lm555 timer problem

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LED

5-VDC-S.P.S.T... I

Which way did you wire the protection diode? The cathode goes to the 555 output and the anode goes to ground. The diode should only conduct when the relay is switched off, not on.

-Bill

Which way did you wire the protection diode?

The cathode goes to the 555 output and the anode goes to ground.

The diode should only conduct when the relay is switched off, not on.

I'm doing a one second on 10 off.

I'm applying the current directly to the relay as it would output from pin 3 and it pulls

Yes

This may be the case my pack is at 5 volts and I'm getting a little over 4 at the output.

I'm using a cmos 7555 chip if that means anything.

Thats what I figured.

I have a basic off the shelf astable timer circuit with the diode for less than 50% duty cycle thats on this page.

I hooked it up with 4 AAA NiMH batteries. I used an LED to get the resistors correct for the timing I need and it all works.

I'm going to drive a relay with this circut so I hooked up a 5v reed relay with a protection diode and I get nothing. Replace the relay with the LED and it still works. I apply the 6v to the relay directly and it works.

Here is the relay

Any thoughts? Am I just not putting out enough juice to drive the relay? I tried another regular 5v DIP realy with the same results. Maybe a transistor?

thanks

------------

You need to put the relay from the output to the plus supply. IIRC the 555 has more pull-down capacity than pull-up.

Need a counter EMF diode across the relay to protect the chip from the release voltage spike.

mike

You didn't mention how fast you are pulsing the output? I can only assume you have it slow enough to pull it in.

Also, some DC Relays, (like I said, some) have a diode in them and if you connect them incorrectly you'll load the output of the timer and the relay won't pull in.

How ever, I am going down a different path here, I'll wager that you are attempting to turn on the relay using the ON state of the timer directly on the output? If that being the case, there is one thing you should know about these little buggers, on the high side of the output state there are darlington transistors and you get a loss of ~ 1.5 volts on the output.. So, it's Vcc-1.5 I think the last time I checked? This maybe enough to cause your issue.

The Cmos version has much less output current than the non-cmos version. I can only assume you're working with the old stand by non-cmos bipolar type.

The solution to your problem is to use a NPN transistor in common emitter mode and the collector to load the low side of the coil. Use a R to drive the base of the NPN into saturation from the 555 output.

Jamie

The Cmos does not have the voltage drop like the bipolars do how ever, they have less current handling at the output. Your relays maybe may need more than it's getting. Yes, you still need a transistor in this case. Especially with CMOS!

Jamie

Not all 7555's were created the same. Some "brands" can source or sink

100ma while others are only 15ma - TI comes to mind.

To use that particular 7555 you will need to use a transistor

-or-

buy another brand of 7555 (check specs first)

-or-

use a standard 555. These may put some switching spikes on the power supply and that might be a problem with other circuits.

The relay wants about 50ma to operate so this isn't going to be a problem for any but the lamest 7555

correction IIRC some 7555's will only source 10ma but sink 100ma - changing the direction of current through the relay may work but you have to adjust your timing

"Cliff Hartle"

** Then why the hell did you head your dumb post with the

WRONG NUMBER ???

.... Phil

LM555 can drive 200mA loads, an 5V reed relay is likeyl to require much less than that. but if you've got one of the CMOS variants (7555, TLC555 , UA555 etc) instead of the LM555 that could be the problem, those parts can't provide current sufficient for much more than a LED.

103 ohms, it's going to want around 50mA with with 5V supply,

you didn't leave the LED resistor in the ciercuit did you?

If that's a CMOS 555 all is not lost you can connect the relay between pin 3 and pin 8 and it should work, as the CMOS parts can sink current stronger than they can source it.

--
?? 100% natural

--- news://freenews.netfront.net/ - complaints: news@netfront.net ---

Thanks for all the proper replies. (Sorry I thought the LM meant cmos)

I'm going to put a n2222 transistor in the circuit and move on.

It looks like I just need a 470 ohm resistor between pin 3 and the 2222 to drive it properly and I'll make sure all the protection diodes are installed.

thanks

"Cliff Hartle"

** AFAIK - Natsemi's " LM " prefix stands for " Linear Microchip ".

It began with the LM101 op-amp released in 1967.

The so called " CMOS versions " of the LM555 are only functionally similar but otherwise un-related.

.... Phil

--
LM = "Linear Monolithic"; DM = "Digital Monolithic".

Wee, I see flames, back fire, explosions coming now!..

Jamie

"David Eather"

** The Intersil one is pretty lame:

I source is only a couple of mA with a 5 volt supply.

.... Phil

OK here is the latest.

I replaced the CMOS 555 with a non CMOS 555. Don't ask me which one, its the one Radio Shack had in their bins. (sorry only source for one or two parts)

Still a problem.

I narrowed it down to the diodes, N4001's, I was using to protect against the spikes. I have one going from the output to the relay and one going across the coil like in this circuit.

I double and tripled checked the polarity of the diodes and they are correct. Its like its something so simple I can't see the problem.

As soon as remove the second diode. Works like a charm.

Some circuits I have seen only use the one from the output to the coil, some have the one across the coil and some have both.

Do I need the one across the coil?

The circuit that you posted (3a) should work like a charm. Try that - there is only 1 diode. IFF still difficulty try a replacement diode. IN4001 will work fine in this app.

Yes. That is the only diode you need in the output circuit. Remove the diode that goes from the 555 output to the relay.

Ed