555 power-up output state

[imix500]

Hello all,
Searched but didn't find the answer to this-
I have a 555 used as delay-on for a relay. The output is high for ~5 seconds then goes low triggering a pnp darlington and a large relay. The circuit seems to work great except that once in a while the output is stuck low when power is applied. Cycling power solves it, but is there a simple way to nearly guarantee the output is high on power up?
I can post a schematic if needed, but it's just the generic config with Trigger tied to Vcc via 10K and RC of 50K/100uF.
Thanks!

 

[MikeMl]

Post your schematic. I want to see how you did the trigger.

 

[BrownOut]

A capacitor connected to the reset pin and ground, a resistor connected to VCC and to the capacitor. This will keep the 555 reset until VCC stabilized. Only a few ms should be required, so make the cap like 1uF or so and the resistor a hundred or so K ohms. Pay close attention to the capacitor polarization.

 

[imix500]

Here's the schematic.

 

[imix500]

Hows this?

 

[MikeMl]

Why is R4 where it is????
It should be tied to +12V; not to ground. Your output Darlington is partially always on????

 

[imix500]

Sorry, mistake in the schematic. Corrected below:

 

[ljcox]

Why do you need the Darlington?

From memory, the 555 can source & sink 200 mA, so that should be more than enough to operate the relay.

Don't neglect to connect a diode across the relay coil to limit the back emf to a safe level.

 

[MikeMl]

So, the timing cycle is supposed to be initiated when 12V is applied to the circuit?

 

[imix500]

The relay draws about 175mA and I didn't want to push it. The diode is indeed there.

 

[imix500]

MikeMI, correct. Unfortunately it is not possible to have 12v standby power.

 

[ljcox]

I rarely use the 555, it is usually simpler to use a CMOS Schmitt Trigger.

Here is one I designed for someone else.

It could be adapted to your purpose.

 

[imix500]

ljcox,
I don't think I have any Schmitt triggers, but I can look tomorrow.
I do have 4541 timers if they would be a better choice.

 

[ljcox]

The 4541 would need a "power on" reset.

The crude way to do it would be to connect a resistor from MR to Gnd and a capacitor from MR to Vcc.

The better way would be to use a 555 or a Schmitt Trigger.

So it would be much simpler to use the circuit I attached previously.

 

[MikeMl]

This should do it: Leave the Discharge pin open. Hook TRIG to THRESH. Use diode to discharge timing network when power is removed.

Your circuit didn't have a reliable trigger mechanism.

 

[BrownOut]

Post deleated.

 

[imix500]

So I built the circuit MikeMi posted this morning and it has been running for over 100 power cycles without missing a beat. Unless something happens with it in the next few hours I think the problem is solved.
Thanks for the help!