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Problem controlling relay with monostable 555

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DriverX

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Hey guys,

Not sure if this is the correct section for this, but feel free to move it if it's not.

So, I'm having a little trouble with controlling a relay using a 555 timer connected in monostable (one-shot) mode. Basically, I have everything hooked up just like in the schematic below, except:

1) I'm using a 10k pull-up resistor, instead of 2k (in the left corner)
2) I'm using 100k resistor and 10uf cap, instead of 47k and 22uf
3) I omitted the 2k resistor between the 47k resistor and 22uf cap
4) I'm using a 0.01uf cap, instead of 0.1uf, to go from pin 5 to ground
5) Both diodes are 1N4004
6) It's powered by 4 1.5V batteries connected in series (6V)

Now, the circuit works (I think...), but it's not consistent. For example: I connect the power, hit the momentary switch, and the relay clicks and un-clicks like it should (in approximately a second, so that's fine), then I hit the switch again, nothing happens, hit the switch for the third time, relay acts just like the first time, hit the switch for the fourth time, nothing happens, hit it for the fifth time, works just like the 1st and 3rd time, etc etc.

What should I do to make the relay respond the same way every time I press the momentary switch? Are the resistor and capacitor values that I'm using fine? Is there a better way to achieve the same task?

Here is the schematic:

**broken link removed**

Any advice would be greatly appreciated.

Thanks,
Adam.
 

Are you sure, that relay can be reliably engaged by the output voltage? What's it's specification?
 
Try connecting another capacitor, I suggest 100uF between the supply and ground lines, mounted as close as possible to the 555. You may be seeing instability on the supply as the relay currents turns on and off.

Brian.
 
Hmmm, I'm actually not sure what the specs on the relay are. There is no writing on it, but I have a feeling it might be an automotive relay. Should I try to power the circuit using 12 volts then (instead of 6)?

I'll try to add the 100uf cap to the circuit and I'll let you know what happens.

Thanks a lot for all the replies so far!

---------- Post added at 17:51 ---------- Previous post was at 17:03 ----------

I tried connecting an 100uf cap just like betwixt suggested, but now the relay wouldn't click at all.

I also ordered a 12 volt battery because maybe it is a 12V relay...
 

Leave the capacitor in place, it should be there anyway.
Almost all automotive relays are 12V so you are right to increase the supply voltage, be careful that you don't exceed the current output limit of the 555 though. In an automotive environment there is usually lots of current available and most of the relays are operated by mechanical switches. The output of some types of 555 is limited to only a few mA which is almost certainly not enough to fully operate the relay. If that is the case, I recommend you feed the 555 output through a 1K resistor to the base of an NPN transistor, ground it's emitter pin and move the relay and diode to between the positive rail and it's collector pin. The diode cathode (+) end goes to the supply line.

Brian.
 
When you say 555, then I expect bipolar NE555 or LM555, that can drive 200 mA, not a low power CMOS device. Most regular relays should consume considerably lower current, for 12V relays usually up to 30 or 40 mA. But measuring the coil resistance would do no harm.
 
I am using the NE555. I'll try using a transistor to bump the current up a bit and use a 12V power source. Would any NPN BJT work, or is there a specific one that would be more suitable for this application?
 

The transistor type is not really important as long as it's collector current rating is greater than the relay current. Most small signal transistors will be OK.

Brian.
 

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