Weak Signal, need 1.2v to 5v

I've tried googling and searching this site for a few days now. So forgive me for asking.

I have a weak signal, 0-1.2v logic that I need to be boosted or amp-ed up to 0-5v logic. The incoming signal is being sent from a hex inverter (7404). The signal is a 100ms wide pulse. These two things are things I can't change. Unfortunately the signal is being sent via BNC cable about 25 feet and the voltage by the time I get the pulse, is down to 1.2 volts.

Before I was just using a 7404 to invert and give a slight boost so that my computer can read the signal. I read the signal on the DB25 port on the computer. Someone did something, since this was working for over a year... and that is no longer the case now. Looking back on it, it was a fluke that the project was working as long as it was.

Now I've tried to use the same spects for another project box, which does not work. The previous box was created using USB power (gnd and 5 volts.) But if I needed to have a wall wart, transformer, whatever you want to call those ugly brick things, is okay. Also, not being limited to save power or space is also a nice thing.

I've managed to make a pulse-width modulation as discribed at http://www.allaboutcircuits.com/vol_3/chpt_8/3.html Which would be perfect, if instead of dropping down to -12v it only went down to 0 or GND. I'm sure there is a elegant solution. Its just been a few years since I've played with this stuff.

Thanks for your time. Peter

The schematic would be helpful.

Also, is there a pullup resistor on the output of the 7404? Have you considered using an open collector device like the 7405. Gates may be placed in parallel and reasonably small pullups used, which might let the full signal transit the coax.

Unfortunately I am unable to provide full schematics. Multiple people currently use the signal. I only have access to the panel where the signal comes through the wall.

More background, interesting, but not really important. Currently the device sending the signal is a fmri chamber. An optical signal from the scanner about 10μ is stretched with a 555 timer and ran into an inverter, 7404. That signal is sent to multiple devices. Apparently everyone else's device is able to deal with it just fine. Everyone except my department... Yes, I stared blankly when I head that the first time. But it is what it is... and unfortunately there isn't much I can do about the origin of the signal.

Thanks beenthere, I'll ask the lab owner if he is willing to try something different. I still would prefer to deal with the signal that I get.

-Peter

The real problem here is threshold. Normal TTL parts like a 7404 have maximum logic "0" of around 0.8V and a minimum logic "1" of around 2.0V

This is a serious misapplication of a TTL part which was never intended as a line driver. I suggest that you use an analog comparator (LM393, LM339) configured as a Schmitt Trigger for a receiver. Use split power supplies and set the thrshold midpoint at 0.6V (= 1.2V/2) and maybe 200 mV of hysteresis.

The outputs are open collector and can be pulled up to any convenient logic supply less than or equal to the Vcc to the comparator chip.

Hi Papabravo,

Thanks! I will give that a shot. It seems like an elegant solution. I found this site and will try to build to its specifications.

http://www.random-science-tools.com/electronics/inverting-schmitt-trigger-calculator.htm

I've included the resistor values in the attachment.

Thanks again!
-Peter

I used an Arduino device. I can simply set the input to the analog in. A threshold of .7 volts causes a digital line out to go low, else its high. It's always annoyed me when I see posts that have no solution. It was a rather elegant solution. Probably overkill.

Thank you again for your help, and your continued help to others.