Ground a astable 555 at startup

[darrins]

I have a circuit that uses a monostable 555 timer to control an astabable 555 timer; something like the first circuit in the following image



My circuit works ok, but occasionally, I'm getting some strage behavior when I first apply power. Sometimes, when I turn on my power supply, the astable 555 starts sending output pulses, even though I haven't pressed the trigger button for my monostable 555. Is this some stray capacitance or possibly a noisy DC signal?

Does anyone know of a way to make sure pin 4 of the 2nd timer is completely grounded when I first apply power?

Thanks.

Darrin Smith

[Kain]

I believe you can try to add a simple transistor based switch to do this for you. You should place it so it will shunt the 2.2K pull down resistor of the monovibrator. Its base should be connected to RC circuit to time a short interval when you power up the circuit. What is the power supply that you use though? I mean is the output voltage is well filtered because this might be one of the main causes for this strange behavior :( I see a filter capacitor is missing in your circuit, so you are totally relying on the stability of the DC power supply. It won't hurt you if you put 100uF and few nF capacitors to filter the voltage no matter if you think that the PSU is good or not. Just my oppinion :)

[Guest Alun]

I believe you can try to add a simple transistor based switch to do this for you. You should place it so it will shunt the 2.2K pull down resistor of the monovibrator. Its base should be connected to RC circuit to time a short interval when you power up the circuit.

I wouldn't recomend this because if pin 3 is high and the transistor shorts it to ground bad things could happen!

What is the power supply that you use though? I mean is the output voltage is well filtered because this might be one of the main causes for this strange behavior :( I see a filter capacitor is missing in your circuit, so you are totally relying on the stability of the DC power supply. It won't hurt you if you put 100uF and few nF capacitors to filter the voltage no matter if you think that the PSU is good or not. Just my oppinion :)

I agree with you here though, connecting a 100uf capacitor and a 100nf ceramic capacitor in paralell with the power suply is a very good idea.

Also connect all of the pin 5s to 0V with a 10nf capacitor, and all of the pin 4s that you've left not connected to anything should be connected to +V. In general it's bad practice to leave pins unconnected as they can act as antennas and pick up noise thus causing it to mal-function. Always connect all un-used pins on ICs to either +V or 0V you can find out by looking at the datasheet.

Try using the NE556 timer its 2 555s on one chip its cheaper and you're circuit will be smaller and as a result be less affected by noise in general.

I've attached the 556 datasheet for you

FC_NE556.pdf

[Guest Alun]

You could go for a 7556 it uses less power and puts less of a strain on th power supply.

IS_ICM7555_ICM7556_.pdf

[Guest Alun]

And here are some handy 555 application notes.

Sorry for triple posting but as far as I'm aware there's no way of attaching more than one attachment.

PH_555_Applications.pdf

[Kevin Weddle]

Just as a side note, there is normally a correct way of doing things. The two circuits only produce similar results. Besides, why don't you just use the switch to supply power to the 555. This way it will oscillate the moment you press the switch.

[Guest Alun]

Just as a side note, there is normally a correct way of doing things. The two circuits only produce similar results.

As far I can see all three circuits should give identical results:

Circuit 1:
pin3 (output) is connected to 555-B pin4 (reset)
When the reset is low the circuit is continuously being reset so it will never trigger thus oscillation will not occur, when the reset is taken high 555-B acts as a normal astable and oscillates.

Circuit 2:
When pin3 on 555-A is high the diode is reverse-biased so it has no effect on 555-B, when it goes low the diode is forward-biased so C1 is shorted to ground and it never charges so the circuit won't oscillate.

Circuit 3:
The RC circuit on 555-B is connected the the output of 555-B so when pin3 is high it charges as usual but when low there's no power for it to charge.

You could also connect pin8 (VCC) of 555-B to pin3 of 555-A but I wouldn't recommend this as it couldn't be implemented with a 556 chip, also the output resistance of 555-A could affect 555-B.

I would recommend circuit 1 because the internal resistance of the output of 555-A won't affect the RC circuit in 555-B.

Besides, why don't you just use the switch to supply power to the 555. This way it will oscillate the moment you press the switch.

This would work, but I assume that darrins wants a 1.1 second tone.

Try replacing the 100K resistors with 10Ks and the 0.1uf connected to the switch with 1nf, as these are the values used on the datasheets and it's better to stick with them.

[trigger]

I think you can make use of the LM555 pin 4 reset pin to reset the ICs while they are powered up.
Just simple RC can do.

[darrins]

Thanks to all who responded.

The power supply I'm using is one I built using the following schematic.

http://www.uoguelph.ca/~antoon/circ/vps.htm


I do have a small cap (0.1 uF) b/n pin 5 and ground of both 555's, although the schematic I posted doesn't show them.

I will try the suggestion of putting some filter caps between my power supply and my circuit.

The problem usually arises when I flip the main switch of my power supply. I wonder if I'm getting some "bounce" from the switch? If so, will the filter caps take care of this issue, or do I need to create a debouncing circuit?

Thanks again.


Darrin

[Kevin Weddle]

A simple trigger can be constructed with a resistor and inductor to ground. When you turn the power supply onto this circuit, the result is a pulse. This is a better way to trigger your oscillator. Just design the oscillator as usual and use this trigger to generate the reset. The pulse will enable the oscillator for that amount of time.