Hand machined mechanical pencils

Wide-range adjustable pulse generator (19/06/12)

This gadget produces an adjustable-duration 12V pulse. Repetition rate is selectable from 0.2, 1, 5, or 60Hz, and the duration range can be selected in twelve steps from 2.5µs to 500ms. The output is from a UCC37322 gate driver, so can be used directly to power MOSFETs and thyristors. It's very useful for testing inductors to determine their saturation current.

A ZIP file containing a schematic and board diagrams is available here. A 555 astable is used to provide a clock for a 74HCT123 monostable. I didn't use a 555 for the monostable since the 555 isn't edge-triggered, and the capacitor+diode clipper trick didn't work very well. Your mileage may vary! Two rotary switches are used to select a resistor for the 555 and a capacitor for the 74HCT123 - values are given in the text file in the ZIP.

The 12-way rotary switch was modified from a 4-pole 3-way switch as described here.

Here's the obligatory photos. It's a rather messy fit inside the box, thanks to all the timing capacitors, most of which had to be made up from smaller caps in parallel.