was made using a Autometer tachometer and the shiftlight output activating a relay that grounds the ignitor coil. But I have a volt meter so I can tell you guys some stuff if you tell me how to test it.

Here is a diagram of how the tach is connected. However its no longer the full tach, just the circuit inside a project box so that it looks better.

2 potentiometers, one circuit -- posted image.

Here are the wires that connect to the potentiometer. Turning it clockwise decreases the revs and counter clockwise increases it. It is a 10K pot. 2 potentiometers, one circuit -- posted image.

oldspark

Gold - Posts: 4,913

Joined: November 03, 2008

Location: Australia

Posted: August 19, 2010 at 7:02 PM / IP Logged

If that pot is grounded at one end, the "new" pot or resistor may be able to be connected in parallel with the grounding switch grounding it. But if that switch is connected to something else, then a relay (SPST) etc may be required.

micrors4racer

Copper - Posts: 88

Joined: August 18, 2009

Posted: August 19, 2010 at 7:51 PM / IP Logged

Well i tested the voltage on the contact points. Orange wire is 7v yellow is 2.9v and green is 1.8v so i guess none of the points are ground?

i am an idiot

Platinum - Posts: 13,667

Joined: September 21, 2006

Location: Louisiana, United States

Posted: August 19, 2010 at 8:19 PM / IP Logged

The output range of your device is from 1,8 volts to 7 volts. The yellow wire, is the adjustable wire. Depending on the position of the shaft the output will be somewhere between those 2 voltages.

It may affect the device if you were to parallel a second pot and use the relay to determine which center leg the system would see. I really think you should use a 3PDT relay. Radio Shack should sell one. Get one and post a picture of the diagram of the relay and I can lead you in the right direction.

micrors4racer

Copper - Posts: 88

Joined: August 18, 2009

Posted: August 19, 2010 at 8:45 PM / IP Logged

When searching their website nothing comes up with 3PDT or TPDT. No way to do it using a SPDT or a SPST relay?

oldspark

Gold - Posts: 4,913

Joined: November 03, 2008

Location: Australia

Posted: August 20, 2010 at 12:25 AM / IP Logged

Even if not grounded, it only needs an SPST relay.

If there is a resistor from green to gnd (and nothing else) that could be moved to the orange side, then only the switch is needed.

Maybe only the switch is required anyhow since - when the switch is open - the parallel resistor will be disconnected.

You don't have the circuit diagram, or a pic of the PCB?

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