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Looking for help - 555 not triggering
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Looking for help - 555 not triggering

by shobley on Wed Apr 01, 2009 11:28 am

Hi,

I have a 555 timer in Monostable mode that delivers a short measured pulse on the falling edge of the input signal (5v -> Gnd).

I hooked up a regular 6pin optoisolator (NTE - 3400 series, single transistor) and connected a 10K resistor between pin 4 and gnd - essentially copying the isolator input signal from a remote 555 in astable mode.

The mono 555 is not triggering, I suspect that this is because there is not enough current finding it's way into the 555. I read that the input impedance of the the 555 is > 2MOhms, and so I think that all the effective "power" is routing through the 10K, as the path of least resistance.

The obvious thing to try would be to up the 10K to 100K or greater, but before I do that, does anyone know how I should calculate a correct resistance value.

Or maybe I don't need one at all?

BTW - when I couple the output of the astable 555 directly, everything works OK. I just need to isolate these two 555s.

Steve
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Re: Looking for help - 555 not triggering

by shobley on Wed Apr 01, 2009 12:58 pm

Here's the (rough) schematic.

http://www.stephenhobley.com/Tesla/opto555.gif

Usually I can these things to work OK, but for some reason this one is not doing the job...
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Re: Looking for help - 555 not triggering

by zener on Wed Apr 01, 2009 1:19 pm

Well the first thing to do is to verify that the signal is making it through the optocoupler. Have you verified that?

Why are you using the optocoupler?

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Re: Looking for help - 555 not triggering

by Amberwolf on Thu Apr 02, 2009 3:56 am

shobley wrote:Here's the (rough) schematic.

http://www.stephenhobley.com/Tesla/opto555.gif

Usually I can these things to work OK, but for some reason this one is not doing the job...

It looks like you have the output from the opto setup to pulse high, normally held low, right?

Unfortunately, your receiver-end 555 is setup to hold the trigger high, thru the 1K R3, implying that it must be pulsed low to trigger it.

If the 555 is correctly setup for what you want it to do, then you should setup the opto's output to pulse low instead, by putting it's resistor between the collector and the 5V supply, and then connect the C9's open end to the collector rather than the emitter of the opto. The emitter will then go directly to ground.

You can put a much smaller resistor on the emitter of the opto to ground, if you need to limit the current further for any reason, as long as the value is such that the voltage divider created by the 5V side resistor and the ground side resistor create a voltage at the collector of the opto that is sufficiently low to trigger your 555.


Otherwise you need to remove R3 so it doesn't hold the trigger input high.
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