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555 and relay

Greetings. I'm building a single traffic light controller circuit for a friend... basically it's three 555 timers in monostable mode (actually a 555 and a 556), capacitively coupled to trigger each other so that each stage can be individually timed. I've got it built with LEDs on the outputs, and everything's working fine. Unfortunately, it falls apart when I try to drive a relay in place of one of the LEDs.

At first, since I've not really worked with relays before, I hooked it directly to the coil. After that didn't work, I did some research. Eventually, based on a couple of documents I found (see Ref 1 and Ref 2 below), I set it up with a few diodes as shown here:

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I've tried a couple of different 555s. And I've tried it without the zener. Unfortunately, in all cases the symptoms have remained the same:

Whenever the timer in question is triggered, the relay engages as expected. When the timer's interval expires, it triggers the next timer, but does not return to low output. Listening carefully, at the time it should have disengaged, the relay starts a quiet buzzing, as if it's rapidly cycling. As far as I can tell, this goes on indefinitely.

This only happens with the relay in the circuit. I can return to just the LED on the output, and the circuit performs as expected.

In doing some searching of this newsgroup, I've noticed some references to using driving transistors... is this really necessary? The relay I'm using (datasheet Ref 3, nominal coil voltage 5V) only draws up to 40mA (and in practice,

Reply to
glibdud
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Keep using LEDs

Use Solid State Relays for your lights. They are basically a LED in series with 1k resistors. You can get SSRs that will drive up to 40A @

250vac in a small package. I am using them to control 5500 watt spa heater elements directly off of 741 OP amp outputs (comparator circuit as a thermostat)
Reply to
gfretwell

Your relay circuit looks fine; in fact it has more protective diodes than are actually needed; but I have 2 questions.

(1) How much current does it take to pull in the relay? Measure it.

(2) What kind of 555 are you using? CMOS or bipolar? A real NE555 can source considerably more current than a CMOS 555 (7555, LMC555, TLC555).

You may be simply overloading the output of the 555.

Reply to
mc

When the 555 stops supplyig current to the realy coil, the inductance of the coil drives the voltage at the driven end of the coil some 8V below ground, and draws current out of the 555 output via the 1N4001 diode.

This pulls charge carriers out of the silicon substrate on which the

555 was made, disturbing the operation of the integrated circuit. If you threw away the 7.5V zener diode, this wouldn't happen, but the relay would turn off more slowly.

A driving transistor would allow you to isolate the 555 from the relay coil more effectively.

--
Bill Sloman, Nijmegen
Reply to
bill.sloman

Did you put a .01 uF on pin 5 to ground? Do you have a big electrolytic across pin 8 and ground? Can you post a schematic?

Ed

Reply to
ehsjr 

--
Try this: (View in Courier)

           +V    +V  
            |     |K      U1
           [R1] [CR1]  +-------+ 
            |     |    |_      |
 +----[C1]--+-----+---O|T   OUT|--+----+-------+
 |                     |       |  |    |       |K
 |                     +-------+  | [COIL1]  [CR4]
 |                                |    |       |
 |  +-----------------------------+   GND     GND
 |  |      +V    +V  
 |  |       |     |K      U2
 |  |      [R2] [CR2]  +-------+ 
 |  |       |     |    |_      |
 |  +-[C1]--+-----+---O|T   OUT|--+----+-------+
 |                     |       |  |    |       |K
 |                     +-------+  | [COIL2]  [CR5]
 |                                |    |       |
 |  +-----------------------------+   GND     GND
 |  |      +V    +V  
 |  |       |     |K      U3
 |  |      [R3] [CR3]  +-------+ 
 |  |       |     |    |_      |
 |  +-[C1]--+-----+---O|T   OUT|--+----+-------+
 |                     |       |  |    |       |K
 |                     +-------+  | [COIL3]  [CR6]
 |                                |    |       |
 +--------------------------------+   GND     GND 
                              

The capacitors are 0.1µF, the resistors are 10k, and the diodes are
all 1N4148s.

Also, connect a 0.1µF capacitor across each timer\'s power terminals,
as close to the package as possible.

I haven\'t shown the timing components, but since you\'ve gotten it
working with LEDs, you know where they go, already. :-)
Reply to
John Fields

This can occur with a relay from output pin 3 to ground, operating with output high. A diode, 1N4148 etc, anode to pin 3 and cathode to the relay coil will fix this. The relay coil needs a diode across it as usual to snub the turn off transient.

As a source, the 555 drops around 2 volts, so if your supply is only 5V you may not have enough to drive the relay. 8 volts would be good.

all the best Ian Macmillan

Reply to
Ian Macmillan

Thanks to everyone... there are some good suggestions in there. I'll play with it some more this weekend and post what I find.

To answer a few questions:

- The relay drew about 25mA when energized.

- The chips are NE555P and NE556N

- There's a cap on pin 5, but I may have forgotten my filter cap across the power supply. Power's pretty clean, though. (Sorry, don't have a schematic handy nor the time to draw one at the moment. Might do one up this weekend.)

Thanks again,

David Murphy snipped-for-privacy@yahoo.com

Reply to
glibdud

At that small current you can use the 555 internal DISCHARGE transistor, and this will put nearly all of the VCC across the relay. You will need to reconfigure the 555 for positive edge triggered as shown below. This is a less common configuration and actually works better because the 555 will go into SET at power on, keeping the relay OFF, the TRIG input overrides any voltages applied to THRESH. The positive going edge applied to THRESH resets the 555, causing DIS to pull low, turning the relay ON, OUT also goes low which discharges CT through RT. This state continues until CT is discharged to VCC/3 in a time 1.1*RT*CT, which sets the 555 through TRIG, causing DIS to turn OFF, and OUT to quickly recharge CT. The relay turns OFF and the voltage at DIS can be used to positive edge trigger the next 555 in the chain. The 5.6K is used to top off the charge on CT to VCC. The 560 quick recharge resistor should be negligible compared to RT and limits the OUT current during the recovery phase. RST input is not shown, tie it to VCC if you're not going to use it. View in a fixed-width font such as Courier.

. . . +VCC . | . .---------+---+-------------+---+---. . | | | | | |+ . | === | | _ === . | 0.1U | 1 | | | |1U . | | | N - |/| --- . | --- | 9 ^ |/| . | | 1 - |/| . | -------------- 4 | - . | | VCC | A | | . [5.6K] | | ---+ . | | | | ___ ___ . | | | | | | . .--------------|TRIG DIS |--------+----> | | . | | | | | | . |1N914A| | | ------- . +-|

Reply to
Fred Bloggs

I'd make the change, shown below, for how the input pulse is handled:

--
--
kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

Whats a 555? :)

Reply to
Brian

Good one, kills a good 1/4ma quiescent.

Reply to
Fred Bloggs

--- The capacitor across the 555's supply terminals isn't to help clean up the supply, it's to provide the very high current the 555 needs, momentarily, when it crowbars the supply during output transitions. See page 6 of:

formatting link

Therefore, in addition to any other cap(s) you may have in there for gross filtering/decoupling of the supply, you'll need a separate, at least 0.1µF ceramic cap located as close to each chip as possible.

---

--- OK.

-- John Fields Professional Circuit Designer

Reply to
John Fields

Tom Tom, Sorry, I missed it, but what is IOCCC? Thanks.

Let's say I want to hook it up backwards, the relay will energize, closing the N/O (Normally Open) relay contact and completing the circuit from the charger - it just seems to hope that it won't work as i thought.

Reply to
Aristotle Eisenglas

Hmmmm- in your header:"Comments: This message did not originate from the Sender address above. It was remailed automatically by anonymizing remailer software. Please report problems or inappropriate use to the remailer administrator at ."

Whatcha scared of, pussy? Why all the hiding? Damned chickensh_t riffraff using an anonymous remailer...probably a pontificating loud mouth asswipe who realizes he will lose all credibility by publicizing a pea-brained ignorance of electronics that will belie scant intelligence approaching disability.

Reply to
Fred Bloggs

Fred, the guy is a troll-pest; he is posting randomized lines of text all over this and, I think, other newsgroups.

--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
Reply to
John Woodgate

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