Pulse Trigger
Module
This
module produces a fully variable 12V square pulse over three ranges; 0.1ms - 1ms,
1ms - 10ms, 10ms -100ms. It is designed around the 555 timer and uses a custom
delayed negative edge trigger based on a PIC12C509 microcontroller. The microcontroller
programming allows a 5s delay between pressing the fire button and triggering
the 555 timer. The circuit diagram of fig 1 shows this:

Fig
1. Pulse trigger generator
The
module operates by pressing the Prime button (SW1), this illuminates the LED indicating
that the 5s countdown has started. At the end of the 5s period the LED is extinguished
and the BC108 is pulsed for 10us, generating the negative edge which fires the
555. The on-time of the 555 output is determined by the range selector (SW2) and
pulse width dial (VR1). There is a slight overlap in the ranges so that the full
range of pulse widths can be generated. The output pulse is 12V and can source
about 200mA maximum. The output and circuit ground are brought out to a screw-less
terminal block.
Fig
2. Module parts ready for final assembly
Fig
3. The completed module (the text is easier to read on the real thing)
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