Alt-BEAM Archive

Message #01978



To: Wilf Rigter Wilf.Rigter@powertech.bc.ca
From: Steven Bolt sbolt@xs4all.nl
Date: Fri, 2 Apr 1999 09:18:32 +0200 (CEST)
Subject: [alt-beam] Opamp SE's WAS: 74HC240


On Thu, 1 Apr 1999, Wilf Rigter wrote:

> It has very respectable current requirements at 2-3V and also at
> much higher supply voltages ie 5V-12V which makes it quite flexible for
> different applications.

The use of opamps is very appropriate for linear applications above
3V, and you've found margin to go a little lower. Good news for
those who want the low parts count of Dave's Photopopper, but can't
find the 1381.

> Having said that, your sampling method for the threshold detector
> is very nice and will reduce current at higher voltages as well
> but the 74HC cmos oscillator could be made more efficient with
> the addition of a 5x resistor at the input of the first stage
> forcing a larger input voltage swing (2 times the power supply)
> and longer time constant for a given RC.

I had a look and was able to gain about 1uA. At 0.005, the
pulse/pauze ratio is already very small. The dominant factor is
actually the chip manufacturer. With two of three I tried, the
SunEater_II/IV approach results in a wasted current (not going into
the storage cap) of about 10uA. The third (Motorola) needed
something like 36uA. To that we have to add the storage capacitor
leakage, which for my 47000uF `supercaps' is about 2uA.

I don't think too much should be made of those numbers.
SunEater_IV's adjustable switch-on level and motor burst duration
do much more for (direct drive) photovore performance. Being able
to use a physically small, light supercap and still get short motor
bursts helps to keep size and weight down, which also is rather
important; SunEater_V is supposed to use Dave's 4mm diameter, 1.5
grams "Tiny Pager Motors." The large capacitance is an additional
advantage - it results in little voltage drop during a burst, which
gives more efficient acceleration from stop.

Best,

Steve

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# sbolt@xs4all.nl # Steven Bolt # popular science monthly KIJK #
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