Just another monostable 555 timer question

[RobertRobot36]

Just another monostable 555 timer question :D

I have a 555 monostable circuit with a relay attached to + and pin 3.
I tried attaching it to pin 3 and - but it didn't work. Anyway, its very temperamental and keeps going that when you push the button it will stay on for about 2 seconds or less, and sometimes it stays on just for the time the button is pushed.

I was wondering if it was the battery, but i was planning to connect a 9V plug to where the battery should be. Would this be a problem and blow the chip? If it would how could i fix it?

So yeah, my two questions:

Can i use a 9v powersupply instead of a 9v battery?
Why does the relay not stay on for the 15 seconds (ish) that it should?

Thanks for your help :D

Edit:
On the relay i have a motor. When the motor circuit is turned off the timer circuit works fine, but when i turn on the motor, the timer does not function correctly. The only way the two circuit are connected is by the relay

 

[schip666!]

Please post a schematic.

Off the top of my head though...if things work right without the motor connected then either the motor is drawing too much current (if on the same power supply) or it is glitching the supply somehow. It might also be that the 555 output can't supply enough current to reliably run the relay, you'd need to check the specs and the specific relay you are using and see if they match up OK.

For the "9v powersupply" question, I presume that you mean a 9v "wall-wart" kind of thing. This _should_ be ok, but be aware that many such warts are not regulated so the actual un-loaded voltage might be around 15v -- which is still ok for a 555 I think. Also the output may not be filtered very well, so adding a big electrolytic capacitor might help.

 

[RobertRobot36]

This is part of the circuit. The other part is completely separate so we won't worry about that. I changed the capacitor and it worked fine, but now its going funny again. I don't know if it is bad soldering but i have checked the joints and they look ok to me.

Thanks

 

[schip666!]

You should probably have a bypass cap from pin 5 to ground in case you are getting noise on the control pin. A reverse-bias diode across the relay coil is always recommended, and if you're not reversing the motor you could put one there as well.
And a bypass cap on each power supply too.

The specs say the 555 can source or sink 200ma, so check that your relay is not drawing more -- it should be quite a bit less. And the max supply voltage is 16, so check your wall wart first. If you are not using it to run the motor you might find a nominal 6v wart to keep the max voltage down, or even a 5v regulated one.

Also check the motor supply wiring. If there's any chance of it interfering with the 555 supply YMMV.

 

[RobertRobot36]

Sorry that went straight over my head
The relay says:
Res. Load 10A
240VAC or 24VOC
Coil DC 6V

Im not going to use the plug on second thoughts.
Im also confused at why the relay does not work from pin 3 to - because it worked once, but then i re made the circuit and i can't get it to work :S It just clicked on and off

Im not very 'advanced' with electronics so could you explain in simple terms please :D (Sorry to be a pain )

 

[schip666!]

I think those specs are for the relay's switch contacts. But you need to verify that the coil, which is/should-be operated by the 555 output, doesn't draw more than 200ma. A simple thing would be to measure it's resistance and use the magic of OhmsLaw: At 9 volts a resistance of 45 ohms will draw 200ma. So you want your relay coil to have a resistance higher than 45ohms, better to be way higher. Also, your power supply needs to be able to deliver at least that much current, if not, operating the relay will cause the circuit to loose power.

A "reverse bias diode" is a power rectifier diode connected across an inductive load, like your relay and motor, in the reverse direction of the normal current flow. So a diode between your relay's coil pins, pointing "up"...here's a picture: http://www.industrial-electronics.com/image/slnd-diode-protect_fig4.jpg

"Bypass caps" are just small capacitors, usually .1uf ceramics, connected near the power supply pins and ground. They "filter" high-frequency noise and spikes on the power. The 555 reference schematic also shows one from pin 5 (which is a control voltage input) to ground, and you don't show that in your schematic.

The wall-wart problem is that most are not regulated, so when they say they are (for instance) 9v and 200ma, they mean that they will supply 9v when 200ma is being drawn from them. If you don't draw the whole 200ma the voltage will be significantly higher, and could be higher than the 555's maximum. You can measure things. Put your voltmeter on the output of the wart and try some different resistors across the output. Again 9v at 200ma is 45 ohms... Be careful because that is close to 2 watts, the resistor will heat up quickly and your typical 1/4w resistor will burn out.

At this point I would back out to the simplest configuration. Just the 555 with no relays or motors. Get that to work reliably, then add the next component. If it stops working you probably have a power-supply problem -- likely too little current to run the relay or noisy junk on the power.

 

[RobertRobot36]

Thankkss for that, i understand now :P

I have started from just the 555 circuit and it seems to be ok, but i think it randomly turned on earlier, but i think some of the contacts touched.

Thanks for your help :D

 

[vk6kro]

Relay coils generate large voltage spikes when they are switched on or off, so it is a good idea to have a diode across the coil to clip off the spike to avoid damage to the device driving it.

[PLAIN]http://dl.dropbox.com/u/4222062/relay%20driver%202.JPG

This circuit shows the connection of the diode, but it would also be a better way of driving the relay. Only a small current would be supplied by the 555 and a cheap power transistor could be used to do the actual switching.

 

[RobertRobot36]

Can an LED be used as a diode because i don't have any diodes at this minute :P

 

[schip666!]

You need a 1amp or greater power rectifier diode, an LED might blow out on the first kickback. It is a refinement to the circuit so you might get away without using it.

The transistor circuit posted by vk6kro may be handy if you find that your relay _is_ drawing too much current from the 555 output. Note that the "from micro" input needs to go high to turn on the transistor and relay.

 

[RobertRobot36]

When i turn on the 555 circuit and the motor circuit it works , but if i leave it in the off state it comes back on after a while. Will putting the diode across it solve this?

(I measured my plug and it was 15v)

 

[vk6kro]

No, the diode is there to stop damage to the 555.

It is possible that this damage may have already happened.
The 555 should not be turning on by itself.

Maybe you could start with a new 555 and use the circuit above to switch the motor.

 

[RobertRobot36]

Ok, ill get a diode tomorrow and a new 555.
I think it turned back on because the battery was running low

 

[RobertRobot36]

Hi again,
I've done the thing with the diode, but i don't think a 9v battery is enough volts for the circuit?
The Volts coming out of the battery is about 8.20
The Volts going to the coil when the button is not pressed is about 2.40
The volts going to the coil when the button has been pressed is about 5.50
The volts coming out of the coil when the button is pressed is 0 (or 0.01)

Would it be a good idea to increase the voltage of the battery, because the coil is rated at 6v, but only 5.5V is entering, and the switch bit inside does not operate.

Any ideas?
Thanks

 

[vk6kro]

It sounds like the battery is already flat. That 8.2 V must be the voltage of the battery with no load, so if it is, the battery is no good.

If you have a power supply, use it.

Could you measure the DC resistance of the relay coil? You can use this to calculate the current flowing.

Also, please use the circuit above which shows a transistor doing the switching.

 

[RobertRobot36]

Yes that was with no load

On the 200 ohms part of my multimeter thing it says the resistance is 6.52

My power supply is 9V, but i measured it with no load and it was 15v (DC), would that blow the chip?

With the circuit that included the transistor, is the resistor coming from the chip important, if so what resistance should it have?

 

[RobertRobot36]

I have just done the transistor circuit but it doesn't work

 

[vk6kro]

Yes that was with no load

On the 200 ohms part of my multimeter thing it says the resistance is 6.52

My power supply is 9V, but i measured it with no load and it was 15v (DC), would that blow the chip?

With the circuit that included the transistor, is the resistor coming from the chip important, if so what resistance should it have?

Are you sure about that resistance? A 3½ digit multimeter would normally give a reading of 6.5 ohms (not 6.52) on that scale. Could you have the decimal point wrong?

Assuming it really is 6.52 ohms, then the current if you had 15 volts across that coil would be 15 volts / 6.52 ohms or 2.3 amps.
A relay like that could not be driven directly from a 555 and it may have damaged the second 555.

If possible, I would look for a relay which had a lower current requirement. Something like 600 ohms resistance, or higher if you can get one. At 15 volts this would draw about 25 mA. Depending on the contacts on the relay it should still be able to switch a small motor on and off.

The resistor you asked about depends on the transistor you can get. Transistors have current gain and this varies from one transistor to the next, even if they have the same number on them.
The base current of the transistor has to be calculated by dividing the relay coil current by the current gain of the transistor. The resistor has to be calculated from the base current of the transistor. It isn't difficult, but you need to do it to get results.

I see you already tried a transistor. What kind is it and what resistor did you use?
Does your multimeter have a transistor tester on it?

You may also need to check that your 555 is still working without the relay connected to it.

 

[RobertRobot36]

I didn't use a resistor, and yes i tested my transistor and it came up as 500 something, not too sure cause i didnt note it down

 

[vk6kro]

It is possible to blow up a transistor by doing that.

The resistor is necessary to limit the current into the base of the transistor.
So, you should test the transistor again and if it works, write down the Hfe figure you get.
The resistor needs to be about 4.7 K, but this will vary with the transistor and the relay coil resistance.

Also write down the number on the transistor. It might start with 2N or BD or something like that.

Also re-measure the resistance of the relay and write it down.

Can you get another relay?

 

[RobertRobot36]

Sorry the relay was 65.2

The transistor has a hFE of 563

on the transistor it says BC548 and underneath it says -G21 ( i think )

I do have another relay

 

[vk6kro]

Ok.

That relay will still draw 230 mA with 15 volts across it. So the BC548 will not be suitable.

You may be able to get a BD139. This is a small power transistor which can carry a much greater current than the BC548. It is normally used with a heatsink which can be just a small piece of thick aluminum about an inch square in this case. Use this if the transistor gets too hot to touch.

It has different wire connections to the BC548, so be careful. Looking at the writing where it says BD139, with the wires pointing downwards, the connections are: from left to right, EMITTER, COLLECTOR, BASE

How much resistance does the other relay have?

If you can get a BD139 you would also need a 2.2 K resistor for a supply of 15 volts and 1 K for a supply of 9 volts.
Don't use the battery, though, because the current is too large.

 

[RobertRobot36]

The other Relay is about the same as the first relay
I don't think i will be able to get the other transistor, but why can i not connect the relay from pin 3 to ground? I worked once before but for some reason i couldn't get it to work

 

[vk6kro]

Connecting relays to a 555 is a bad move because there is a risk of blowing up the 555 which is not capable of delivering the current needed to run it and also the relay may produce voltage spikes even with a diode protection fitted.

The BD139 or most other power transistors are much more rugged than the 555 and probably cheaper if you did manage to blow them up.

If the cost of the BD139 is a problem, then you should be even more cautious about doing anything that will destroy more devices.

Old TVs, VCRs and computer power supplies have several power transistors in them and you may be able to recover some of these free. You only need one good one.

 

[RobertRobot36]

Is there any possible way of using the one i have?

 

[vk6kro]

The transistor?
Not really.

One possibility is that the motor you are using might use less current than the relay.
If so, I guess you could switch it directly from the 555.

If you live in the US, Radio Shack have TIP31's at $1.69 and Darlington transistors TIP120's at the same price.

 

[RobertRobot36]

No i live in the UK
But thanks for your help :)
I might change the 555 circuit to a latching circuit, it should work with what I am making

 

[Dmytry]

Just measure resistance of the relay coil and divide 6 volts by it to get current.
I'd just use a transistor if i were you... connect it's base to 555's output through resistor. Also I always just use opamp based oscillator or timer instead of 555, 'cause opamp is more universal and it's cheaper to just buy 10x lm324 once and use them for this sort of stuff.

Ahh and also. Relay may have damaged your 555 's output stage already. When you disconnect a relay you can get high voltage spike; you need a protective diode connected in parallel with coil so that it does not normally conduct (when you are powering the coil).

You need a 1amp or greater power rectifier diode, an LED might blow out on the first kickback. It is a refinement to the circuit so you might get away without using it.

The kickback current is equal to relay coil current...

 

[vk6kro]

If you REALLY can't get a power transistor, you could try this:
[PLAIN]http://dl.dropbox.com/u/4222062/555%20driving%20relay.PNG

It just uses a resistor to reduce the current to the relay coil to a safe level for the 555.
Note that the resistor has to be 1 watt as it will get warm.

A diode is still required across the relay coil.

 

[vivekB]

visit

http://www.circuitshelp.com/2012/07/555-monostable-vibrator.html

this might help you...!