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Aaaargh! 555 circuit works well with LEDs but not with relays...


bmachine
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I built a monstable 555 timer based on John Adams' shematic which stays on for 20 seconds.  The output of that one feeds power to an astable version  of the 555 circuit which blinks every two seconds.  I have LEDs at each stage and everything works perfect.  But when I add a relay in parallel with the LED (sending the ground and pin 3 to the coil of the relay) the relay just stays seems to try and close but does not (I hear a slight buzzing) and the LED does not blink anymore.

When I hook up the 9v battery directly to the relay, it closes easily so the problem is not there.

I thought it might be because there was not enough juice getting to it so I used a 9v dc wall wart but the results are the same.

Any idea of what I might be doing wrong?

Thanks very much.

Bmachine

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The max output current of an ordinary 555 (not a Cmos 555 which is less) is 200mA and with a 9V supply its output voltage is only 6.5V or 2.5V. So if the battery is brand-new then the relay coil gets much less voltage than the battery.
When the relay is turned off it developes a very high voltage across its coil which can damage the output of the 555 unless a suppression diode is used.

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Hello all,

I was always taught that I should drive a passive part with a transistor. (old school)

I have included a quick drawing.  I drew it because I don't know Bmachine's background, and Audioguru's explaination could be a little confusing to a newbie.  The Damper Diode (supression diode) is hooked up in  parrellel to the coil, and is hooked up in 'reverse bias' as shown.  (Audio, a newbie MAY hook the diode up in series with the 555 and the coil, or, hook it up in forward bias accidently)

I didn't go crazy with the drawing, leaving the additional designing to the builder!!!

oldgrandpainmi

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Thank you audioguru and oldgranpa.  Very good info.

Your assumptions are correct oldgranpa in that I am reasonably new at this.  So I very much appreciate your schematic.  I am assuming that the "thing" on the right is the relay (right above the OR).  So all I need is to add that transistor in between.  As far as said transistor, is NPN its full description?  In other words, if I go to Radio Shacsk, will the clueless helper be able to give me exactly what I need when I say NPN or is there a more specific part number to ask for?

Thank you very much again.

Bmachine

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Connect a resistor between the output of the 555 and the base of the transistor to limit the current. Use about 1k ohms.
The current ratings of the diode and the transistor should be the same as the relay coil's current. We can't recommend which diode or transistor because you don't tell us the relay's current. 

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Your relay uses a lot of current for a little 9V battery to supply, so its voltage quickly drops to below the voltage required from a 555 to operate the relay. The battery will last longer when a transistor is used to drive the relay.
I would use a 2N3904 or a BC547 transistor and a 1N4148 diode. The 1k base resistor is fine.

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Bmachine,

Sorry about the drawing.  Had I known that you were a real newbie, I'd have added the base resistor to the transistor, the current limiting resistor to the LED, and etc., but I see that Audioguru has it under control.  The intent of the drawing was to give you ideas for you to design your own circuit.  (it's a bad habbit, from helping my kids and grandkids with school work.  Give them help, not the answers.)

Fog maching and light machine?  Does "B" stand for "bowling"?  I did the same basic thing for a bowling alley I used to work for years ago.

Good luck!!!!

oldgrandpainmi



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Thanks oldgranpa and audioguru.

Guru, Apologies for the 100k/1k mix up.  When you fill out the "Reply" screen, you can't see  the previous replies and I could not see the schematic so I erroneously thought it was 100k.  I will follow your 1k directions of course.

The reason I wanted to use two transistors is that the fog machine has actual 120V going through that relay while the lights just need a very low voltage short.  I fear that if I hook up both my to output jacks in parallel to the one transistor,  the 120v might fry  the inners of the light controller.

Bo

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