I should mention that this is a 12v DC circuit.I want to use a linear actuator to tension a cable. I can use a limit switch on retract but the travel can vary on extend. I need a load based way of opening the circuit when max tension is reached. I'll be using a momentary rocker switch.
I'm glad you answered. When I was first talking with my buddy about this project I mentioned garage door openers but didn't know how they worked.If you want a way to measure tension, you just need an idler and a spring loaded arm attached to a potentiometer. The angular displacement is proportional to tension.
Les, THANK YOU! That looks like what I need. I thought I needed an amperage based circuit.You could monitor the current to the motor in the actuator. Just have a low value resistor in series with the actuator and monitor the voltage developed across it using an Op amp or comparator IC. I have used this many years on motorised curtains were it would be difficult to fit limit switches. This is how I did it. You could just use the output of the comparator (Via a transistor.) to unlatch a relay. The capacitor on the comparator input is to prevent the starting current of the motor from triggering the comparator. I used an LM339 comparator but you could use an LM393. NOTE you will need a pull up resistor on the comparator output. (In the above schematic I have activated the internal pull up resistors in the PIC.)
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by Jake Hertz