Need help with designing a 3 second delay on circuit

[Matthew10_28]

Right now, I have a 3V 1500mAh CR123 battery connected to a switch and a small length of nichrome wire. When it is switched on, the nichrome wire heats up until it breaks as it should. Naturally, the circuit is open at this point and it's served its purpose. From my understanding, it takes about .25 amps for about a second to heat the wire until it breaks.

I'd like to add a 3-5 second delay (user adjustable would be nice) to this, such that when the switch is turned on, nothing passes through the nichrome wire until the allotted time has passed. Then the nichrome would heat up and break as before, opening the circuit. Once the nichrome wire breaks, the device won't be interacted with for a long time, so having the battery continue to drain would be unnecessarily problematic. This has to be resettable (loaded with another nichrome wire). I've looked into using a 555 delay on circuit but I don't have a good understanding of how it really works. There seems like a lot of variations to it as well. I've got a degree in mechanical engineering, but I just can't wrap my head around how to design this. Any help would be appreciated.

 

[andre_luis]

Check if this could help you:

**broken link removed**


+++

 

[Matthew10_28]

Sorry, that really doesn't help at all. I even read the webpage you got that from. Thanks for the information though.

 

[phobos1]

https://www.edaboard.com/threads/305923/

 

[albbg]

Sorry, that really doesn't help at all. I even read the webpage you got that from. Thanks for the information though.

Why ? What's exactly the problem with this circuit ?

 

[rusty81]

The easy way is, as previously posted, a 555 monostable. What's wrong in a 555 monostable?
Do you prefer a bjt solution maybe? I can sketch one for you if u want.

What is your operating voltage?
3V isn't enough for a standard 555, unless you use a low voltage version, like LMC555

Regards

 

[BradtheRad]

When power is applied, a small current charges the capacitor at left.

When the capacitor reaches 1.3 V, it turns on the darlington transistor array.

The nichrome wire is located so that when it breaks, no current drains the battery.



The potentiometer adjusts the time delay, but it also changes bias current through the transistors. You'll probably need to experiment with capacitor values, to get the delay you want.