Simple vibration activated LED light - Capacitor?

Discussion in 'The Projects Forum' started by JamesMade, Jul 9, 2014.

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  1. JamesMade

    Thread Starter New Member

    Jul 9, 2014
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    Hi all, this is my first electronics project and I need a bit of help getting started.

    I'm looking to make a very simple vibration activated LED and cr2032 battery combo light to illuminate the water tank in my espresso machine.

    So far I plan on using a normal white 5mm led as well as this vibration switch. I understand that the simplest circuit would be to use just the battery, vibration switch and the led. This would light the led while the switch was vibrating. BUT, if it's not too complicated I'd like to have the LED stay on for a while... maybe 30 seconds or so? My extremely limited electronics knowledges suggests that a capacitor might help with this but I have no idea what the circuit would look like and I haven't been able to find anything similar online.

    One last thing, I'd like the battery to last for as long as possible, the LED won't be on much other than when I'm using the machine so it should be able to last a very long time (I think). Also, the overall size is a bit on an issue as, ideally, this would all fit in the (dry) space underneath the water tank. That space is about 1"x1"x.5".

    So, any chance that one of you electronics gurus could suggest a simple circuit to keep the LED on for a while? Any help is really appreciated!

    Thanks!!
     
  2. wayneh

    Expert

    Sep 9, 2010
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    You need a "monostable multivibrator" or "one shot" circuit. That's a classic application for a 555 timer IC. Trigger once, and it stays on for a length of time you can set.
     
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  3. JamesMade

    Thread Starter New Member

    Jul 9, 2014
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    Okay, there are definitely no shortage of tutorials online for 555 circuits so I should be able to figure that out. Thanks for the suggestion.

    I was hoping for something a bit more simple though. Is there no way the vibration switch could complete a connection to "fill up" a capacitor from the battery? Then the capacitor could slowly drain into the LED?
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The forward voltage (Vf) of a white LED (depending on type and luck) may be more than the CR2032 can provide. Could you tolerate an ultra-bright red LED instead?
    In view of the limited space available you may have to use tiny surface-mount components. Not ideal for a beginner's project.
    Unfortunately you would need a physically large cap for anything longer than a few milliseconds of light if you don't use a 555 or similar circuit.
     
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  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    CR 2032 measured 3 V, stuck a 1 W white LED across for med bright light.
    Think a darlington driver might work.
     
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  6. JamesMade

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    Jul 9, 2014
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    Okay, I guess a 555 is the way to go. I'll post again once I draw a potential circuit.

    Thanks again.
     
  7. Bernard

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    Aug 7, 2008
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    For 3 V, a C555 might be best, but still need a driver for LED as for + output, current limited to 10 mA.
    A charged C works but gradually dims over one minute, I'll throw the drawing in just for kicks. Tried with MPSA14 but led brighter with BS170.
    A low RDS logic level FET migh be best in either case.
     
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  8. JamesMade

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    Jul 9, 2014
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    Thank you very much for the drawing Bernard! Am I understanding it correctly that this circuit doesn't use a 555? Does the 1uF Capacitor keep the LED on after the switch stops vibrating or does that functionality still need the 555 to trigger the driver?
     
  9. Bernard

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    A switch closure of 1 or 2 ms charges the cap. which discharges thru leakage which keeps the LED on for over 30 sec.. Could use a C555 to drive the FET which would keep the LED at full brightness for the set time. BS 170 came from Radio Shack.
     
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  10. JamesMade

    Thread Starter New Member

    Jul 9, 2014
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    Amazing, thank you very much! This is exactly what I was hoping for. Clever solution!

    Thanks again for your help. Now I have to find the time to build this...
     
  11. SgtWookie

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    A 555 timer has current flowing through it even when idle, so the battery life will be short.

    A Darlington transistor would drop too much voltage; even with low collector current it wouldn't get below 0.8v.
     
  12. Bernard

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    Aug 7, 2008
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    Something strange going on, started to test with a red LED & ckt would not shut off. Replaced BS 170, & with red piranah LED, 70 mA, 2.38V, reasonablly bright which stays on untill C is discharged. Added 66MΩ for time out in about 20 sec. 100MΩ might be better, but that bin was empty.
    Re tried 1W white, same result but not as bright. Wish we had another 1/2 V.
    V drop with MPSA14 darlington is .6V, with BS170 is .16V
     
    Last edited: Jul 10, 2014
  13. wayneh

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    One thing to note is that you could easily boost (~2X) the battery voltage by borrowing the boost circuit from one of those single-cell solar lamps. They take the 1.5V cell voltage up enough to light the LED, something a bit over 3V. Might help open up more options.
     
  14. wayneh

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    I'm surprised that either conducts with such a low gate voltage. Always learning stuff here.
     
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