MS 555 timer false trigger

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ptribbey
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MS 555 timer false trigger

Post by ptribbey » Sun Oct 03, 2004 10:48 am

Hi All, I've run ito a problem I could use some input on. What I've got is (4) 555 monostabe circuits each on their own board, 12V supply properly bypassed and decoupled at the chip, wired for one second pulse (10uFd-2K,68K), The output is diode protected, goes by a set of 1 foot wires to a 12V relay which controls a small 110VAC motor. The inputs are microswitches with 2 foot 18 awg wires to the boards which then feed .1 caps charged with 2K resistors. Furthermore, the pin 2 input is diode protected, and pulled up through a 100K resistor.
All circuits work great one at a time, but when the others are added, they trigger each other randomly when activated. I've tried pulling up the input harder (all the way down to 330 ohm), using a smaller coupling cap, charging the input cap faster, etc. The relays were originally on the boards, but taken off in an attempt to solve this.
The object of this circuit is to press one or more of four microswitches and get a one second spin of the respective AC motor, Whether you tap the switch or hold it down.
Hope somebody has an idea or two to try.
Thank You
Paul Tribbey

toejam
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Re: MS 555 timer false trigger

Post by toejam » Sun Oct 03, 2004 11:35 am

Those 555's have a very high input impedance. You might try putting a 1 meg or smaller resistor between the input and ground.

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Re: MS 555 timer false trigger

Post by Enzo » Sun Oct 03, 2004 12:00 pm

You didn't mention either way, but your relay coils are inductors, and when they are turned off they generate a large voltage spike trying to keep the current going. Proper practice is to put a diode across each relay coil in reverse direction. In other words if the relay runs off a positive supply, then wire the diode with the cathode - the line end - on the end of the relay coil that gets the positive supply voltage. That way the sipply voltage will not go through the diode normally, but the voltage spike will, and it shunts it off. 1N4007 or something similar works fine.<p>Look on any commercial piece of gear that uses relays and you will see a diode next to each relay and wired across its coil.<p>If you have no suppressor diodes on your project, those spikes might be what is triggering your circuits.<p>ANother thought is that your power supply might be marginal. It has enough juice to run a relay or two, but not for a whole pile of them. So when several try to energize at once it draws down the power supply voltage enough to confuse the circuits.<p>As an experiment, remove the relays from the circuit and put LEDs in their places. Add a current limiting resistor in each case. Now see if it still false triggers. If not, that verifies the relay coils are responsible one way or the other. If it still does it even with the LEDs, then something darned odd is going on.<p>When I design relay control circuits I usually wire them up to drive LEDs first anyway. That gives me a visual indication of the circuit function and is a lot less bulky.

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ptribbey
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Re: MS 555 timer false trigger

Post by ptribbey » Sun Oct 03, 2004 1:57 pm

Thanks for your replies, but,
as I mentioned in the third line of my original post "the output is diode protected" and my power supply is more than adequate at 12 volts at 32 amps. (computer style). Yes the input impedance is very high on these 555's, but it's looking for a voltage of 1/3 supply to trigger, and I'm pulling it up currently with a 100K ohm, but I've tried all the way down to 330 ohm. I,m sure the false triggers are somehow getting through to the input from the AC portion. Without the AC powered, all circuits work perfectly, either one at a time, or simultaniously. I'm scratching my head now because the AC portion is completely isolated from the DC part.<p>[ October 03, 2004: Message edited by: Paul ]</p>

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Re: MS 555 timer false trigger

Post by dyarker » Sun Oct 03, 2004 6:39 pm

When you moved the relays off-board, did the coil spike protection diodes move with the relays, or are they still on-board?<p>If still on-board, add diodes at the relays. Otherwise the wires become a transmit antenna for the spike current.<p>Put small capacitors at the relays and on the board across the wires (like .047uF or 0.1uF ceramic, mylar, poly, etc). Twist the wires together to make twisted pair.<p>The wires to the switches should also be twisted pair to reduce noise pickup.<p>Isolate 555 power from relay power. Relays can be connected to 12V from the power supply. Put a diode in series between 12V supply and the 555 power bus. Add around 50uF filter to 555 power bus.<p>"Without the AC powered, all circuits work perfectly, either one at a time, or simultaniously." Does this mean relays connected but not the motors, or relays not connected? I should have asked this first. Above suggestions should help in either case.<p>Cheers,
Dale Y

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ptribbey
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Re: MS 555 timer false trigger

Post by ptribbey » Sun Oct 03, 2004 7:07 pm

I will try using some twisted wire from the switches. About wires being spike transmitters, that may be true but it was the same malfunction when the relays were on board. To answer Dale's questions, 1: diodes on board, 2: relays and motors connected, AC off. I have scrutinized the supply rails with the o'scope, and can't detect any spikes. I have them heavily filtered, then filtered some more.
Thanks for those ideas.<p>[ October 03, 2004: Message edited by: Paul ]<p>[ October 03, 2004: Message edited by: Paul ]</p>

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Re: MS 555 timer false trigger

Post by toejam » Sun Oct 03, 2004 10:02 pm

I assume yuu are using a switching power supply. You might want to check the filter caps in it. if it is a 32 amp supply you may not be loading it enough for it to regulate properly.

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Re: MS 555 timer false trigger

Post by rshayes » Sun Oct 03, 2004 11:05 pm

Another possible problem lies in the NE555 timer itself. The bipolar version (the original NE555) uses a TTL-like totem pole output stage. With TTL, and also with the NE555, there is a brief interval of time when both transistors are conducting. This is due to the fact that recovering from saturation is a slower process than turning a transistor on. For a few nanoseconds, both transistors are on, and a pulse of current flows directly from the supply to ground. This pulse is very short, but can reach fractions of an ampere. This might be enough to trigger an adjacent device if it is coupled through the power supply or through a ground loop.<p>There are later versions of the NE555 made with CMOS. Both Intersil and Texas Instruments have made them and possibly other manufacturers as well (Digi-Key lists an TS555IN timer made by ST Microelectronics). These use a CMOS output stage that is not as strong, but also does not draw the high pulse current that the bipolsr device does. The bias currents on the input are also much lower, so these are better for long time applications. They are usually pin compatible with the original NE555.<p>With the CMOS versions, you will probably need a transistor buffer stage to drive a relay coil. The original NE555 can drive some relays directly, but I doubt if the CMOS versions can.<p>One advantage of the CMOS versions is that the output swings from rail to rail for light loads. This makes it simple to get a 50% duty cycle square wave by connecting the timing resistor to the output rather than to the reset terminal.

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ptribbey
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Re: MS 555 timer false trigger

Post by ptribbey » Mon Oct 04, 2004 7:47 am

Update:
Thank you all for your valuable input.
I had been aware of some but not all of your advice. Last night I wired up some 74c14 schmitt triger inverters for a one second delay. They each drive an mps a64 darlington which drives the relay. The kids have been playing the game all morning with no failures. While I wish I could have figured out the interference problem, The new circuits have a lower parts count and are therefore cheaper. You know: time and money.
Anyhow, thanks to all.
ps the supply is brand new, and I was monitoring it on one of my o'scopes. no sign of the skinniest pulse.
Steven, I suspect you figured out the problem. It seems to me to be the only logical answer. The part number was Texas Instruments NE555P.
Thanks
Paul<p>[ October 04, 2004: Message edited by: Paul ]<p>[ October 04, 2004: Message edited by: Paul ]</p>

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Chris Smith
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Re: MS 555 timer false trigger

Post by Chris Smith » Tue Oct 05, 2004 9:03 pm

RF in long wires can false trigger a 555.<p>The leads of one foot in length, are the perfect antenna transmitters. <p>Twist them for starters, shield them if possible.

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