lm555 timer problem

[Cliff Hartle]

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

http://www.doctronics.co.uk/555.htm#more_astables

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
http://www.allelectronics.com/make-a-store/item/RLY-617/5-VDC-S.P.S.T.-N.O.-REED-RELAY/1.html

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

 

[Cliff Hartle]

Which way did you wire the protection diode?

I double checked the diode direction by connecting the led after the 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.

 

[Cliff Hartle]

You didn't mention how fast you are pulsing the output? I can only

assume you have it slow enough to pull it in.

I'm doing a one second on 10 off.

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.

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

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?
Yes

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.

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

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.

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

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.

Thats what I figured.

 

[Jamie]

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

http://www.doctronics.co.uk/555.htm#more_astables

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
http://www.allelectronics.com/make-a-store/item/RLY-617/5-VDC-S.P.S.T.-N.O.-REED-RELAY/1.html

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 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

 

[Jamie]

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




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.

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

 

[Phil Allison]

"Cliff Hartle"

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

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

WRONG NUMBER ???



..... Phil

 

[Jasen Betts]

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.

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.

Here is the relay
http://www.allelectronics.com/make-a-store/item/RLY-617/5-VDC-S.P.S.T.-N.O.-REED-RELAY/1.html

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?

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?

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.

 

[Cliff Hartle]

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

 

[Phil Allison]

"Cliff Hartle"

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

** 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

 

[Jamie]

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

Jamie

 

[Phil Allison]

"David Eather"

Not all 7555's were created the same. Some "brands" can source or sink
100ma while others are only 15ma - TI comes to mind.

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

** The Intersil one is pretty lame:

http://www.intersil.com/data/fn/fn2867.pdf

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



..... Phil

 

[ehsjr]

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. http://www.eleinmec.com/figures/001_03.gif
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?

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