This project is a simple pulse extender. The circuit is extracted from the pulse extender that is part of the ThunderBolt Monitor kit described on this site.
It takes a narrow 5V TTL level positive going pulse and generates a wider (nominally 1mS when powered from 9-12V), RS-232 level pulse of the same polarity on the output. The circuit expects a relatively low impedance source, below 1kOhm will give the best results.
The circuit triggers on the leading edge, the falling edge timing is not well controlled. This is not critical as long as the receiver (typically a PC running NTP) also triggers on the leaqding edge.
The printed circuit board is available in my Projects page at OSHPark.com for $9.50 for 3 boards directly from OSHPark. All the parts can be purchased from Digikey.
Resistors and capacitors are 0805, the transistor can be just just anything, an MMBT2222A will work fine.
The regulator is in SOT-89 package, the full part number is MCP1703AT-5002E/MB.
The RS-232 level translator also gives you a lot of options. I have found that the low power devices tend to have longer delay and slower rise time, so I actually recommend the SN75C3232E from Texas Instruments. It is specified to 1Mbit/s.
The unused gates of the SN75C3232E are connected to pads so they can be used for other purpose.
The total cost if you make 3 boards should be well below $10 per board.
Click here for the parts list.
I make no money on your purchases.