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Author Topic: Simple 555 timer circuit.  (Read 6605 times)

Bill D203

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Re: Simple 555 timer circuit.
« Reply #25 on: January 25, 2011, 04:12:27 PM »

Cheers PMK / Pete.
The timer got here today and is just the jobby :-)) :-)) :-)) :-)) :-)) :-)) :-)) :-)) :-)) :-)) :-)) :-)) :-)) :-)) :-))
Must buy you a pint  very soon :} :}
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PMK

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Re: Simple 555 timer circuit.
« Reply #26 on: January 25, 2011, 05:35:34 PM »

Would love to take you up on your offer of a pint, but the man from Delmonte (el cardiologist), he say NO!
Just glad to hear that the timer gizmo does what it's supposed to do.
Health permitting, give me a couple days and I should have your KillBill failsafe gubbins working.

Later.....
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malcolmfrary

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Re: Simple 555 timer circuit.
« Reply #27 on: February 05, 2011, 04:16:22 PM »

Yet another thought after finding the Practical Electronics Handbook download.
This one uses an SCR, not often used in DC applications because they don't like switching off. 
At switch on, the capacitor charges up to the zener voltage.  At that time, Q1 passes current to the gate of the SCR, which switches on, and stays on until power is disconnected.
At switch off, the capacitor will discharge via whatever was also connected to the battery (a 1M resistor across a and k of the SCR would ensure discharge)
Sadly, craplins, while they do have one SCR listed, don't want to sell any.  They can't be bothered to show any useful information, including the type(!).  Fortunately, ESR have a few listed, and data for these can be found.
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PMK

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Re: Simple 555 timer circuit.
« Reply #28 on: February 05, 2011, 06:09:34 PM »

Ah, that's nice. The beauty is in the simplicity. I have a few SCRs in the junquebox - TO92 types, but no idea of their exact specifications.
If I may, just two (albeit very minor) questions:

1) Should the Zener read 4v7, or 4.7v - as opposed to 4k7?
2) I'm not familiar with seeing the 10k resistor tied to the wiper of the pot in this particular instance. Could I pick your brains and ask what are the benefits of doing it that way? I'm more used to seeing the 10k being in series with the leg of the pot on the 'hot' rail, and the wiper usually connected to, say, the Base of Q1... if that makes sense.

The 1-meg res' across A and K would be my prefered choice of discharging the cap'.
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malcolmfrary

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Re: Simple 555 timer circuit.
« Reply #29 on: February 06, 2011, 10:48:50 AM »

4V7 is just another way of saying 4.7v, usually thats what appears on the zener anyway, decimal points tend to get lost on the component, which can be tricky.  It also helps confirm that its a zener diode under consideration.
The wiring of the slider resistor is purely arbitrary - its just there to make sure that the capacitor does take some time to charge.  The results would probably be the same if the connection between the top of the pot and the +ve line.  The pot is what adjusts the timing of the charge, Q1 is an emitter follower observing the voltage and presenting it to the zener diode at a lower impedance, so Q1 does need to look at the capacitor voltage.  The pot should give a good range of times, changing the capacitor would give a further range, as would using a different voltage zener.
There are lots of ways of skinning this particular cat, and its quite possible that the result could be achieved with yet fewer components, (741, pot on one input, R on the other, transistor output springs to mind) but unless I accidentally have a large cheese butty and a flagon of something potent for supper one night, probably not me.
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PMK

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Re: Simple 555 timer circuit.
« Reply #30 on: February 06, 2011, 06:04:27 PM »

"4V7 is just another way of saying 4.7v...."

Absolutely. It's just that your "4K7" kinda threw me. I guess I'm more used to seeing the "K" when it comes to resistance.
(Okay, okay - I know. I'm just being nit-picky again is all.)

<10K wiper resistor>
"The results would probably be the same if the connection between the top of the pot and the +ve line."

Ah, that's what I wanted to hear. Thanks for the heads-up. The rest of the circuit is pretty much self-explanatory, but thanks for the detailed info. just the same.
Concurred, ref the 741 - in which case hysteresis wouldn't even rear its ugly head.

Here's to a cheese sarnie and a flagon of scrumpy.
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malcolmfrary

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Re: Simple 555 timer circuit.
« Reply #31 on: February 06, 2011, 09:39:02 PM »

Quote
Absolutely. It's just that your "4K7" kinda threw me. I guess I'm more used to seeing the "K" when it comes to resistance.
Quite right, "K" should have been a "V".  Don't know where that crept in.  Probably a  Silver moment  {:-{ (mental note to self - just the one quart of absinthe on the breakfast wheaty-bangs).
Hysteresis shouldn't really be a problem with a power on delay.  When power goes off, the relay should release immediately - the only theoretical problem is making sure that the capacitor is sufficiently discharged before switching on again to keep the required delay.
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