I have attached a simple PSHEAD v1 schematic for a brief explanation. It
still is the simplest and most elegant of the PShead designs. The actual
component values don't matter very much as is explained below.

The PD1/PD2 photobridge acts as a voltage divider with the midpoint at Vcc/2
when the light on each PD is equal. The response is ;inear over a wide range
of ambient light levels. Some PDs are very efficient and may need some light
shielding (ie heat shrink) to reduce the photo current level in very bright
light.

The hi/lo oscillator is uses R1/C1 to set the basic frequency. Resistor R3
connected from the photobridge midpoint to the R/C node of the oscillator is
used to influence that frequency. That frequency depends on the voltage of
the midpoint as well as the absolute resistance of the PDs. In general, the
frequency is the lowest when the PD bridge center voltage is near the
threshold of the 7AC240 inverter input.

With a R1 to R3 = 10 to 1 ratio, the frequency and the duty cycle varies
when the PD output voltage is less than +/-10% above or below the threshold.
Voltage levels above and below that +/-10% range will cause the output of
the oscillator to stop and will be steady high or low, inverted with respect
to the input voltage. The band of oscillation can be made more or less by
decreasing or increasing the ratio of R1 and R3 values. (ie if R1=R3 it
always oscillates). I use typical ratios of between 2 to 1 and 10 to 1. If
the ratio is high, the sensitivity is greater but the tendency to wiggle is
also greater. Remember a shaky head is not power efficient.

For some applications that have lot's of build in damping (ie Submarine
head, Mazola head) R3 can be 0 ohm 8)

The Nv/Nu driver (also called Nx driver) both differentiates and integrates
the complementary outputs from the oscillator. When the oscillator
oscillates, the Nx driver AC couples the output pulses noninverted as the Nx
time constant is longer than the oscillator period. With AC coupled
(non-inverted AC) pulses in phase across the motor, the PSHead is in the
power save mode but with the brake on! That is important to be able to stop
the motor rotation quickly to avoid mecahnical overshoot when the head is
aligned with the light source.

When the oscillator stops oscillating theres are no pulses to AC couple and
instead, the steady output is DC coupled (and inverted) to cause a
differential voltage across the motor which then rotates.

The ratio of the R1/C1 and R2/C2 time constant should be about 1 to 10 for
low standby power. Smaller ratios can be used for faster and more "varied"
response but sometimes higher standby current. Since the R1/C1 time
constant is influenced by R3 and the photo bridge output voltage, that ratio
will vary with light level. The ratios of those two RC timeconstant
component values cause a variety of behaviour, sensitivity and efficiency.
For solar powered PS Heads, the time constant of R2/C2 should (probably) be
much shorter than the duration of the SE pop or some unpredictable (perhaps
interesting) side effects may occur.

Hope that short description helps,

wilf


----- Original Message -----
From: "Mike Robertson" <mikerobe@...>
To: <beam@yahoogroups.com>
Sent: Saturday, April 05, 2003 6:55 PM
Subject: Re: [beam] SPSH with inverted 1381 SE


> all right
> ver 1-2 is what i have built right now and it works seemingly quite well
> but i haven't found any caps of the same values as those in your original
> circuit
> the two caps...do they control the on and off time? (off charge, on run)
> if so then i could just drop the 10 M down to 100k and keep usign the same
> caps i have right?
>
> maybe i should start working through other circuits and convert them to
use
> the inverted 1381
>
>
> mike
>
> m
> ----- Original Message -----
> From: "Wilf Rigter" <wrigter@...>
> To: <beam@yahoogroups.com>
> Sent: Saturday, April 05, 2003 7:14 PM
> Subject: Re: [beam] SPSH with inverted 1381 SE
>
>
> > Hi Mike,
> >
> > Ori's suggestion is good.
> > Some tiny comments:
> > You're correct to switch the PDs with pin 1. Otherwise those can leak a
> lot
> > of current during charging.
> > Leave pin 19connected to 0V as you suggested to avoid floating inputs.
> >
> > The right hand PD should be reversed. The 1uF cap should be 0.01uF and
> the
> > 0.1 uF should be 0.001uF to make the response time 0.1 seconds (With 10M
> and
> > 1uF the timeconstant would be 10sec!).
> >
> > wilf
> >
> >
> > ----- Original Message -----
> > From: "Mike Robertson" <mikerobe@...>
> > To: <beam@yahoogroups.com>
> > Sent: Saturday, April 05, 2003 12:53 PM
> > Subject: Re: [beam] SPSH with inverted 1381 SE
> >
> >
> > i wasn't exactly clear on what changes you meant
> > but here's what i did
> > 13812 out to pin 1 on XX240
> > 19 stayed on ground
> > 20 to Vcc
> > 10 to gnd
> > 13812 pin 2 to pin vcc
> > 13812 pin 3 to pin gnd
> >
> > and the last inverter was added to the driver portion
> > pin 9 to 7 (pin 1 to 9 removed)
> > pin 11 to 13
> > 10M resistor connecting pin 10 and 11 removed
> > and cap values were changed
> > ----- Original Message -----
> > From: Ori
> > To: beam@yahoogroups.com
> > Sent: Saturday, April 05, 2003 1:16 PM
> > Subject: Re: [beam] SPSH with inverted 1381 SE
> >
> >
> > That will work just fine, but... An improvement from someone you
> wouldn't
> > expect (Wilf ;).
> >
> > Connect all of the parts as you would if the cap was a battery, except
> do
> > not connect the enable pins of your 240. Then, all you need to do is put
> the
> > 13812 in, and connect the output to the enable pins.
> >
> > Skip the solarengine! ;)
> >
> >
> >
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