One-Shot Circuit with Constant "Triggers" 1

[mhenry79]

Hi,

I am trying to design a temperature-controlled circuit for my vehicle and I can't figure out how to get a one-shot circuit from constant inputs...

I have a remote starter which supplies a (-) output as long as the engine is running remotely. I also have a Maxim DS1620 IC which supplies a (+) output as long as the temperature is below 50 degrees. When both of these conditions are met, I want to output a 500 millisecond (-) pulse, only once, to trigger a seat heater.

I would use the (-) remote start output as the (-) power source and the ignition circuit as (+), so this would handle selectively powering the circuit. But how do I turn the constant temperature input into a one-shot output? The only idea I had so far was a 555 monostable, but it just re-triggers every time the output cycle ends.

What am I missing? Thanks in advance.

Mike

 

[itsmoked]

You need a NON-retriggerable Monostable Multivibrator..
Check this part out. Both are the same but the data sheets are different so you get more explanation.

http://eshop.engineering.uiowa.edu/NI/pdfs/00/53/DS005325.pdf

http://www.semiconductors.philips.com/acrobat_download/datasheets/74HC_HCT221_CNV_2.pdf

 

[mhenry79]

itsmoked,

Thank you for the help!

I think I mis-stated my original post. It is not that the 555 "re"-triggers, but repeatedly triggers; it doesn't interrupt its own output, but just produces the timed output over and over again. I think this happens because the temperature chip provides its positive output as long as the circuit has power. (I invert it to a negative trigger input using a 2222A NPN transistor.)

Maybe I am not understanding, but to me the data sheets show that the 221 would also re-trigger after the output ends? Could I use a latch, maybe with an inverter, to change one of the inputs after the first output?

Mike

 

[mhenry79]

itsmoked,

I did the further bit of experimenting I should have done before posting the question, and... I already have a timed circuit! Since the ignition does not shut off, the seat heater responds while the starter is turning.

So, negative "remote starting" input, plus positive "starter turning" input, = about a 1 to 3 second power source and only when remote starter is running; plus a noise reduction capacitor to supply a LM7805, plus diodes to prevent feedback to and from the truck.

Temperature IC only supplies positive if temperature is low enough, so that could drive the 2222A NPN transistor to output the ground I want. The output pulse doesn't need to be 500 ms -- just didn't want 5 minutes.

So (hopefully) I should be good to go!

 

[itsmoked]

mhenry79; Glad you've stumbled onto your solution.. I was having a bit of a problem deciphering your system.

Good luck.

 

[mhenry79]

itsmoked,

Your pointing out the non-retriggerable led me in the right direction, so I submitted your reply as a valuable post. Since I have never posted on here, I don't know what that does... but thanks again!

Mike