In Reply to: Re: How Can I Produce a Triggerless One-Shot? posted by David Ashby on 08/06/02 at 7:56 AM:
I've created a higher drive one-shot by combining the RC circuit with a transistor. I now get the high-current one-shot that so far is 100% reliable.
The collector of a 2N3904 transistor is connected to Vcc (+5 VDC). The base is connected through a 10uF capacitor to Vcc, a 1.0M-ohm resistor to ground, and a 1N4148 diode to ground. The emitter is connected through a 100-ohm resistor to ground, and to the LED array and 74HCT193 Reset pin. When the circuit is powered up, the LED array lights up then switches off and counting begins from zero.
However, I cannot seem to get the pulse long enough. I cannot go higher than 10uF for the capacitor or counting will fail to consistently begin from zero. (It typically begins at two with 22uF and up.) Varying the resistor from as low as 100-ohm to as high as 1.0M-ohm has no visible effect on the pulse length. Is the Vt of the 2N3904 too high to keep the gate open? Would another transistor be better suited here, or is there some other way of increasing the pulse length?