Design Help 2 - Please NO PIC

I'm still waiting for my newark order so I went ahead and worked on the 555 solution. Based on the suggestions I got I came up with this. It has a few more components than some of the other solutions but it is working pretty well. A 100uf capacitor is giving me just about exactly 2 minutes of delay before off. I imagine there may be ways to eliminate the front transistor but for me it helped to have each active component doing one task. The transistor on the front deals with my normally closed, open is on, pressure switch. The 555 gives me 2 minutes of delay. And the transistor at the end drives the relay. The release is very clean and consistent. The relay is one of those 12 volt "ice cube" ones and is drawing about 150ma so I suspect I could get rid of the second transistor. The circuit draws about 14ma when "off" and I think I can live with that. I'm still planing to try some of the other suggestions once I get more parts. Thanks to everyone for all the help. I have learned a bit more :-)

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Reply to
jamesgangnc
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The relay driver transistor should be an npn too.

John

Reply to
John Larkin

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Isn't that going to reverse the on off?

Reply to
jamesgangnc

The pnp, as drawn, will never turn on, because it needs negative base drive and there's none available. If you want the relay on when pin 3 is high, the driver should be an npn.

And you could eliminate two more parts, the 10K and its diode. Just let the first transistor discharge the cap.

Or replace the 555 with a simple schmitt trigger inverter.

John

Reply to
John Larkin

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No. If you really had a conventional PNP output transistor in the circuit as your drawing shows, the collector base diode would be forward biased and pull the base high enough to reverse bias the base- emitter junction into avalanche breakdown, which would probably have destroyed the transistor.

For the long delay time that you want, a CMOS version of the 555 might be attractive.

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I hope your 100uF electrolytic capacitor is a tantalum device - back in the 1970's, when I would with these sorts of time constants, tantalum electrolytic capacitors offered much lowr and more stable leakage currents than aluminium-based devices.

-- Bill Sloman, Nijmegen

Reply to
bill.sloman

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These might help:

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Have fun,

Michael

Reply to
mrdarrett

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Guess I'd better check my parts again because I have something on the breadboard that works. I thought I had a pnp there but maybe I forgot what I put there. I'd better look at the part number on the transistor in question :-)

I'll try it without the 10k and the diode. Easy enough to pull them off. I'm assuming you mean on the output of the first transistor.

Working with what I have in the parts bin at the moment, 555s,

2n2222s, and 2n2907s :-)
Reply to
jamesgangnc

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Not with the 1K in the way. There might be a very slow beta degradation, which is irrelevant in a circuit that doesn't work anyhow.

Common aluminums have self-discharge time constants measured in hours or days, sometimes weeks. Try it.

John

Reply to
John Larkin

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This guy integrates 10nA on a 4,700uF electrolytic to drive a motor:

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Cheers, James Arthur

Reply to
James Arthur

For the long delay time that you want, a CMOS version of the 555 might be attractive.

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But it does not have the drive capacity of the bipolar device, so you need a sensitive relay or a transistor driver.

I hope your 100uF electrolytic capacitor is a tantalum device - back in the 1970's, when I would with these sorts of time constants, tantalum electrolytic capacitors offered much lowr and more stable leakage currents than aluminium-based devices.

You can get aluminum electrolytics with very low leakage and high stability for much lower cost than tantalum.

Paul

Reply to
Paul E. Schoen

555

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Yup, note his capacitor quality check: no significant self-discharge after "a few days."

John

Reply to
John Larkin

555

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No, now I see what you mean. The c-b-e path could conduct through the relay coil in addition to the base drive through the 1K.

That might zap the transistor, but pnp's seem to often have higher b-e zener voltages than npn's, so the current may be low. Again, it doesn't matter much if you damage a transistor that doesn't work anyhow.

John

Reply to
John Larkin

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"Common aluminums have self-discharge time constants measured in hours or days, sometimes weeks. Try it."

I will, I always thought that the aluminums had a high leakage current. I've got a Nichicon HE series 1000uF and 50V. The leakage current is listed as 500uA. This would imply a 'resistance' of 100 k ohm and a time constant of 100 seconds.

OK in 20 minutes the voltage has dropped by less than 10%! (And typically there is a 'fast droop in voltage after the initial charge.) Will this get worse with age? Why such a conservative spec if they are much better than this?

George Herold

Reply to
ggherold

55

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I suggest you don't need a transistor to invert the trigger pulse. Just connect the switch to V+ and 2 resistors to create a voltage divider to ground. Thy can be quite large as the trigger current of the 555 is very low. The midpoint of the divider should be at the specified trigger level (about 5 volts I think). When he switch opens the resistor to ground pulls the trigger line low. The NE555 should be able to run a 150 mA relay directly (200 mA drive capability).

Cheers

Cheers

Cheers

Reply to
Varactor

You evidently missed the part where the switch has one terminal accessible, and "ground" is bolted to an engine block.

Hope This Helps!

I've seen his latest 555 rendition, and I think I like it. ;-)

Hey, OP! Does it work?

Cheers! Rich

Reply to
Rich Grise

Because they're NOT much better than this at their maximum rated temperature, which is sometimes what they're run at. If it's rated to

105C (I checked below, it is), put it in boiling water (put it in a baggie meant for boiling food) and see what the leakage current is. It will surely be a lot higher than at room temperature.

Here it is:

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Leakage current is .01CV = .01 * .0001F * 50V = 500uA. Leakage current is guaranteed to be no more than that over the temperature range of

-40C to +105C.

I heard Bob Pease speak a few years ago, he said there are engineers who call him (or National) to solve problems - they were getting bit because they designed based on what's in the "typical values" column of data sheets. He said you should always design based on the worst case figures, as they're the only ones that are guaranteed. I was shocked, shocked I tell you! to learn that engineers would design based on anything other than worst-case specifications.

Remember this leakage current thing when this thing is in the engine compartment of your idling car in a traffic jam on a hot summer day.

Reply to
Ben Bradley

the 555

=A0It has a few

pretty

s of delay

ransistor

=A0The

pressure

or at the end

he relay is

I suspect

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for all the

We did, back in the early 1970's at Kent Instruments in Luton, England. Tantalum was quite a bit better. At that time, I knew the price of every transistor I was designing in, and we wouldn't have paid the extra for tantalum capacitors if they hadn't been worth the money.

-- Bill Sloman, Nijmegen

Reply to
bill.sloman

Thank you. It does work :-)

Reply to
jamesgangnc

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Ahh high temperatures, thanks Ben. I'll try a bit of heat on the cap and see if that makes it discharge faster. BTW I left the cap discharging through a 1G ohm resistor over night and came in to find the voltage had dropped from 18.3V at 5:30 pm to 15.6V at 8:30 AM. It's nice to know you can make a cheap RC time constant of a day or more if you ever need such a thing. A few years ago I 'designed' (stole from H and H) a variable ramp generator with periods from 1 ms to 1000 seconds. I used a 100uF tantalum for the long times. I looked at the leakage specs on the aluminum caps and never even thought to try them in the circuit.

George Herold

Reply to
ggherold

--
That was thirty years or so ago, Bill, so while your advice may have
been sage then,  it seems you believe that aluminum electrolytics are no
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Reply to
John Fields

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