Very low power relay

[TimFR]

It probably is a pot as the tractor has a readout between 0 & 100, fully down to fully up, but the manufacturer states that the socket output is 0v down & 12v up. I cant see a pot or any other devise, assume it is mounted internally. I have re checked the socket and on down I do get 0.3v, would that be enough to power the transistor?

The reason I need to power a relay is that I have a Trimble sat steering box and for coverage logging I need an open and close circuit. Trimble state that there must be no voltage just open & closed. So initially I thought a simple relay would suffice but I have tried from automotive relays to the lowest power relays I can find but there is not enough power for them.

 

[MikeMl]

Ok, now we are getting somewhere. Bear with me, only a couple more questions:

My circuit pulls-in the relay if the Lift is up, and allows it to drop out when the lift is down. If you use a relay with a 1C contact configuration (SPDT, Common, Normally Open, and Normally Closed contacts), you can choose if the Trimble gets a CLOSED contact if the Lift is Up or if the Lift is Down. Which way does the Trimble want it to be?

The sensor voltage is proportional to Lift position. As originally drawn, my circuit will pull-in the relay when the sensor voltage is between 2V and 12V, or about 15% Up to 100% Up. I can adjust my circuit to switch the relay at any particular position of the lift. At what point in the Lift travel would you prefer to start the data logging?

 

[ronv]

Way back at post # 17 something is wrong with the circuit as it should have turned off with the sensor disconnected. Probably should understand that before we go to far. It would seem like the relay is not tied to ground on the side oposite the transistor.

 

[TimFR]

Which way does the Trimble want it to be?

I does not matter as the Trimble can be programed either way logging circuit closed or logging circuit open.

Data logging would be ideal at 50%

 

[MikeMl]

Ok Tim, here is my final circuit

You will build only the circuit inside the box on the right. Not counting the relay connections to the Trimble, there are only three connections; the sensor, a switched source of tractor power (13V), and frame ground. I would power the circuit from an accessory circuit which is switched with the tractor key, so that it doesn't drain the battery when the tractor is parked.

If the circuit is more than 1m from the sensor connector, I would put the sensor wire inside a shielded wire, where the shield is connected to ground at the same place as where the circuit gets its ground. Do not make any connection to the sensor ground pin...

I am using a 500K potentiometer to feed the gate of an NMOS FET, like a IRF530. This allows you to match the relay pull-in point with the lift position. Simply position the lift to the 50% point, and adjust the pot so that the relay pull-in. There will be a little hysteresis due to the relay. The plot shows the current through the relay at different pot settings as the lift sensor voltage goes from 0 to 12V. Looks like a Pot Position of ~70% will let you set the trip to a 50% Lift position.