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Annoyingly simple 555 problem

Annoyingly simple 555 problem

Annoyingly simple 555 problem

Iv'e spent ages trying to solve what should be a ridiculously easy problem but it's got me stumped.
A standard 555 monostable circuit, output drives a small relay that controls a small motor - it should turn on for 2.5 seconds when the 555 is triggered but just turns back off again - 555 doesn't seem to latch high.  Without the motor all is fine so I assume it must be drops on the power supplies turning the 555 off?.  Tried putting capacitors across the power supply and the motor but no good.  One solution is to separate the motor and 555 power supplies but would rather avoid this.
What am I missing?
Thanks for any help.

RE: Annoyingly simple 555 problem

What is the output of the 555 actually driving, i.e. FET, relay, etc.?

RE: Annoyingly simple 555 problem

The output is driving a 12V SPST relay used to turn a 12V motor on/off.  I have already added a flywheel diode across the relay coil for back EMF protection.

RE: Annoyingly simple 555 problem

What is the current requirement for the relay you are using?

RE: Annoyingly simple 555 problem

I think you should put a diode immediately after the 555... Have had a problem like that before... The 555 'sees' a higher voltage than its trying to output...

RE: Annoyingly simple 555 problem

Isolate the 555 power from the motor with a diode and cap.  

RE: Annoyingly simple 555 problem

"separate the motor and 555 ...."
Is the motor using the same 12 V supply? How low does
it drop when the motor starts ?

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RE: Annoyingly simple 555 problem

Maybe the large starting current from the motor is radiating some RF.  Or there is some contact arcing when the relay closes (more RF noise).

RE: Annoyingly simple 555 problem

Sorted it!
A combination of the diode (thanks BMNET) and experimenting with various capacitors across the motor terminals fixed the problem - I suspect a particualrly noisy motor.
Thanks to all who answered.

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