problem using a 555 - help!

I'm trying to use a 555 to generate a reset pulse ~1ms on power application. Have the "usual" configuration with timing cap from pin 2/6 to pin 1 (ground), charging resistor pins 2/6 to Vcc pin 8. As this is a once-off pulse, DISCHARGE pin is open (no function required). RESET is at Vcc, CONTROL is open/bypassed to gnd.

Schematic at

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There is also a reverse-biased diode across the timing resistor to discharge the cap when Vcc is removed, to assure that the cap starts below 1/3Vcc.

According to the 7555 sheet, on application of Vcc this *should* give a high output until pin 6 reaches 2/3Vcc, then low output. That assumes that the chip gets its ducks in line before pins 2/6 rise past 1/3Vcc, which shouldn't be a challenge.

In my test, the output is low from the outset. No blip.

Apart from using a 555, what am I doing wrong? I could have done this with a bipolar, two resistors and a cap but I wanted a decent fall time :-(.

Reply to
skyline2000au
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application.

(ground),

DISCHARGE

open/bypassed

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discharge the

high

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I'm not sure why your circuit won't work. I've pasted two more conventional approaches below. (You'll need to view them in "Courier" font. Copy and paste into Notepad if necessary.)

Power-up Reset Circuit +V o--+----+----+----+--------+ | | | | | \ \ | | _|_+ Rtrig / / Rt | | === 100uF 10K \ \ 10K|8 |4 | / / +-+----+-+ | \ \ |Vcc RST| | ~1mS | | 2| |3 | Reset +-------+TR OUT+-----------o Pulse | | 7| (7)555 | | Out | +--+DIS | | | | | | | | +--+THR + | | | 6| |7 | | | | | | | --+-- = Connected Ctrig _|_ _|_ +-+----+-+ | | 0.01 --- --- 1| |5 | | |Ct | _|_ | | | |0.1 | --- 0.1uF | ----- = Not connected | | | | | | 0V o---+----+----+----+--------+ T=1.1RtCt

+V o | \ Rrst / 150K \ / \ |\ ~1mS | | \ Reset +----+---+ >o---o Pulse | | | / Out | \ |/ | / Crst _|_ \ Rdis 0.01 --- / 2M2 | \ T=0.7RC | | (A Schmitt Trigger is best, +----+ but any gate will work. To | use a non-inverting gate, -+- connect Rrst to 0V and Crst/ GND Rdis to +V.)

Hope this helps, ... Steve

Reply to
Steve Carroll

There are several easy to find 555 application notes on the web which describe in detail how to tailor pulse width to whatever you might need.

Reply to
ian field

Indeed there are, and I probably have them all here - including a Signetics paper one from the 80's. None of these provide a direct solution to the requirement, although the configuration I trialled - a hybrid of astable and monostable - seemed that it *should* work.

The requirement is that this unit MUST generate one and only one reset pulse (active high) once Vcc is applied to the circuit. Apart from controlling Vcc to the module I have no other control inputs possible. Also, the footprint available won't permit a DIP14 package, hence the selection of a 555 as a possible solution.

Reply to
budgie

The traditional method of generating a reset pulse is to use a simple C/R across Vdd & GND, if sharp transitions are important a Schmidt trigger can be constructed with a couple of transistors and a few resistors.

Reply to
ian field

I appreciate that. But in the same footprint I can squeeze the 555.

The circuit I am using matches fig 9b on page 8 of the Philips AN170, *except* (don't you love the way that always appears in places like this) instead of a switch shunting the cap as the start control I am relying on the cap being discharged below 1/3Vcc whn Vcc is first applied to the curcuit.

Incidentally I placed a 10uF cap across the 10nF and the circuit works fine, giving ~2.2secs of high pulse. So at some point between the two values it would seem that the 555 is not getting its act together before the cap charges to

1/3Vcc - about 1mS - which sounds incredible. Will be playing with more values to home in on the critical area, but it means the circuit's repeatibility is in question.

Thanks for your ongoing input.

Reply to
budgie

Why not use Maxim's series of microprocessor supervisors. They're available in SOT-23, or SC-70 packages, and you don't need an electrolytic cap that takes up valuable real estate.

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And they're low power.

Microchip has a range as well.

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dmm

Reply to
dmm

would

SOT-23,

valuable real estate.

Fair coment David. Unfortunately I have the pcb's to hand for the 555 effort, and also 555's are on the shelf here. I tend to think (aka design) with the bits on hand unless I can see a major issue with them or a major advantage in going another way.

But if it works with 10uF, why not with the original 10nF value?

Reply to
budgie

would

values

in

SOT-23,

valuable real estate.

A slow risetime Vcc would likely have a deleterious effect on the timing of the

555 output. First I'd check the risetime of the output of your intended power supply and ensure that the voltage doesn't make any unintended changes, like dropping to lower voltages while on its way to its final voltage. You could check if the 555 is affected by using a signal generator with a 5V sawtooth waveform to power a sample 555 and check at what point the trigger is made. I'd check some from other manufacturers as well, especially if you are intending to manufacture a number of your project.

dmm

Reply to
dmm

*except*

would

values

in

in SOT-23,

valuable real estate.

555 output.

ensure that the voltage doesn't

to its final voltage.

sawtooth waveform to power

other manufacturers as well,

Just to provide background, this module will live between two existing production assemblies.

DRIVING DEVICE MODULE TARGET DEVICE

The "target" device requires a reset pulse to initialise an on-board counter, and the driving device contains no spare directly controllable lines. The module's only role is to generate that reset pulse, and the only way to achieve this with existing pinouts and without redesigning and reworking driving device is a "power-on reset pulse generator".

The Vcc supply to the driving device's output connector (where the module connects) is switched by a relay. Vcc caps are on the relay input side except for a couple of .01's on the load device and one on the module. The rise time on the Vcc on the module is quite fast. Bear in mind the reset pulse duration isn't critical - the target of 1 or 2mS was established simply to allow software delays in the driving device to outlast it, and to be an order of magnitude (or more) slower than any Vcc rise issues.

I only hope that relay bounce won't intervene. But again, the "10uF works but

10nF doesn't" bit is what has upset my thinking.

The requirement is for about a dozen of these modules, so I can reasonably settle on a single manufacturer's 555 at this stage, but your point is noted re any variation with brand.

Reply to
budgie

On Sat, 26 Jan 2008 18:37:31 +0800, budgie put finger to keyboard and composed:

Can you replace the relay with a transistor switch, at least for troubleshooting purposes?

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.
Reply to
Franc Zabkar

software

Certainly can, Franc. Will try that out later on and report if anything different noted.

Reply to
budgie

achieve

device

except

software

(or

That didn't take long. I actually used another 555 as the switched source for Vcc, and the module behaved as planned. Went back to the CRO and I could see the Vcc bounce on the relay-driven configuration - although I had been

*triggering* the CRO on Vcc I hadn't been *viewing* Vcc, only the reset pulse and the target counter.

Apparently the bounce causes the 555 some issues. Certainly even with the bounce the module circuit works as predicted with 10uF, so I tried some other values and found that 100nF would reliably give a ~10mS blip.

As bounce can be unreliable especially from unit to unit, I will probably go with a more conservative 1uF and simply allow a longer software delay on startup.

Thanks to all who offered suggestions.

Reply to
budgie

It occurred to me that the 555 might be suffering the same problem that makes the logic need a power on reset - unpredictable operation while Vcc stabilises.

A radical solution, admittedly a bit like using an A-bomb to kill an ants nest, use an 8-dil PIC 12F series MCU to generate the pulse, the PIC has built in power on delay specifically to avoid the problem I suspect you might be having, if you need a longer delay for Vcc to stabilise, its only a few lines of code delay loop and the same again to programme precise pulse duration, its a little more expensive than a 555 but some of the cost will be offset by not needing any external components.

Reply to
ian field

Ian, I don't know if you had seen my later posts when you despatched this. The problem *appears* to be caused by contact bounce on the relay which switches the already_establiished_and_rock_steady Vcc to the subject module. When I use an RC combination which takes longer to charge from 0 to 1/3Vcc than the bounce takes to settle (worst case) I get good and repeatable behaviour.

I had actually considered those hard_to_source 8-pin minilogic gates as a simple way to achieve my requirement - I often use XOR gates to generate pulses. However the ubiquitous nature of the 555 won me over, and particularly as they are on the shelf here.

But certainly an 8-pin micro would have been a workable solution too.

Reply to
budgie

the

simple

555s are notorious for (re)triggering on the narrowest of spikes. My old Yellow Light car alarm was based on 555s and turning off the parking lights after arming it would make it trigger. I had to install a choke in series with its 12V feed to fix it. A mate of mine in Wollongong made a sprinkler timer with 555s and it would switch on by itself when he saw lightning in Sydney, 80km away. If the supply is bypassed really well and you've got an appropriate RC network on the trigger input so tiny contact-caused spikes can't get through, then it'll probably work OK. My 3c worth (it was 2c, but inflation pushed up the price).
Reply to
Bob Parker

Yes, retriggering in the normal sense is "managed" by the configuration (see URL in my original post). The Vcc supply itself is solid and thoroughly bypassed, but of course the weak point in my scheme is always going to be the relay which applies that rail to the 555 and its RC circuit.

With the revised value of C, the proto module is now happily behaving itself.

Reply to
budgie

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