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DPRG: 555 Program

Subject: DPRG: 555 Program
From: Charlie W Youngblood chaz128 at juno.com
Date: Fri Sep 17 08:27:58 CDT 1999

I have that mimi-notebook, and several others( nand gates, ....).

I also have a Tandy 1000 HX with a RS 232 ad on board. It is fully pro-
gramable. I have all the programming data, specifications, and scemat-
ic. If some one has a Q-Basic program, I could pick it up at the next
mtg.
I would like to experiment with this as a controller. I like this system
because it boots on ROM. Any help would be appreciated.

			Charlie Y.
>
>Radio Shack used to sell Forrest Mims notebooks, which were good
>sourcebooks for circuits, including 555 circuits.  I think there was a
>dedicated 555 mini-notebook.
>
>To generate PWM suitable for controlling an R/C servo, you have to use 
>a
>one-transistor inverter downstream of the 555.  I don't remember the
>circuit offhand, but basically you set it up to give a NOMINAL total 
>period
>of "about" 20-50 msec (noncritical), and then you use the variable 
>resistor
>in the discharge arm, plus some fixed resistance, to give you your 1-2 
>msec
>control pulse.
>
>OK, from memory:  Tie 555 pins 2 and 6 together, and tie the capacitor
>between that junction and ground.  The discharge resistor (fixed in 
>series
>with variable) goes between the junction and pin 7.  The charge 
>resistor
>goes from pin 7 to the positive rail.  The charge time is given by 
>0.693(Rd
>+ Rc)C, or some such, R in ohms and C in farads.  The discharge time 
>is
>given by 0.693 Rd C.  What happens is that the 555 will ground pin 7 
>when
>the voltage on pin 6 goes above the threshold.  The capacitor then
>discharges through the discharge resistor, through pin 7.  Meanwhile, 
>the
>charge resistor is also grounded, giving you a little flow directly 
>from
>the plus rail through the charge resistor into pin 7.  The edge on pin 
>2
>caused by the discharging capacitor is what recycles the 555 in 
>astable
>mode.
>
>What you get out on pin 3 is a nice clean high for the length of the 
>charge
>cycle, and a nice clean low during the discharge cycle.  Because the 
>series
>resistance to the capacitor is much higher during the charge cycle 
>(pin 7
>is floating during the charge cycle), the high period is MUCH longer 
>than
>the low period.  This is precisely wrong for servo PWM, so you have to
>invert pin 3.  When I did it, I used a 2N2222 inverter; any inverting 
>gate
>would work just as well (provided it could drive the servo).
>
>NOTE WELL:  You MUST have adequate bypass capacitors in this circuit, 
>or
>the servo will "buzz" (hunt, chewing huge amounts of motor current).  
>I
>used 200 uF directly across the servo supply leads, and 10 uF 
>(tantalum - I
>had it) and 0.01 uF across the 555 supply pins, and that worked pretty
>well.

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