555 and relay

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 @

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.

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

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

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

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

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

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| | | ------- . +-|

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

Whats a 555? :)

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

--- 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:

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

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.

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.

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