Annoyingly simple 555 problem

[TopRigger]

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.
TopRigger

 

[melone]

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

 

[TopRigger]

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.

 

[melone]

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

 

[bmnet]

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...

 

[OperaHouse]

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

 

[nbucska]

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



Plesae read FAQ240-1032
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http://geocities.com/nbucska/>

 

[BobM3]

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).

 

[TopRigger]

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.