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Forum: µC & Digital Electronics Stretch 25ns TTL pulse to >100ns


Author: Photons in Biochemistry (Guest)
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Dear ladies and gentlemen,

we are trying to read out a single photon counting module that sends us 
TTL pulses of a width of 25ns.
Our counting module can count up to 5MHz, but only at a pulse width of 
100ns.

We want to operate the detector at much lower count rates ~10kHz, so 
stretching the TTL pulse we would not loose too much information.

In the biochemistry departement, we do not have a lot of knowledge in 
electronics.

Is there a "simple" way to reach our goal?

We are very much looking foreward to your replies.

Author: ::: (Guest)
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monoflop ?

Author: Photons in Biochemistry (Guest)
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Thank you for the reply.

I just tried to learn about "monoflop" by reading the wikipedia article 
and also found a monoflop e.g. on reichelt.de.

But that means I will actually have to build a circuit on my own, 
doesn't it?

I am not familiar with electronics.

Author: MWS (Guest)
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@Photons,

a fast Google search got me the Berkeley Nucleonics Corporation website, 
the model 575 would likely suit your requirements, could also be it's 
overkill. You may want to check.

In contrary, to make your unit work as little as one diode, two 
resistors and one capacitor could do the trick. Depends on the counter 
input circuitry.

But even for that you'd need somebody in your institute to try, test, 
and set it up for you, as it would be no out of the box solution. And if 
you have not even a slight idea about electronics, there's high risk 
you'll fail.

Author: Reinhard Kern (Guest)
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Photons in Biochemistry wrote:
> Is there a "simple" way to reach our goal?

Hello,

maybe, but before you should specify your detector pulse: length 25 ns 
ok, but what is inactive voltage level, active voltage level, source 
impedance (50 Ohm Coax?)?

Probably you could use a triggerable high speed pulse generator, trigger 
it with the 25 ns pulse and switch the output to 100 ns pulse legth - 
but this is total overkill, sensefull only if you have one.

regards Reinhard

Author: faustian (Guest)
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Check out the 74LS221 or 74HCT221 ICs, which should be available at a 
low price, around 50 cents apiece.


In case you are unfamiliar with 74xx ICs: Always put a small ceramic 
(around 100nF) and an electrolytic capacitor (around 100uF) across the 
VCC and GND pins, using the shortest wiring length possible between IC 
and ceramic capacitor. Do not ever leave any input open circuit, always 
connect to VCC (optionally via a 5-10K resistor. In practice, with a 
clean 5V supply you should be fine without), GND or an active output of 
another IC. Never directly connect outputs together (unless the 
datasheet explicitly allows so), never connect an output directly to 
VCC.74f221

Author: Photons in Biochemistry (Guest)
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@ Reinhard

Thanks for the input:
The TTL pulses are ~0V low, ~3V high (as estimated from an 
oscilloscope).

I can't afford overkill.

I read more about this monoflop. If I can get someone from one of the 
workshops to build it this might actually do the trick.

Greetings from Zurich

Author: ::: (Guest)
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In not too familiar with this TTL Family anymore, but I guess a 74LS123 
should do.

Greetings from Zuerich too.

Author: MWS (Guest)
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As you can't afford overkill, you may try the cheap side :D

Counter needs high input resistance for that.

Author: schon (Guest)
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What values should he use with this? The decay constant should be 
T=2Pi*R*C, right? Meaning that if he wants 100 ns for a 1 M Resistor he 
needs such a small cap that the stray capacitance will already be too 
much...

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