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Topic: 555 timer not resetting when connected to two relays... (Read 11149 times) previous topic - next topic

twang

Hello World!

I need to bug you again on this recent issue I have.
Attached is the circuit diagram.

The basic function is that a 555 timer oscillates a slow square wave to turn two relays ON and OFF. The relays serve as an H - Bridge to the motor. If it is ON it turns one way, if it is OFF it turns another way.

The problem that I have been running into is that when I turn on the circuit, it turns ON the relays
Good.
Then after the right amount of time (100 uf), the relay does not switch OFF but instead a faint buzzing sound is heard.

I tested the 555 circuit without the relays: it read a good 9v to 0v.
I tested the 555 circuit with one relay. It worked fine.
When I connected the second relay, both turned on, but they did not turn off. Voltage was 9v, then 2-3v, not 0v.

I've done some reading and is it a possibility that it has to do with pin 4? I tied the pin to GND and to 9v. It did not solve the problem.


I want to thank the person in advanced who will answer this question. Whoever answers with a working solution first gets 5 Dogecoins!  :smiley-mr-green: :smiley-mr-green: :smiley-mr-green: :smiley-mr-green: :smiley-mr-green:

Thank you.

michinyon

check how much current your 555 device can provide,   it is unlikely to be enough to power a relay coil.

TomGeorge

Hi, check where pin 4 should be connected, it is the reset pin, lookup data sheet.
Also 0.1uF cap from pin5 to neg power supply to help with bypassing.
Also diodes should be fitted across coils of relays to suppress back emf.
Diode Cathode to positive end of coil, Diode Anode to negative end of coil.

Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

MarkT


check how much current your 555 device can provide,   it is unlikely to be enough to power a relay coil.


200mA is the output capability of a 555, check the rating of the relay.

Never ever drive a relay coil without a flyback diode across the winding, you will
probably destroy your circuit.  Each winding needs a flyback diode across it, connected
anode to ground.  Lose the other diode in your circuit, its a mistake.

The 555 itself needs decoupling, and you need to wire it up right as has been described,
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

twang

#4
Jan 11, 2014, 07:59 pm Last Edit: Jan 11, 2014, 08:06 pm by twang Reason: 1
HI all,

  • So I tried tying pin 4 to Vcc already. It did not change anything. Isn't the 555 timer pin 4 tied internally to Vcc with a 100k resistor?

  • I'm sure the 555 can supply both coils. It can turn them both ON, just not OFF. I tried controlling both relays with the "Blink" sketch on Arduino. It seemed to work fine, so I expected it to work on the 555 timer.

    Here is the relay I am using.
    http://www.te.com/catalog/pn/en/8-1419130-3#features

  • Yes, I do notice I drew GND the same way as 9v. Sorry if that was confusing.

  • The other diode in the circuit from pin 3 to the relay was just something I was trying to solve this. I only added it in the drawing because I wanted to show what the current circuit was: The diode did not fix anything, you are right.



Now, I still need to try adding flyback diodes and a 0.1 uf capacitor; thanks for telling me!
I'm assuming I only need one diode for the two relays? The ends of the coil are connected to each other

Don't forget to include your dogecoin address!

twang

OK.

So I added one flyback diode for the two coils (because they are connected in parallel) and removed the diode from pin 3.

Sadly, it did not change anything :(

Does anyone know why it would work on Arduino blink but not the 555 timer?

MarkT

As I said you've likely destroyed your circuit by not having the diodes present, you'll
have to check everything is working without the relays, then check the relays take 200mA
or less when powered direct from the supply, and always always have those diodes across
the terminals - you can destroy your power supply even.

A relay coil stores a lot of magnetic energy when active, trying to switch it off without
providing an alternate route for the current (via the diode) can lead to 100's or 1000's of
volts spike, enough to trash most semiconductor devices immediately.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

twang

#7
Jan 11, 2014, 10:45 pm Last Edit: Jan 11, 2014, 10:50 pm by twang Reason: 1
Quote
As I said you've likely destroyed your circuit by not having the diodes present, you'll
have to check everything is working without the relays, then check the relays take 200mA
or less when powered direct from the supply, and always always have those diodes across
the terminals - you can destroy your power supply even.

A relay coil stores a lot of magnetic energy when active, trying to switch it off without
providing an alternate route for the current (via the diode) can lead to 100's or 1000's of
volts spike, enough to trash most semiconductor devices immediately.


Thanks MarkT for your recommendation!

I disconnected pin 3 from the relay. It still goes from 7v to 0v (My 9v battery seems to be dying). So that is still working

I tested the pin 3 with each of the coils individually with a flyback diode. IT WASN'T WORKING.
I apologize for the confusion at the beginning of the forum post; my memory was jogged wrong. It worked on the Arduino for one relay, but I never tested 555 with one relay. The 555 could turn on the one relay but it could not turn it off.

SO I found another clue

  • First of all, I've used low voltage relays a lot and have never run into a problem when not using flyback diodes. I guess I should start using them, however this time the issue was not with the backwards current

  • In the Arduino sketch Blink, whenever you do
Code: [Select]
digitalWrite(13, LOW);
    you are actually connecting the pin to either Vcc or GND (with registers and all that stuff I do not understand).
    If you use a continuity tester on pin 13 to ground, it will beep every time the LED turns off. The same applies vice versa, with Vcc and when the LED is on.
    I then tried the continuity tester on the 555 timer, pin 3 and GND. It worked,  but not vice versa, with Vcc. I do not think this really matters, as the relay has the ability to turn ON, just not OFF.

Even though that does not really matter to me, I wanted to say it just in case the forum made any use of this.
I should also mention that I have tried using pulldown and pullup resistors to the relay / pin 3. That did not help.

I don't think this will help but will it help if I mention that the chip that I am using is rather old? The HA17555 is what I'm using, and I'm assuming it's pretty much compatible with any other NE555 or NM555 or LMC555 or whatever555.

SO in conclusion from what MarkT recommended me to do, the 555 circuit is not damaged, but I realized that the 555 circuit does not even work with ONE relay. I can turn on ONE relay but I cannot turn it off.

I hope there are more problems with the circuit that people can find, because I feel like we are running out of solutions!

TomGeorge

#8
Jan 11, 2014, 10:49 pm Last Edit: Jan 11, 2014, 11:01 pm by TomGeorge Reason: 1
Reply #2
Quote
Also 0.1uF cap from pin5 to neg power supply to help with bypassing.

Have you tried that?
You could also put 100uF Cap across the supply pins of the 555 as well.
See you are trying to get 50% duty by using 1K for Ra, from what I have found when you have high values of C the discharge pin 7 has problems discharging the cap through 10K with 1K to supply.
There is a special circuit configuration for 50%.
Replace the 1K with a 10K, the frequency will be lower and the duty cycle will not be 50%, but it will check the theory

Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

MAS3

HA17555 was Hitachi's (hence the HA in the typenumber) equivalent to NE555.
Hitachi has put it's chip division in a new company called Renesas some time ago, and this company has merged several other brands into Renesas since.
You can find this information in the datasheet for this part.
Have a look at "blink without delay".
Did you connect the grounds ?
Je kunt hier ook in het Nederlands terecht: http://arduino.cc/forum/index.php/board,77.0.html

twang

#10
Jan 11, 2014, 11:01 pm Last Edit: Jan 11, 2014, 11:10 pm by twang Reason: 1
Hey Tom,

I just tried that a minute ago, attaching 104 from 5 to GND and a 470uf from Vcc to GND (I ran out of 100 uf).

It did not affect it, except the buzzing/whining sound starts of more articulated than it did before... I'm not sure if that is progress but if it is then cool!  :P

EDIT: Then I tried switching 1k to 10k. It is definitely slower, and the buzzing/whining noise is much lower. It sounds like the relay is stuck or something; some oscillation going on of course.

Thanks MAS3 for verifying that I can use this 555 timer!
Still not solved :(

twang

IMO, I think that the relay not resetting is because the 555 timer is for some reason not grounding pin 3 when it is connected to the coil
It usually grounds after the set amount of time, but when connected to the coil, even with a flyback diode, it doesn't.

So I'm assuming it has something to do with the relay or coil... And I am not sure what.

TomGeorge

Hi, the relays you are using are they 9V or 5V, the spec you linked to says 5V.
Put the relay coils in series, they might operate with 4.5V on each.

Tom..... :)
Everything runs on smoke, let the smoke out, it stops running....

twang

#13
Jan 11, 2014, 11:38 pm Last Edit: Jan 12, 2014, 03:47 am by twang Reason: 1
Quote
Hi, the relays you are using are they 9V or 5V, the spec you linked to says 5V.
Put the relay coils in series, they might operate with 4.5V on each.


Thanks Tom,

I just did that. I definitely notice a extremely short delay between relay clicks when compared to parallel.

After that click and the amount of time, instead of hearing a whining/buzzing I barely hear this extremely faint sort of hiss that lasts for a fraction of a second, then nothing.

The relay is still not turning off...

EDIT: I'm pretty sure it's not a (lack of) power issue, or my floating (now not floating) pin 4 and 5.

Still an enigma...

polymorph

Is that one of those 9V transistor radio batteries? Sorry, don't know what else to call it. A rectangular 9V battery with snap connectors on the end?

If so, the relays are probably drawing the battery down too far for the circuit to operate correctly.

What you posted was a pictorial. Could you post either a redrawn pictorial or preferably a schematic, the version of the way your circuit is connected now?

The way you had the diode in series with the coils provides no protection at all. The diode must be in parallel with the relay coil, placed so it is normally reverse biased when the relay is powered. With the relays in parallel, you'll only need one diode.



Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

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