# Help designing a Schmitt trigger please

Discussion in 'The Projects Forum' started by eugen39, Mar 2, 2012.

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1. ### eugen39 Thread Starter New Member

Mar 1, 2012
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Here is what I am trying to achieve: a circuit that will have an output of 0V when the input is 8V and below, and 12V when the input is 12V. My limited knowledge tells me this is a Schmitt trigger but I don't know how to approach it.
Any help deeply appreciated.
Eugen

2. ### Brownout Well-Known Member

Jan 10, 2012
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What is the output when 8V<Vin<12?

3. ### eugen39 Thread Starter New Member

Mar 1, 2012
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The input is either 0V, 8V or 12V

4. ### eugen39 Thread Starter New Member

Mar 1, 2012
15
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Would this make more sense: when 0V < input < 10V then output is 0V and when input > 10V then output is 12V ?
Thanks

5. ### Ron H AAC Fanatic!

Apr 14, 2005
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Schmitt triggers have upper and lower threshold voltages. What would you like these to be?

6. ### eugen39 Thread Starter New Member

Mar 1, 2012
15
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Lower threshold = 8V
Upper threshold = 12V
Thanks

7. ### Ron H AAC Fanatic!

Apr 14, 2005
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You should probably leave a little room for voltage tolerances.
8.5V and 11.5V might be better.
What is the impedance of your source?
Do you have a steady 12V supply available for powering the Schmitt trigger?

8. ### eugen39 Thread Starter New Member

Mar 1, 2012
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I do have 12V available and the circuit will drive a relay with an impedance of about 70ohm

9. ### Ron H AAC Fanatic!

Apr 14, 2005
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OK, but where does the voltage come from that you are sensing?

10. ### eugen39 Thread Starter New Member

Mar 1, 2012
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My son and I are working on a project on a Lego Mindstorms. We are trying to build an add on and the output from the Lego is 8V in one state and 12V in another state but the motor we are using doesn't work well at 8V. I tried using a 12V relay and an external 12V power source to eliminate the 8V state but the relay is activated by the 8V source too. So basically we need that relay turned on at 12V but stay off at 8V.

11. ### Ron H AAC Fanatic!

Apr 14, 2005
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Here are two ways to do it. The threshold is about 10V in both of them, with a few hundred millivolts of hysteresis.
If it is unstable due to noise on the input, you can raise the hysteresis by reducing the value of R5 to a value as low as 200k.
The relay turns off when the input is high, so you will need a normally closed relay. If you don't have one, we can modify the design.

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12. ### Wendy Moderator

Mar 24, 2008
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A simple 555 also make a high drive Schmitt Trigger, inverting of course.

555 Schmitt Trigger

13. ### THE_RB AAC Fanatic!

Feb 11, 2008
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Two transistors will make a schmidt trigger, the benefit of course is that one of the transistors can be the one driving the relay. You can easily set it up for 8v and 11v thresholds.

Total cost 2 transistors and about 4 resistors, and a diode for the relay coil.

14. ### eugen39 Thread Starter New Member

Mar 1, 2012
15
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I tried it and it didn't work, I probably did something wrong... I used a 556 because Radioshack was out of 555s and I might have messed out the terminals. I will try again today.
Thanks

15. ### eugen39 Thread Starter New Member

Mar 1, 2012
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Thanks. I will order the parts and put it together. Radioshack doesn't carry all of them. I will let you know how it turned out.
Thanks again.

16. ### eugen39 Thread Starter New Member

Mar 1, 2012
15
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I need a little bit more than that, I am a beginner.
Thanks

17. ### eugen39 Thread Starter New Member

Mar 1, 2012
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What did I do wrong: I connected the input to 2 and 6 pins (and the 8 and 4 pins to +12V and 1 to ground, of course) and the relay to the 3 output and ground but nothing happened. The relay didn't click whether the input was 8V or 12V. At one point I must have shorted another pin because the 555 became very hot.

18. ### MrChips Moderator

Oct 2, 2009
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How about using an analog comparator?

19. ### eugen39 Thread Starter New Member

Mar 1, 2012
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That sounds good but please remember that I am a novice so I would need more info.
Thanks

20. ### Ron H AAC Fanatic!

Apr 14, 2005
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Check post #11.