One-shot design needed

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haklesup
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One-shot design needed

Post by haklesup » Wed Jul 02, 2003 4:41 pm

I need to design a simple circuit that will give me a 20us square pulse with variable amplitude (about 0V to 1V and at least 1mA). The event will be triggered by a button push.<p>I am familiar with variable pulse-width designs using the 555 but I am not sure how to vary the output amplitude without adding another stage like an op-amp to scale the signal.<p>At this point I am looking for suggestions for any similar design not just 555 based ones.<p>Chris

desterline
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Re: One-shot design needed

Post by desterline » Wed Jul 02, 2003 9:17 pm

The simplest idea I can think of is just a voltage divider. Two resistors in series between the output of the 555 and ground, take your output from the connection between the two resistors. The output voltage will be in proportion to the ratio of the two resistors, i.e. two 1k resistors will give you half voltage, ect.<p>Other ratios are redily creatable. If you use a pot, say 5k, with the wiper connected as your output, you can adjust it.<p>Good luck
Denny

bruinbear714
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Re: One-shot design needed

Post by bruinbear714 » Thu Jul 03, 2003 9:19 am

I don't have my magazine in front of me, but in this month's issue of Nuts & Volts, there is a section on transistor circuits. In it, there are a few oscillator circuits that uses discrete devices, I believe with two transistors, a couple of caps and a couple of resistors.<p>Or you might want to look into ring oscillators.

Chris Foley
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Re: One-shot design needed

Post by Chris Foley » Thu Jul 03, 2003 3:17 pm

desterline has a good idea. You're already familiar with the 555; you might want to start with a standard 20 microsecond one-shot with a CMOS 555. Let's assume a regulated 5VDC supply. If you load the output pin with a 40K resistor in series with a 10K pot, you can adjust the wiper of the pot to give you pulses of 0 to 1V amplitude (the lightly loaded output of the CMOS 555 is practically equal to the power supply rails). If you then place that 0 to 1V signal as the input to a "rail-to-rail" op-amp configured as a voltage follower, you will have a low-impedance output. If you don't have a "rail-to-rail" op amp handy, you might want to try the LM6132 dual. It's 8 pins, and is available in the standard 8-pin DIP package. Don't use an LM358 -- it will accept your signal, but the rise time and fall time will probably be several microseconds. Happy hunting.<p>LM6132 Product Folder<p>[ July 03, 2003: Message edited by: Chris Foley ]</p>

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