Hi Lindy,

Using that calculator just plug in 1uF, 2.2 Megohms and 10 Ohms

It shows 6 us for the low time but that will actually be larger since
the on resistance of the 555 discharge will add a bit. So you'll about
10 us every 1.5 to 2 seconds.

If you don't want negative pulses (which is basically what 555 timers
do) you need to invert the output, either with another 555 and no parts,
or an inverter, or a transistor and a couple of resistors.

Best regards, Bob

PS to use a 555 as an inverter, tie 4 and 8 to positive, 1 to ground.
Use pins 2 and 6 tied together for the input and your output is on 3.


On Wed, Dec 11, 2013, at 08:28 AM, Lindy Mayfield wrote:
> I wanted to try to create an analogue circuit that would trigger a sonic
> range finder (SR04) so that I could check things out with a scope.  It
> requires a 10us pulse to start it.  (Then the output line goes high
> relative to the distance.)  I don't think I have the ability (yet) to do
> much more with it though.  The intent is to connect it to a Pi.
>=20
> What I wanted was a 10us pulse every second or so, and I found a handy
> 555 laying around.  However, using the nice calculator I found below, I
> couldn't manage to get anywhere near the results I wanted no matter what
> I put in.   (The largest capacitor I have is 100 uF.)  Is this possible?=
=20
> I know it should be.
>=20
> Thank you!
> Lindy
>=20
> http://www.ohmslawcalculator.com/555_astable.php

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