1
\$\begingroup\$

I have been trying to design a trigger lockout circuit and I have been having some difficulties.

The circuit is taking as an input a 3.3v pulse. These inputs can happen randomly and what I would like is a circuit that acts as a gatekeeper to determine which trigger signals get through. The criteria that determines weather or not a given signal gets transmitted is if 3 seconds have passed since the last trigger that went through successfully.

Ignoring component values the circuit that I have been messing around with is in the image below.

enter image description here

This design doesn't quite work, and I was wondering if anyone here had any thoughts on how I might make such a circuit? It is also a requirement that the output signal also be 3.3 volts or at least close to it.

\$\endgroup\$
  • \$\begingroup\$ There's three numbers that might help you: 5, 5 and 5. \$\endgroup\$ – Majenko Dec 5 '14 at 20:19
1
\$\begingroup\$

I would say: Stage 1: monostable 555 circuit then take the output of that into Stage 2: invert the monostable output and "and" it with the 555 output (DeMorgan to simplify as needed). Then if you want to control the output pulse width you can use the other half of a 556 to make a "one shot" of the desired pulse width.

There are some ill-defined details to work out about leading/falling edges and so forth but the above architecture should serve as a good starting point.

\$\endgroup\$
0
\$\begingroup\$

It's not quite clear to me what your trigger pulse looks like, but you could probably do it with a 74HC123, perhaps using one half to hold the other in reset and generate a controlled length trigger output pulse from that half.

There's a lot of possibilities (are runt trigger pulses an issue? what if trigger pulses arrive at 1MHz?) so you'll have to do some analysis to see if it's a suitable solution.

The maximum resistance allowed for the HC123 is 10M, so you could use something like 1uF and 6.8M.

\$\endgroup\$
0
\$\begingroup\$

This is potentially a very tricky question, and you need to define the problem much more carefully. For instance, let's say that your trigger pulses last 0.5 seconds, and you get two of them, A and B. The rising edge of B occurs 2.75 seconds after the rising edge of A. A literal interpretation of your question would give a 0.5 second A followed by a 0.25 second B starting 3 seconds after the start of A. How many pulses do you expect to get out? Or are you only interested in the rising edges of the pulses? Although you specify a 3.3 volt pulse amplitude, are you requiring the lockout circuit to operate from 3.3 volts as well?

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.