# Need help designing a circuit

Discussion in 'The Projects Forum' started by Skarkull, Sep 21, 2010.

1. ### Skarkull Thread Starter New Member

Sep 21, 2010
2
0
Good day, I'm an electronics tech in the military, but I have mainly been doing LRU troubleshooting, and my understanding of circuit designs has not been used since I was trained on it over 7 years ago. Any help would be greatly appreciated.

I'm trying to design a warning system that will activate either a LED or a small buzzer. I need this warning system to activate when there is an absence of current. Here's the basics:

I have an electrical motor that will be powered by 12v here is how it is wired up:

Now what I need is a warning to let me know when the circuit between pins 85 and 87 is broken and there is no current indicating that the pump has gone bad. I was thinking of logic gates, and a 4000B CMOS chip because of the high voltage requirement, 12v. However, I'm not familiar with IC chips too much, I only received training and have no practical experience. I know on the 4000B pins 8 and 9 are a NOT logic gate with 8 being the input and 9 the output. So when I have no voltage on pin 8 I should have a logic high on 9 correct? What should this high consist of? 3V? 5V? would it be enough to run an LED off of, or would I need to use the voltage coming off of pin 9 to activate a transistor? Any help in this matter would be greatly appreciated, as I said it has been years since any training, and even with my training, I have no practical application.

Thanks,

James

2. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,165
1,120
The problem is the number of different ways the pump could go 'bad'. The relay might stop funtioning, so no voltage gets to the motor. The motor coils might go open, so voltage would be good, but no rotation of motor. The motor coils could short, so current is drawn, but no motor rotation. The fuse from the battery might open, so no voltage.

See where I'm going?

3. ### tom66 Senior Member

May 9, 2009
2,613
215
When motors rotate, they often draw pulses of current at a fixed number of pulses per cycle. Detect these pulses, and you can have a solution to your problem. If there are no pulses, the motor is not turning.

4. ### nuckollsr Member

Dec 17, 2009
16
1
Most motor failures present as failure to run because the motor has died or has no power. Simply watching for voltage at the motor terminals catches the second condition but not the first. Some simple way to detect both presence of voltage -AND- reasonable current flow may accomplish what you need to do. Here's an article that speaks to some simple ways to combine current and voltage detection to drive a warning light.

http://tinyurl.com/27hexqs

Bob . . .

5. ### Skarkull Thread Starter New Member

Sep 21, 2010
2
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Yes, but I'm not caring why it's not working, just that it isn't. The only one that would be a problem would be if the motor shorted and current was still going across the motor. However, if current was going across the motor unrestricted, like if it shorted, the fuse would blow, and then there would be no current going across the circuit again, turning on the light or buzzer.

That reed switch seems too complicated, I would have to experiment to find out how many wraps I would need for a motor that draws 11-15 amps of 12V DC. Would the CMOS idea not work?

6. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,165
1,120
perhaps you should go to the source. A small magnet on the shaft or pulley could be sensed by a Hall effect device. That way you wouldn't care WHY the motor stopped, it would just trigger whenever the motor stopped turning no matter what the reason.

7. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
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555 missing pulse detector?

8. ### tom66 Senior Member

May 9, 2009
2,613
215
Something like that.

9. ### siddhu27 New Member

Sep 17, 2010
2
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you can try using battery,whose backup is good and long lasting.

10. ### Wendy Moderator

Mar 24, 2008
20,993
2,731
There was a similar thread, where I designed a current alarm. For what it's worth, I like the idea of a shaft tacometer better.