Alt-BEAM Archive
Message #05220
To: beam@sgiblab.sgi.com
From: Steven Bolt sbolt@xs4all.nl
Date: Sat, 10 Jul 1999 11:03:01 +0200 (CEST)
Subject: [alt-beam] Re: Something funny with the 1382 voltage triggers?
On Fri, 9 Jul 1999, Bob Shannon wrote:
---8<---
> Either of these approaches can resolve the problem without
> resorting to other SE circuits.
I can't help but wonder why you are so reluctant to replace that
2-transistor latch/driver, while keeping the voltage trigger.
Note that a monostable would not merely avoid all reset trouble.
If you stop and think, you'll realize that using the storage cap
and the load as a crude RC timer isn't such a great idea in most
applications. The monostable gives you an independent RC constant,
easily adjustable for maximum efficiency.
You might connect the voltage trigger output to the active high
input of a monostable (1/2 74HC221, for instance). A cap and
a resistor determine the output timing (about 0.7 * C * R).
Use an adjustable resistor to make burst duration continuously
adjustable; replace with a fixed resistor when you've found the
optimum value.
1K connects a BC817-25 to the active high output. The transistor
drives your motor.
The smallest supercap (47000uF) would provide sufficient current
in a light and compact package, and the voltage would hardly drop
during motor bursts, unless you want them rather long.
If your motor does well at low voltages, efficiency will be
increased if you use for instance a 2V (instead of 2.7V) trigger
level, or go even lower. You still get good performance, because
the voltage won't drop much and certainly not rapidly, while the
motor is running.
That's five and a half components for a single SE, or eleven
for a double version. The 74HC221, all resistors, the 100N cap and
the driver are SMT, which means that weight and size will be mostly
determined by the solar panel and motor(s).
Best,
Steve
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# sbolt@xs4all.nl # Steven Bolt # popular science monthly KIJK #
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