Hi Johnny ,

any photodiodes will work. The trigger level is about 3 times the photo
voltaic output of the PD. Add a series Si diode to raise the trigger level
by 300mV (3x 100mV) . Since the PD voltage rises with the log of the light
level this means the PD voltage changes little over a large light range
making the trigger level and the "pop" relatively constant. The 555 with
the PD with the lower output voltage will of course trigger first. The cap
should be sized to cause the motor to turn the bot no more than 45 degrees
each pop. Any larger cap and the bot does "twirls".


However your question "triggered" a new version that can use larger caps for
longer pops by driving both motors at the same time but with a turning bias,
which depends on the side triggered.
As shown in the attached, the two diodes connect an active low output which
drives a primary motor directly, also to the opposite motor. The diode drop
reduces the voltage applied to the secondary motor so that it turns more
slowly than the primary motor.

regards

wilf


----- Original Message -----
From: "Johnny Flores" <jflores6@...>
To: "Wilf Rigter" <Wilf.Rigter@...>; <beam@yahoogroups.com>
Sent: Wednesday, March 12, 2003 1:45 PM
Subject: [beam] Re: 555 for a voltage sensor in a SE


Looks cool! Are the diodes marked "PD" standard photodiodes or something
else? Any specific part number? Also, I'm assuming that a larger cap
(e.g., 6800uF) would cause a longer charge time but also a longer discharge
time?

Thanks again!

Johnny
----- Original Message -----
From: Wilf Rigter
To: 'J Flores'
Sent: Wednesday, March 12, 2003 12:04 PM
Subject: RE: 555 for a voltage sensor in a SE


oops, try this for that 555 article.

http://groups.yahoo.com/group/beam/message/24457
-----Original Message-----
From: J Flores [mailto:jflores6@...]
Sent: Wednesday, March 12, 2003 7:28 AM
To: Wilf.Rigter@...
Subject: 555 for a voltage sensor in a SE


I read a "letter to the editors" in the current issue of Nuts and Volts
that described the use of a 555 chip as part of a voltage sensing circuit in
a battery charger. It was set up with a voltage divider across the input
and trigger pins (3 and 6?) such that when the power available from the
battery dropped to a preset threshold, the 555 triggered a solid state relay
to turn on the charger.

I'm wondering if a similar (albeit reverse) set-up could be used as the
voltage sense trigger on an SE? In other words, wire the 555 such that
power is switched to the motor when the 555 trigger senses that the
capacitor is sufficiently charged. Does this make sense? Has anyone tried
this before?

Thanks!

Johnny Flores
University of Texas at San Antonio



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