afesheir

hi guys ...
I am connecting a relay with 555 timer output (pin 3) configured in monostable mode ... when the output come 1, i hear the sound of relay connection, but when the output turns back to 0, i hear a sound (tick tick tick tick ....) as if the relay is connecting and disconnecting ... also the capacitor doesn't discharge and pin 3 voltage still HIGH ...

i am using a power supply of 5.5 V and the relay rating is 6VDC ... plz i need help urgently

I²R

Try a ~10k pulldown resistor on the 555 pin 3 to ground.

afesheir

Thanks I²R ... but what do u mean by pulldown ..? resistor in parallel with the relay ...??

and if yes, how will this solve the problem physically ...?

thanks again

I²R

Exactly what I said. A 10 k resistor from pin 3 on the 555 to ground. I can't state it any more clearly than I already did, so I don't understand the source of your confusion. The term "pulldown" means literally what it says:

Its purpose is to pull the output of pin 3 down until it is driven high by the 555.

eng1

You need to place a clamp diode accross the coil of the relay, or pin 3 will be driven negative. Actually that diode will clamp the voltage at -0.6 V; an old app note suggests to place another diode in series with pin 3.

afesheir

I²R ... i got what u mean, but the 10k pulldown resistor didn't fix my problem !!!

eng1 ... i have already tried to place a diode in series with pin 3 and the relay coil to prevent any reverse currents that may be due to the coil, but in this case, the circuit stopped completey, i.e., even the sound of the relay (tick) wasn't heared on starting ... please i still need the help

eng1

See the attached image. The diode accross the coil is the most important.
The diode in series with pin3 produces a voltage drop of about 0.6 V and the voltage that drives the relay could be too low.

Attached Images

555dr.png (2.1 KB, 2 views)

mytech

What is your actual ON time?... Can you upload your schematic?..