Hello all, Hop someone can help me out here. I've been tasked by a friend to fix an issue for him. The solution I wont go into unless required, but it involves using a 555 in an astable circuit.

What I need to know is, upon power up, does the 555's output pin (3) start HI (even for a ms) and get pulled LO and stay LO until the first output pulse (at which point it would go HI). OR Does the output pin start LO on power up and only get pulled HI on the first pulse.

I know these two options are very similar, but for what I'm hacking this into, the output CANNOT go HI before the first timed output pulse (which is timed for about 3.5seconds). It MUST stay LO at all times before that first pulse.

Now naturally I'd assume that it says LO at all times until its pulsed HI. But whats causing my confusing here is that I've got the circuit in a simulation program (ISIS Pro). And with a load on the output pin (a BC108 switching in 12v to a relay), ISIS is clearly showing the output going HI for a split ms, this is long enough to cause the transistor to close, and the relay configuration to latch. I need the relays to latch AFTER the delay til the first pulse (3.5seconds).

With NO LOAD on the output pin, as in, just a bus wire, and a voltage probe on it in simulation, the output pin doesnt go HI on powerup and the fall LO until its pulsed. It stays LO at all times before the first pulse. So I'm now not sure if this is just a bug in the ismulation software, or wether this will actually happen if I build the circuit for real.

Can anyone clarify the characteristics of a 555 and its output pin??

Thanks all, Dave.

 

We did this recently. Increasing the capacitor on pin 5 and/or using an rc time delay on pin 4 will delay the start so the glitch does not appear.

 

Aah, thanks for the quick reply. So this does actually happen in real life builds, not just in this simulation? I'm currently simulating a 10nF cap onpin 5. Pin 4 is going straight to VCC.

 

The datasheet recommends a 10nF cap from pin 5 to ground anytime pin 5 is not used, but this is often ignored.

 

The 555 is fundamentally a inverter, so if pin 2,6 is high to begin with the output is low, and visa versa.

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The quick answer is it depends entirely on your configuration. Schematics are the true language of electronics.

 

This is beginning to baffle me now. I'm sure these simulations make it up as they go along. The output is now staying HI for en entire pulse length and then dropping LO. Here is my schematic. Forgive the layout, it's done in a rush to test the theory.

Any ideas?

 

Would help if i posted the pic too, one second, lol.

 

thanks guys, dave.

 

Remember what I said about pins 2,6? A capacitor starts off discharged (ideally), so on power up Pins 2,6 are low, the output would be high.

555 Schmitt Trigger

As the capacitor charges the 555 will hit a threshold, and switch to low, which also shorts pin 5 to ground. This discharges the capacitor, until it hits the bottom threshold, and the circuit switches again, starting the capacitor charging again.

CMOS 555 Long Duration Minimum Parts LED Flasher

This explains the theory of operation.

Hope it helps.

 

Hi Bill, thanks for your detailed reply. I understand now why it is functioning this way, but I'm still not 100% on how I'd go about rectifying it. I can't simply take 2 and 6 HI as then of course it would never trigger. I've also probed pin 2 and 6 in simulation, to prove the theory. And on power on, it starts at 0v (with the output HI), climbs to around 7.5v (output goes LO) then drops to 4v (output goes HI).

 

The schematic shown is an oscillator. You are wanting a variable duty cycle oscillator that starts low, yes?

 

There is another approach, use the Vcc as the common for the relay. The 555 sinks current as well as it sources it. If you would like a schematic on how to do this I can show you, but I am about to step out for the day.

 

What I need to know is, upon power up, does the 555's output pin (3) start HI (even for a ms) and get pulled LO and stay LO until the first output pulse (at which point it would go HI). OR Does the output pin start LO on power up and only get pulled HI on the first pulse.

I don't use simulation and cannot comment on that, but I have breadboarded the circuit you attached to post #8. Immediately on power up, pin 3 goes high and stays high for the time period controlled by R2/C3. At the end of that period, pin 3 goes low. The cycle repeats for as long as power is supplied. I believe that is exactly what Bill said would happen.

There is no "glitch" that I can observe. The collector of Q1 follows the same pattern.

 

I think the OP wants his oscillator to start low, which require an inversion of some kind.

 

I think the OP wants his oscillator to start low, which require an inversion of some kind.

Depending upon the current requirements of the relays, no external transistor may be needed; just use the 555 to sink the relay current. Right?

 

A stock 555 can handle 100ma no problem (don't forget the swamping diode), it is rated for 200ma. I suspect the OP has already figured out his problem though.

 

What about using some sort of AND or OR gate to fix this problem? Just shooting out an idea...