Can't Power ON my IC555 Timer with capacitor in trigger pin

[Enrique15]

Greetings, members of this great Forum.

My problem this time is that I designed a controlled timer with a IC 555. Of course, watching basic designs in electronic magazines.

One of those designs mentioned a capacitor in the Trigger pin and a switch to connect this capacitor to GROUND to enable the Power On of the timer.

Here's a look of my circuit:

So, I connected the whole thing in a Protoboard, and with the first push of the button, a led connected in pin 3 (Vout) lighted on as expected, and turned off after the time I chose for the circuit.
But with the second push of the button, and so the next ones, I couldn't get the led to turn on.
Inmediately I thought "maybe the capacitor is too charged". So I connected a resistor in parallel with the capacitor to discharged it. And it sure worked, because the led turned on after the next push of the button.

But everytime I want to turn on the led, I have to discharge the capacitor with the resistor.

Of course, the easy way is to connect the Push Button to the Trigger pin directly. And yes, the IC 555 works great that way.

But I wanted to know how to get a normal operation of the timer using the capacitor, because in those electronic magazines I read, some designs use the capacitor. And they use a 10uF, as I did here.

At first I tried with a 10uF electrolicit capacitor. After seeing that after the first push of the button, it won't turn on again, I change the electrolitic for a ceramic one, 0.1uF. But the same happened.

Are those electronic magazines wrong ?
Am I wrong ? How am I wrong with this design ?

I'll appreciate any help.

See you later. :sm9:

 

[platonas]

Re: Can't Power ON my IC555 Timer with capacitor in trigger

I think u need a resistor on the common pin of switch and capacitor. The other end must be on +6V in order to discharge the capacitor and make it ready for the next cycle.

 

[IanP]

Re: Can't Power ON my IC555 Timer with capacitor in trigger

You should more carefully copy circuits from magazines.
I believe they publish schematics in good will, however I would expect them, if they don't know by themselves, to use experienced person to check contains befere it is published.
In your case it is just 1 missing resistor (previous post explains where it is missing) but in many cases you can come accross quite complicated circuits which are totally wrong or have a lot of missing connections, components, wrong values etc.
So the advice is be more carefull and if possible try to find similar circuit and compare them .. this may save you a lot of time and frustration ..

btw the resistor value can be anythig from 1 to 22k and the cap can be 0.1µF +

 

[Enrique15]

Re: Can't Power ON my IC555 Timer with capacitor in trigger

Thanks Platonas and IanP for your opinion.

But are you sure the RESISTOR should go between the capacitor and the switch ??

Because the main idea of the design is to have the capacitor connected to GROUND "directly" through the Push Button, so in the first moment of this connection, the other side of the capacitor sees this ground or LOW voltage, so the trigger pin sees the low voltage as well, because to trigger the IC555 you need a low voltage there. And the resistor (22k in the diagram) should be between Vin (Power Supply) and this trigger, to get the HIGH voltage in the Trigger pin that will disable the IC555 to start working.

I think that if I put an extra resistor, between the capacitor and GROUND (through the push button), the trigger pin won't see the low voltage needed. Or at least, not as a good low voltage as the one you get having the capacitor connected to ground directly.

That's what I understand of the circuit.

But I'll try what you suggest. And I'll post later if I got any good results.


PD: Yes, I know electronic magazines got a lot of circuit diagrams wrong. I've found a lot of terrible mistakes in diagrams of various magazines I have. But there are also a lot of circuits that are fully correct.
But to build a circuit, I base myself more in the theorical explanation than in just the circuit. Because I'm aware of the many mistakes they have in their diagrams.

See you later. :spoko:

 

[IanP]

Re: Can't Power ON my IC555 Timer with capacitor in trigger

This extra resistor should be connected not to 0V but to +6V.
Its purpose is to discharge capacitor so on both sides of the cap (after SW is released) you have +6V.
Otherwise groundig the cap trough switch will not generate a pulse needed to trigg the 555..