Relay doesn't cut off.

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snug_gles

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Hi all,

Alright, the circuit uses a vibration switch to trigger a 555 timer which runs for about 3.5 minutes. The output of the 555 drives a 5VDC 1A relay to a load. I know that the 555 timer cuts off, but the relay does not, retriggers the 555 output I think. I have put the required 1N4004 and 1N4148 diodes as seen in other circuits, but this does not address the problem.

Any ideas on what else I can do or perhaps some ideas to improve the circuit? I do need to use the 555 and would prefer not to swap to a different IC.

Thanks.

I have attached the circuit below.
 

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Doesn't the line on the diode indicate the direction of current flow?

Well, I've wired it all up as per the circuit, with the protection diode as per the following tutorial that I referenced.


Main issue here is - it all works fine on the breadboard!!! But when I put it on the PCB, it doesn't.
 

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you dont have a ground for your relay!

are all your solders clean and making contact, because if it works on your bread board then it should work on your PCB.
 

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You're absolutely correct, I'm just lost with it all.

The correct circuit is attached below. This follows the correct protection diode diagram as above.

I've checked the soldering and tracks and everything.

I'm wondering if the 1N4148 is not right for the relay? Should I be using a different diode?
 

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If your vibration switch is still open when the 555 times out, the output will stay high until the switch closes. If this is the case, you need a differentiator between the switch and pin 2, so the input pulse is shorter than the expected output pulse. A differentiator for this circuit needs special attention, or it can destroy the 555 with overvoltage on pin 2 when the switch closes.
 
A couple of suggestions. 1. Connect a 47 uF electrolytic capacitor between pins 1 & 8. + of the capacitor to pin 8. I have seen 555 timers act erratic with out a bypass capacitor from the Vcc to ground pins. 2. Change on the 33uf capacitor to a low value like 4.7 or 10 uf and see if the relay deenergizes after 28 sec or 60 sec. Some times the leakage current of electrolytic capacitors on the 555 prevent the capacitor from fully charging. If the relay operates normally with a small value the larger capacitor could be the problem.
 
I think you may have hit the nail on the head. Unfortunately, if you make C smaller, R has to be larger, and you may have the same problem. One way to guarantee that the circuit will time out (although it will be shorter than predicted) is to put the timing capacitor in parallel with the timing resistor. The leakage now aids the resistor, instead of fighting it.
 
I've tried putting a bypass capacitor across +ve and -ve terminals of the 555 timer but still the relay sometimes does not cut off.

I've also reduced the capacitor to a 15uF tantalum capacitor and the circuit appears to work sometimes, but doesn't work at other times.

Interestingly though, whenever I touch my multimeter's -ve probe to the gnd pin of the battery, the relay clicks and cuts off straightaway. So I can't even measure what's going on. The other thing is, if I leave my -ve probe of my mulitmeter connected to the circuit, it works 100% of the time.

So, I still can't figure it out. Do I need to put an npn transistor into drive the relay from the 555 timer? Is that the problem?

I just can't work out why at some times... it works... then at other times... it doesn't... the relay seems to have a mind of its own. Problem is that the behaviour is not repeatable! Battery voltage stays between 5V and 6V as the 4AA's run down after awhile.

Any ideas?

Thanks in advance.
 
hi,
On your drawing where is pin 4 [RESET] and pin 5 [CONTROL] connected to.?
 
Hi Eric,

Pin 4 - RESET - not connected to anything.

Pin 5 - CONTROL - not connected to anything. I read that if noise is not a problem, this pin need not be connected to anything, but if noise is a problem, then cap is required.
 

hi,
The way you describe the effect of the DVM 'aerial' wire, suggests that mains pick up could be a problem.

I would connect pins 4 and 8 in any 555 monostable circuit.

Are you using a CMOS or TTL version of the 555.?

EDIT; The 1N4004 should be across the relay and the 1N4148 from the 555 output to the transistor base.
 
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Hi Eric,

Is the attached circuit what you are talking about?

Swap the diodes over and then connect up pins 4 and 5?

I wasn't sure what you meant by DVM 'aerial' wire.

I'm using the CMOS 555 (NE555).

Soldering as we speak.

Thanks.
 

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snug_gles said:
Hi Eric,

Is the attached circuit what you are talking about?

Swap the diodes over and then connect up pins 4 and 5?

I wasn't sure what you meant by DVM 'aerial' wire.

I'm using the CMOS 555 (NE555).

Soldering as we speak.

Thanks.
Click to expand...

hi,
Ref the diodes, I did misread your circuit, woops!. I would use a 1N400X in both locations.

A would agree with MSU, I would use a transistor to drive the relay.
You dont say what the relay current is.?
 
Thanks for the circuit diagram MSU.

..............................

The load draws about 250mA (max) but fluctuates between about 180mA to 250mA whilst operating.

I will change both diodes to 1N4004.

I will also try to put a NPN transistor to drive the relay. I've got some BC548 NPN transistors. What size resistor should I put for the base? Would a 1k2 resistor be correct?

So collector connected to Vcc, relay coil connected to collector via 1N4004. Base connected to 555 output via 1k2 resistor and emitter connected to ground.
 
snug_gles said:
I will also try to put a NPN transistor to drive the relay. I've got some BC548 NPN transistors. What size resistor should I put for the base? Would a 1k2 resistor be correct?

.
Click to expand...

Hi,
Look at this drawing.
 
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Hi again,

I've done some testing with the recommended circuits, but the relay still at certain times, does not cut off.

I've added a diode to the base of the transistor to try to cut off any reverse current to the 555 timer.

Would appreciate any comments if this is suitable or if there is a better solution?

Preliminary testing with the diode in place appears to be satisfactory.

Thanks.
 

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hi,
What type of capacitor is the 33uF..?

For testing have you tried reducing the 5.6M to say 1M0.?

Lets know what you find.
 
33uF tantalum or also using 22uF tantalum.

Want to get at least 2 minutes from the 555 timer.

Will test with a lower value resistor to increase the current to the cap later.
 
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