Capacitors as timed circuit

R3BORN UK

Jan 15, 2013
8
Making a buzzer wire game. I have a:

12v (although it works down to a minimum of 3v) horn
http://www.ebay.co.uk/itm/140957326028?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

60cm of 12v multicolored flashing LED strip
http://www.ebay.co.uk/itm/170885943093?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

I have the idea of using capacitors to charge, then as soon as contact is made they discharge, powering the horn and lights for 1 or 2 seconds.

What I'm not sure about is the correct capacitors to use?

BobK

Jan 5, 2010
7,682
There is no current draw given for either product, so it is impossible to tell.

But making a guess, the LED strips alone would a good fraction of an amp, lets say 1/3 and the siren would probably take more, so let's say 1 Amp altoghether.

To pull 1 Amp from a capacitor for 2 seconds puts you into the supercapacitor region.

Bob

R3BORN UK

Jan 15, 2013
8
There is no current draw given for either product, so it is impossible to tell.

But making a guess, the LED strips alone would a good fraction of an amp, lets say 1/3 and the siren would probably take more, so let's say 1 Amp altoghether.

To pull 1 Amp from a capacitor for 2 seconds puts you into the supercapacitor region.

Bob

Ek. Ok, I do have some 555 timers and a pile of npn's and pnp's? Not sure of the max rating for the timers though

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Ek. Ok, I do have some 555 timers and a pile of npn's and pnp's? Not sure of the max rating for the timers though

A 555 will easily work with a driver transistor to drive your load.

Chris

R3BORN UK

Jan 15, 2013
8
What other bits would I need to get? Simple as poss is better given the time constraint

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
What other bits would I need to get? Simple as poss is better given the time constraint

Well, as Bob pointed out ...
There is no current draw given for either product, so it is impossible to tell.<snip>Bob

There's also no flash rate or duty cycle given. We need this information.

Chris

R3BORN UK

Jan 15, 2013
8
Apologies, the lights finally came through the post this morning so I could finally test them all.

The lights go through a very lengthy cycle of quite decent programmed flashes/chases,which wouldn't be seen on the original suggested set-up - so I'm going to have them constantly "on" on a separate circuit.

I have a 12v 2A power supply which I used to test the lights and horn. At their brightest program the lights suck just under 500mA. The Horn however seems more lenient, connected to the 12V 2A supply it pulls a full 1A - However, it seems to run fine off of two AA batteries requiring a dramatically smaller amount of 140mA (with only a small change in dB).

So my thinking at the moment is to have the lights on a separate battery (bank of AA's or a moped battery do?) Then if they die they die, the game can still be played.

The horn can then run of it's own, smaller, supply. This is the one I need some kind of "delayed-off" circuit for. If i stick to a 2 or 3 AA for the horn - so as to keep the amps down - is a simple capacitor/series of a viable option again? Or would a 555 still be a more reliable option?

R3BORN UK

Jan 15, 2013
8
Ok, some success

Got this: http://www.technologystudent.com/elec1/5554.htm to work brilliantly. Now is it possible to alter it so if the switch is held closed, the output to the relay stops after the time as set by the VR1? Ideally the switch would need to be release, and then re-pressed to activate the horn for another few seconds.

Thank you guys!

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Ok, some success

Got this: http://www.technologystudent.com/elec1/5554.htm to work brilliantly. Now is it possible to alter it so if the switch is held closed, the output to the relay stops after the time as set by the VR1? Ideally the switch would need to be release, and then re-pressed to activate the horn for another few seconds.

Thank you guys!

Assuming you're referring to the circuit at the top of the page I don't see VR1. That circuit is a monostable and is what you're looking for but it needs some tweaks. As it stands it's level triggered on the Trigger pin 2. If S1 is held closed it will never time out. Google "Edge Triggering a 555". This only needs the addition of a small cap and another pull-up resistor. Edge triggering will insure that anyone with a heavy finger can't hold the output pin high. I would also move D1 with the Cathode connected to pin 3 and Anode connected to Ground.

Chris

R3BORN UK

Jan 15, 2013
8
Sorry! I replace R1 (I think) with a 10k variable. Where should I be looking to add the new cap/res?

(I did look at edge triggering as you suggested but couldn't make much sense of it as what would be applicable and where in my circuit - still slightly noobish)

Last edited:

May 8, 2012
4,960

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