led timer

[salutka]

Hello everyone,

I'm starting a project and I don't quite know where to start. I've browsed through this forum a bit but could not find my exact situation... Anyways I need in my car an led that turns on for 6 seconds. It has to come on when the key is in the on position, stays on for 6 seconds, turns off and stays off until I turn the key back on. I think the 555 timer might work but I know nothing about that chip. Any help would really be appreciated.

Thank you.

 

[Riff Raff]

a monostable 555 timer will probably do the trick in your case.

check this link out: ht tp://ww w.eleinm ec.c om/article.asp?4
(i can't post urls, so remove the spaces)

for an on time of about 6 seconds, you can use a 470uF capacitor and a 12Kohm resistor.

 

[salutka]

Thanks for the info!

I drew up the circuit for final revision but I do have some questions, If the trigger pin is wired directly to the 0v, will the 555 timer run its cycle the moment it gets power? Also I assume that the output will be the same voltage as the VS?

Hope it's good.

 

[Boncuk]

Hi salutka,

you don't need to use a big electrolytic cap for the circuit. Reduce the capacity to 10µF and use a resistor of 570KΩ instead of 12KΩ.

The exact resistance value 546.1KΩi s achieved best by using a combination of a fixed value resistor (470KΩ) and a trimmable resistor of 100KΩ.

570KΩ will result in a time out of 6.262seconds. For an electrolytic I suggest to use a tantalum type capacitor.

Don't forget to smoothen the alternator voltage. (Not included in the schematic)

Boncuk

P.S. There is a small error in the schematic. The total resistance value has to be 546.1K.

 

[salutka]

So the IN with the 6.8k resistor and the TP1 have to be connected to the 12v?
And is your mistake the leg #1 of your variable resistor?

 

[salutka]

Sorry, the resistor is fine, forget what I said about the #1 leg!

 

[Boncuk]

So the IN with the 6.8k resistor and the TP1 have to be connected to the 12v?
And is your mistake the leg #1 of your variable resistor?

TP1 goes to the positive battery terminal and IN goes to the key switch.

 

[Hero999]

Hi salutka,

you don't need to use a big electrolytic cap for the circuit. Reduce the capacity to 10µF and use a resistor of 570KΩ instead of 12KΩ.

The exact resistance value 546.1KΩi s achieved best by using a combination of a fixed value resistor (470KΩ) and a trimmable resistor of 100KΩ.

570KΩ will result in a time out of 6.262seconds. For an electrolytic I suggest to use a tantalum type capacitor.

Don't forget to smoothen the alternator voltage. (Not included in the schematic)

Boncuk

P.S. There is a small error in the schematic. The total resistance value has to be 546.1K.

570kΩ is not a standard value, the nearest standard value to 546.1k is 549k which is a 1% tolerance resistor.

In practise, there's no point in using such a precise resistor value because the 10µF capacitor will be 10% at best so use a 560k resistor.

 

[salutka]

Ah I see so every time the trigger gets a signal it restarts its cycle.

Well thanks a bunch for all your help.

I will keep you posted.

 

[Boncuk]

570kΩ is not a standard value, the nearest standard value to 546.1k is 549k which is a 1% tolerance resistor.

In practise, there's no point in using such a precise resistor value because the 10µF capacitor will be 10% at best so use a 560k resistor.

My statement was: 470K + 100K (not 570K as a single resistor). To compensate for capacitance deviations he should use the circuit as is - with a trimmable resistor.

 

[salutka]

I have a problem with my timer, I have it connected to a clean 12v power supply, it performs great for the first five minutes then it stars to blink constantly, if I kill the power for 1/2 hour and start it again it works for the first five minutes. Any suggestions?

 

[rjvh]

change the capacitor

it could be that you have a monday morning capacitor that sliped through the quality control

Robert-Jan

 

[Boncuk]

change the capacitor

it could be that you have a monday morning capacitor that sliped through the quality control

Robert-Jan

could as well be his private "monday morning". Electrolytic capacitors don't like reverse charging at all!

Go check it!

Hans

 

[salutka]

Turns out it was my Monday morning capacitor, btw the circuit works great.

Thank you all so much for your time and help, I really appreciate it!

 

[Boncuk]

Turns out it was my Monday morning capacitor, btw the circuit works great.

Thank you all so much for your time and help, I really appreciate it!

Always triple-check!

 

[salutka]

I know its been a while but I'm having problems with this circuit, the major one is that when the transistor brings the trigger to 0V for more than 6 seconds the output remains high. I know the circuit works because if I send voltage to the transistor the timer outputs 11v for 6 seconds. Another problem, but not that major is the transistor is too sensitive, meaning that even if I just touch the base (2n3904) with my finger it triggers, so I think I would need a way to stabilize and filter it. One more thing I used a 6.2k instead of a 6.8k on the base, I don't think it makes that big of a deal but U never know. Thanks.

 

[4pyros]

it could be that you have a monday morning capacitor that sliped through the quality contr

More likely a Friday night cap!!

 

[ronv]

Timer

The 555 doesn't like to have the trigger input low when it times out and will re-trigger. You could fix that by AC coupling the trigger to develop a short pulse. The sensitivity is probably because the the base of the transistor is "floating" when the key is off. This could be fixed with a resistor from the base to ground - say 20K.
You might consider the circuit below. It triggers only once when the key is turned on and only draws power when the key is on.

 

[salutka]

I'm not familiar with AC coupling and the "V1" component in your design. I see you kept every thing as-is pretty much, so your design will be easy to implement. If you can send me a part number I would really appreciate it! Thanks.

 

[ronv]

V1 is just there for the simulation. Connect your key switch to that point.