# Different on and off trigger points

Discussion in 'The Projects Forum' started by AdamAnt, Feb 5, 2013.

Feb 5, 2013
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I have been trying to build a circuit for days now on a circuit simulator program, and I have not been able to make it do what I want.

This is a 12V circuit. Control input is a variable resistance between 1KΩ and 10KΩ. Output is a SPST relay.

I want independently adjustable on and off trigger points based on the resistance value of my control input. I want the relay to trigger on when control input resistance exceeds a set limit (limit controlled by a trimpot), and the relay to trigger off when control input resistance falls below another set limit (limit controlled by another trimpot). The off trigger point must always be lower than the on trigger point.

As an example, if the on trigger is set at 5kΩ, and the off trigger is set at 3kΩ, when the control input resistance exceeds 5kΩ, the relay turns on, but will not turn off until the control input resistance drops below 3kΩ.

I can do this with a micro controller easily, and the program logic may help others understand what I am trying to accomplish; "if input 1>5k then output 1 = high; if input 1<3k then output 1 = low". I am trying to avoid a micro controller, because this seems like it would be a very simple circuit, I just cannot seem to get the correct combination to make it happen.

Any help would be greatly appreciated.

2. ### tracecom AAC Fanatic!

Apr 16, 2010
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1,430
Have you tried using everyone's favorite IC, the 555?

3. ### mduren New Member

Feb 5, 2013
2
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Are both pots connected to input 1? Shouldn't the 3k be on input 2 and the 5k be on input 2?
What are you using for your circuit logic?

Nov 3, 2012
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5. ### tracecom AAC Fanatic!

Apr 16, 2010
3,934
1,430
When pin 2 (trigger) of the 555 is taken below 1/3 Vcc, pin 3 goes high. When pin 6 (threshold) of the 555 is taken above 2/3 Vcc, pin 3 goes low. I think that if you tie pins 2 and 6 together, you should be able to use your pot (with a couple of additional resistors) as a voltage divider to achieve your goal.

6. ### crutschow Expert

Mar 14, 2008
19,754
5,525
A window comparator should do what you need. It allows easy, independent control of the high and low trip points.