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From: nahshon@dan.haifa.ibm.com (Itai Nahshon)
Subject: Re: Futaba fp s148 servos
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Message-ID: <NAHSHON.94Nov1162701@dan.haifa.ibm.com>
In-Reply-To: strohm@mksol.dseg.ti.com's message of Mon, 31 Oct 1994 19:40:14 GMT
Date: Tue, 1 Nov 1994 14:27:01 GMT
Disclaimer: This posting represents the poster's views, not necessarily those of IBM.
References: <39229s$3mg@nova.np.ac.sg> <1994Oct31.194014.16696@mksol.dseg.ti.com>
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In article <1994Oct31.194014.16696@mksol.dseg.ti.com> strohm@mksol.dseg.ti.com (john r strohm) writes:
>In article <39229s$3mg@nova.np.ac.sg> jjt@np.ac.sg (Jones Jack Trevor) writes:
>>Question:
>>
>>is it easily possible to reverse the direction on a Futaba FP-S148 r/c
>>servo?

>The easiest way to do this is to hit your R/C hobby shop and buy a servo
>reverser.  This is a dedicated electronic module that plugs between the
>servo and the controller (usually the R/C receiver) and processes the pulse
>to "reverse" it.  Basically, the device uses a 3 msec one-shot and an XOR
>gate.

>If your local hobby shop doesn't have them, check Ace R/C catalog.

The circuit is very simple. I have built one on a breadboard with a
single 4538 and a few passive components. 
One of the "one-shot's" is trimmer set to 3 Msec (I can use the trimmer to
center-adjust the servo) and is triggered by the rising edge of the control
pulse.
The other "one-shot" has a long (>2Msec) time-contant. It is triggered
by the falling edge of the control pulse and reset by the end of the 3msec
pulse. The output from the second one-shot is 3 Msec minus the input pulse.
Note that the second "one-shot" in my citcuit is used as a simple SR 
Flip-Flop.
I'm sure a similar circuit can be built with cheapper components (4001
quad NOR gate for example). The 3 Msec pulse should be very stable
(it should not change with changes in supply-voltage, temperature etc..)
I do not have any specifications for 4001 based one-shots.

Itai Nahshon
Haifa, Israel
nahshon@vnet.ibm.com
